taken from:http://www.youtube.com/watch?v=ea1GIRosBoc

atom parts

Electrons move constantly. Each electron has a negative electrical charge. Electrons can move away from the atom sometimes. They can be shared between atoms, or they can go from one atom to another. Electrons can even move through matter, which is what causes electricity. Electrons are very light and incredibly small.

Neutrons
and protons are held together very tightly in the nucleus, and are very hard to break apart. If you do break a nucleus apart, you release a great deal of energy, because the atom is made of energy.

proton is a subatomic particle found in the nucleus of all conventional atoms. The only place you can find matter without protons is in a neutron star or the core of powerful particle accelerators. The protonprotonproton has a positive charge, which balances out the negative charge in atoms, electrons. If an atom has an imbalance of protons or neutrons, it is no longer neutral and becomes a charged particle, also known as an ion.
external image atomo.jpg

////http://en.wikipedia.org/wiki////

What is an atom?​​​​


The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.
The name atom comes from the Greek which means uncuttable, or indivisible, something that cannot be divided further. The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was divisible. The principles of quantum mechanics were used to successfully model the atom.
taken from:http://en.wikipedia.org/wiki/Atom

external image atom.jpg

atomism

The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny quantities has been around for millennia, but these ideas were founded in abstract, philosophical reasoning rather than experimentation and empirical observation. The nature of atoms in philosophy varied considerably over time and between cultures and schools, and often had spiritual elements. Nevertheless, the basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.
The earliest references to the concept of atoms date back to ancient India in the 6th century BCE appearing first in Jainism The Nyaya and Vaisheshika schools developed elaborate theories of how atoms combined into more complex objects. In the West, the references to atoms emerged a century later from Leucippus, whose student, Democritus, systematized his views. In approximately Democritus coined the term átomos.which means "uncuttable" or "the smallest indivisible particle of matter". Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.taken from:http://en.wikipedia.org/wiki/Atom

external image atom_nucleon.jpg

stucture of the atom
Is a very basic unit of matter is the atom. Although originally thought of as the smallest form of matter, atoms are really made up of even smaller particles—a nucleus consisting of protons and neutrons and electrons in shells around the nucleus. The most common way to picture an atom is the solar system model. The number of protons in the nucleus determines the element's atomic number. The number of electrons typically equals the number of protons.

external image Atom_Structure_3.png

taken from:http://www.school-for-champions.com/science/atom.htm

An atom a fundamental piece of matter. (Matter is anything that can be touched physically.) Everything in the universe (except energy) is made of matter, and, so, everything in the universe is made of atoms.
An atom itself is made up of three tiny kinds of particles called subatomic particles: protons, neutrons, and electrons. The protons and the neutrons make up the center of the atom called the nucleus and the electrons fly around above the nucleus in a small cloud. The electrons carry a negative charge and the protons carry a positive charge. In a normal (neutral) atom the number of protons and the number of electrons are equal. Often, but not always, the number of neutrons is the same, too.
take from:http://www.qrg.northwestern.edu/projects/vss/docs/Propulsion/1-what-is-an-atom.html

external image atom_model_04.gif
taken from:http://education.jlab.org/qa/atom_model_04.gif
characteristics.


Atoms are extremely small. One hydrogen atom(the smallest atom known) is approximately 5 x 10-8 mm in diameter. To put that in perspective, it would take almost 20 million hydrogen atoms to make a line as long as this dash -. Most of the space taken up by an atom is actually empty because the electron spins at a very far distance from the nucleus. For example, if we were to draw a hydrogen atom to scale and used a 1-cm proton (about the size of this picture the atom's electron would spin at a distance of ~0.5 km from the nucleus. In other words, the atom would be larger than a football field!
Atoms of different elements are distinguished from each other by their number of protons (the number of protons is constant for all atoms of a single element; the number of neutrons and electrons can vary under some circumstances). To identify this important characteristic of atoms, the term atomic number (z) is used to describe the number of protons in an atom. For example, z = 1 for hydrogen and z = 2 for helium.
Another important characteristic of an atom is its weight, or atomic mass. The weight of an atom is roughly determined by the total number of protons and neutrons in the atom. While protons and neutrons are about the same size, the electron is more that 1,800 times smaller than the two. Thus the electrons' weight is inconsequential in determining the weight of an atom - it's like comparing the weight of a flea to the weight of an elephant. Refer to the animation above to see how the number of protons plus neutrons in the hydrogen and helium atoms corresponds to the atomic mass
taken of http://www.visionlearning.com/library/module_viewer.php?mid=50


atom identification
The scanning tunneling micr is a device for viewing surfaces at the atomic level. It uses the oscope quantum tunneling phenomenon, which allows particles to pass through a barrier that would normally be insurmountable. Electrons tunnel through the vacuum between two planar metal electrodes, on each of which is an adsorbed atom, providing a tunneling-current density that can be measured. Scanning one atom (taken as the tip) as it moves past the other (the sample) permits plotting of tip displacement versus lateral separation for a constant current. The calculation shows the extent to which scanning-tunneling-microscope images of an individual atom are visible. It confirms that for low bias, the microscope images the space-averaged dimensions of the electron orbitals across closely packed energy levels—the Fermi level local density of states.
An atom can be ionized by removing one of its electrons. The electric charge causes the trajectory of an atom to bend when it passes through a magnetic field. The radius by which the trajectory of a moving ion is turned by the magnetic field is determined by the mass of the atom. The mass spectrometeruses this principle to measure the mass-to-charge ratio of ions. If a sample contains multiple isotopes, the mass spectrometer can determine the proportion of each isotope in the sample by measuring the intensity of the different beams of ions. Techniques to vaporize atoms include inductively coupled plasma atomic emission spectroscopy and inductively coupled plasma mass spectrometry, both of which use a plasma to vaporize samples for analysis.
,

taken from: http://en.wikipedia.org/wiki/Atom#Identification
external image atomo.gif

taken from: http://intercentres.cult.gva.es/intercentres/46019684/atomo.gif


quarks: inside the atom


taken from: http://www.youtube.com/watch?v=SMgi2j9Ks9k




thomson model

Modern atomic modelWave functions of the first atomic orbitalsMain ThomsonArtículo atomic model: Plum pudding modelFollowing the discovery of the electron in 1897 by Joseph John Thomson, it was determined that the material consisted of two parts, one negative and one positive. The downside was made up of electrons, which according to this model were immersed in a mass of positive charge by way of raisins in a cake (from the analogy of English plum-pudding model) or grapes in jelly. Jean Perrin subsequently proposed a modified model from Thompson where the "raisin" (electrons) were at the outside of the "cake" (the positive charge






The name atom
comes from the Greek átomos, α-τεμνω, which means uncuttable, or indivisible, something that cannot be divided further. The concept of an atom as an indivisible component of matter was first proposed by early indian and Greek philosophers. In the 17th and 18th centuries, chemist provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicist discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was divisible. The principles of quantum mechanics were used to successfully model the atom.
atoms
The atom is a basic unit of matter consisting of a dense, central nucleue surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.
The name atom comes from the Greek ἄτομος/átomos, α-τεμνω, which means uncuttable, or indivisible, something that cannot be divided further. The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was divisible. The principles of quantum mechanics were used to successfully model the atom
history:
The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny quantities has been around for millennia, but these ideas were founded in abstract, philosophical reasoning rather than experimentation and empirical observation. The nature of atoms in philosophy varied considerably over time and between cultures and schools, and often had spiritual elements. Nevertheless, the basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.
The earliest references to the concept of atoms date back to ancient India in the 6th century BCE, appearing first in Jainism. The Nyaya and Vaisheshika schools developed elaborate theories of how atoms combined into more complex objects.[8] In the West, the references to atoms emerged a century later from Leucippus, whose student, Democritus, systematized his views. In approximately 450 BCE, Democritus coined the term átomos (Greek: ἄτομος), which means "uncuttable" or "the smallest indivisible particle of matter". Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.

http://en.wikipedia.org/wiki/Atoms

subatomic particles:

In physics, subatomic particles are the particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles. Particle physics and nuclear physics study these particles and how they interact.Exampols of subatomic particles that constitute the atom are: electrons, protons, neutrons.
external image 118a.jpg
http://en.wikipedia.org/wiki/Subatomic_particle


























John Dalton
The unit Evidence for Particles showed how the ancient Greeks had ideas about particles and atoms. But it wasn't until the start of the nineteenth century that a theory of atoms became linked to strong experimental evidence. It was then that an English scientist called John Dalton put forward his ideas about atoms.


From his experiments and observations, he suggested that atoms were like tiny, hard balls. Each chemical
An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.
element had its own atoms that differed from others in mass. Dalton believed that atoms were the fundamental building blocks of nature and could not be split. In chemical reactions, the atoms would rearrange themselves and combine with other atoms in new ways.


In many ways, Dalton's ideas are still useful today. For example, they help us to understand elements, compounds, and molecules.





















J. J Thomsom

At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. This is a tiny negatively charged particle that is much, much smaller than any
An atom is the smallest particle of an element that can still be defined as that element.
atom. When he discovered the
Electrons are tiny, negatively charged particles that orbit the nucleus of an atom in energy levels (or shells).
electron, Thomson was experimenting by applying high voltages to gases at low pressure.

external image 20070924klpcnafyq_29.Ges.SCO.png
external image 20070924klpcnafyq_29.Ges.SCO.png



E. Rutherford

The next development came about 10 years later. Two of Ernest Rutherford's students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University with radiation. They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They expected the particles to barge their way straight through the gold atoms unimpeded by the diffuse positive charge spread throughout the atom that Thomson's model described. However, they got a big surprise.
In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus.



Niels Bohr


The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the
The nucleus is the centre of an atom, containing protons and neutrons.
nucleus in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high energy level to a low one.




external image quimica_001_02p.gif
external image quimica_001_02p.gif


taken from :​ http://www.broadeducation.com/htmlDemos/AbsorbChem/HistoryAtom/page.htm

Atomism:
Main article: atomism
The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny quantities has been around for millennia, but these ideas were founded in abstract, philosophical reasoning rather than experimentation and empirical observation. The nature of atoms in philosophy varied considerably over time and between cultures and schools, and often had spiritual elements. Nevertheless, the basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.
The earliest references to the concept of atoms date back to ancient india in the 6th century bce,appearing first in jainism. The nyaya and vaisheshica schools developed elaborate theories of how atoms combined into more complex objects. In the West, the references to atoms emerged a century later from leucippus, whose student, democritus, systematized his views. In approximately 450 BCE, Democritus coined the term átomos (greek: ἄτομος), which means "uncuttable" or "the smallest indivisible particle of matter". Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.
Corpuscularianism is the postulate, expounded in the 13th-century by the alchemist pseudo-geber (Geber), that all physical bodies possess an inner and outer layer of minute particles or corpuscles. Corpuscularianism is similar to the theory atomism, except that where atoms were supposed to be indivisible, corpuscles could in principle be divided. In this manner, for example, it was theorized that mercury could penetrate into metals and modify their inner structure. Corpuscularianism stayed a dominant theory over the next several hundred years and was blended with alchemy by robert boyle and isaac newton in the 17th century. It was used by Newton, for instance, in his development of the corpuscular teory of ligth.
Origin of scientific theory:
Further progress in the understanding of atoms did not occur until the science of chemistry began to develop. In 1661, natural phylosofer robert boyle published the sepmical chemist in which he argued that matter was composed of various combinations of different "corpuscules" or atoms, rather than the classical elements of air, earth, fire and water. In 1789 the term element was defined by the French nobleman and scientific researcher antonie lavoisier to mean basic substances that could not be further broken down by the methods of chemistry.
In 1803, English instructor and natural philosopher john dalton used the concept of atoms to explain why elements always react in a ratio of small whole numbers—the law of multiple proportions—and why certain gases dissolve better in water than others. He proposed that each element consists of atoms of a single, unique type, and that these atoms can join together to form chemical compounds. Dalton is considered the originator of modern atomic theory
Additional validation of particle theory occurred in 1827 when botanist robert brown used a microscope to look at dust grains floating in water and discovered that they moved about erratically—a phenomenon that became known as "brounian botshier". J. Desaulx suggested in 1877 that the phenomenon was caused by the thermal motion of water molecules, and in 1905 albert einstein produced the first mathematical analysis of the motion. French physicist jean perrin used Einstein's work to experimentally determine the mass and dimensions of atoms, thereby conclusively verifying Dalton's atomic theory.
http://en.wikipedia.org/wiki/Atom#Origin_of_scientific_theory



WHAT IS ATOM?


An atom is a particle of matter that uniquely defines achemical element. An atom consists of a central nucleus that is usually surrounded by one or more electrons. Each electron is negatively charged. The nucleus is positively charged, and contains one or more relatively heavy particles known as protons and neutrons.

external image atom2.jpg




A proton is positively charged. The number of protons in the nucleus ofan atom is the atomic number for the chemical element. A proton has a rest mass, denoted mp, of approximately 1.673 x 10-27 kilogram (kg). A neutron is electrically neutral and has a rest mass, denoted mn, of approximately 1.675 x 10-27 kg. The mass of a proton or neutron increases when the particle attains extreme speed, for example in a cyclotron or linear accelerator.

An early model of the atom was developed by the physicist Ernest Rutherford in 1912. He was the first to suggest that atoms are like miniature solar systems, except that the attractive force is not caused by gravity, but by opposing electrical charges. In the so-called Rutherford atom, electrons orbit the nucleus in circular paths. Niels Bohr revised Rutherford's theory in 1913. In the Bohr atom, the negatively charged electrons orbit the nucleus at specific median distances. These distances are represented by spheres, called shells, surrounding the nucleus. Electrons can move from shell to shell. When an electron absorbs enough energy, it moves to a larger, or higher, shell. When it loses a certain amount of energy, it falls to a smaller, or lower, shell.
The total mass of an atom, including the protons, neutrons and electrons, is the atomic mass or atomic weight. Electrons contribute only a tiny part of this mass. For most practical purposes, the atomic weight can be thought of as the number of protons plus the number of neutrons. Because the number of neutrons in an atom can vary, there can be several different atomic weights for most elements.
Atoms having the same number of protons, but different numbers of neutrons, represent the same element, but are known as different isotopes of that element. The isotope for an element is specified by the sum of the number of protons and neutrons. Examples of different isotopes of an element are carbon 12(the most common, non-radioactive isotope of carbon) and carbon 14 (a less common, radioactive isotope of carbon).

external image atom.jpg


Protons and electrons have equal and opposite charge, and normally an atom has equal numbers of both. Thus, atoms are usually neutral. An ion is an atom with extra electrons or with a deficiency of electrons, resulting in its being electrically charged. An ion with extra electrons is negatively charged and is called an anion; an ion deficient in electrons is positively charged and is called a cation.




//http://searchcio-midmarket.techtarget.com/sDefinition/0,,sid183_gci211610,00.html//





atoms



A very basic unit of matter is the atom. Although originally thought of as the smallest form of matter, atoms are really made up of even smaller particles—a nucleus consisting of protons and neutrons and electrons in shells around the nucleus. The most common way to picture an atom is the solar system model. The number of protons in the nucleus determines the element's atomic number. The number of electrons typically equals the number of protons.
external image atom.jpg
taken from:
http://www.school-for-champions.com/science/atom.htm

taken from:http://www.youtube.com/watch?v=SVyEpeniCzQ


Atom

The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.
external image atomo.gif
taken from:http://en.wikipedia.org/wiki/Atom

The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.[1]
The name atom comes from the Greek ἄτομος/átomos, α-τεμνω, which means uncuttable, or indivisible, something that cannot be divided further. The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was divisible. The principles of quantum mechanics were used to successfully model the atom.[2][3]
Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.9% of an atom's mass is concentrated in the nucleus,[note 1] with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus.[4] Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.

taken form:http://en.wikipedia.org/wiki/Atom


But First, What is Science?

Chemistry is a science. "Science" and the "scientific method" are formal terms for the system and process by which we seek to answer questions about our universe and our place in it. Humans have always been curious about their world. Out of all the animals, we alone were born with a brain that would ask how and why and then search for a suitable answer. Science enables us to study our natural surroundings and acquire new knowledge within an organized system. New knowledge is continually added to our current understandings of the universe. We update and refine our knowledge to give us our "best story". However, the power of science does not lie as much in its current knowledge base as in its continual development of greater learning and understanding. As the 20th century comes to a close, pause to think about all of the new knowledge that has been gathered since 1900. You are surrounded by the results of our natural curiosity. The machine you are looking at is the result of incredible human thought and ingenuity. As we begin a new century, inquisitive people will continue to ask questions and pursue their answers. Curiosity will always motivate new participants to ask new questions. What kinds of questions are being asked by these inquisitive people will determine whether they are considered a chemist, a biologist, a social scientist, a computer scientist, globe.jpga political scientist, an astronomer, etc.

Chemistry:

The discipline of chemistry is a study of the material makeup of the universe. Chemists ask questions such as the following:
  • What is this material made of?
  • How was it made?
  • How long did it take to make it?
  • Will it last or will it change?
  • What causes it to change?
  • Can we control these changes?
  • Is this material useful?
  • Is this material hazardous?
  • In what new ways could we use it?
  • Can we make it?
talldna.jpg
Everything around us (including us) is composed of the basic building blocks we call atoms. There are only about 100 atoms known to exist. However, these building blocks can arrange themselves into an amazing number of different combinations that we call molecules. There are thousands of known molecules and new ones are discovered on a regular basis. Everything that you can see (and can’t see) is composed of atoms and molecules. So the original question "What is Chemistry?" would be better stated as "What isn’t Chemistry?"
By studying chemistry and learning about the composition of our world, what can we understand about our environment and ourselves? How can we use this understanding to improve our lives? These are motivating questions for chemists. In viewing our world though an atomic lens, chemists make their own contributions to the pattern of discovery, knowledge, and change that has defined human history.


http://www.allatoms.com/


the atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutralneutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines thechemical element, and the number of neutrons determine the isotope of the element.[1]
The name atom comes from the Greek ἄτομος/átomoshttp:www.youtube.com/watch?v=SVyEpeniCzQ α-τεμνω, which means uncuttable, or indivisible, something that cannot be divided further. The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was divisible. The principles of quantum mechanics were used to successfully model the atom.[2][3]
Relative to everyday experience, atoms are minuscule http://www.youtube.com/watch?v=SVyEpeniCzQ with proportionately tiny masses. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.9% of an atom's mass is concentrated in the nucleus,[note 1] with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus.[4] Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magneticproperties.


Nuclear properties

Main articles: Isotope, Stable isotope, and List of elements by nuclear stability
By definition, any two atoms with an identical number of protons in their nuclei belong to the same chemical element. Atoms with equal numbers of protons but a different number of neutrons are different isotopes of the same element. For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons hydrogen-1, one neutron (deuterium), two neutrons (tritium) and more than two neutrons. The hydrogen-1 is by far the most common form, and is sometimes called protium.[[#cite_note-66|[66]]] The known elements form a set of atomic numbers from hydrogen with a single proton up to the 118-proton element ununoctium.[[#cite_note-67|[67]]] All known isotopes of elements with atomic numbers greater than 82 are radioactive.[[#cite_note-sills-68|[68]]][[#cite_note-dume-69|[69]]]
About 339 nuclides occur naturally on Earth,[[#cite_note-70|[70]]] of which 256 (about 76%) have not been observed to decay, and are referred to as "stable isotopes". For 80 of the chemical elements, there is at least one stable isotope. Elements 43, 61, and all elements numbered 83 or higher have no stable isotopes. As a rule, there is, for each element, only a handful of stable isotopes, the average being 3.1 stable isotopes per element which has any stable isotopes. Twenty-seven elements have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten, for the element tin.[[#cite_note-CRC-71|[71]]]
Stability of isotopes is affected by the ratio of protons to neutrons, and also by presence of certain "magic numbers" of neutrons or protons which represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. Of the 256 known stable nuclides, only four have both an odd number of protons
and odd number of neutrons: hydrogen-2 (deuterium), lithium-6, boron-10 and nitrogen-14. Also, only four naturally occurring, radioactive odd-odd nuclides have a half-life over a billion years: potassium-40, vanadium-50, lanthanum-138 and tantalum-180m. Most odd-odd nuclei are highly unstable with respect to beta decay, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects.[[#cite_note-CRC-71|[71]]]

Mass

Main article: Atomic mass
Because the large majority of an atom's mass comes from the protons and neutrons, the total number of these particles in an atom is called the mass number. The mass of an atom at rest is often expressed using the unified atomic mass unit (u), which is also called a Dalton (Da). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately 1.66 × 10−27 kg.[[#cite_note-iupac-72|[72]]] Hydrogen-1, the lightest isotope of hydrogen and the atom with the lowest mass, has an atomic weight of 1.007825 u.[[#cite_note-73|[73]]] An atom has a mass approximately equal to the mass number times the atomic mass unit.[[#cite_note-74|[74]]] The heaviest stable atom is lead-208,[[#cite_note-sills-68|[68]]] with a mass of 207.9766521 u.[[#cite_note-75|[75]]]
As even the most massive atoms are far too light to work with directly, chemists instead use the unit of moles. The mole is defined such that one mole of any element will always have the same number of atoms (about 6.022 × 1023). This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element will have a mass very close to 0.001 kg, or 1 gram. Because of the definition of the unified atomic mass unit, carbon has an atomic mass of exactly 12 u, and so a mole of carbon atoms weighs exactly 0.012 kg.[[#cite_note-iupac-72|[72]]] Other nuclides have atomic masses and molar masses very close to whole numbers in their usual units, such as hydrogen-1. However, except for carbon-12, they cannot be exactly integer numbers, because the masses of different nuclides are not exact integer ratios of each other, although they do not differ from whole number ratios by more than 1%, and often much less.[[[wiki/Wikipedia:Citation_needed|citation needed]]//]

Size

Main article: Atomic radius
Atoms lack a well-defined outer boundary, so the dimensions are usually described in terms of the distances between two nuclei when the two atoms are joined in a chemical bond. The radius varies with the location of an atom on the atomic chart, the type of chemical bond, the number of neighboring atoms (coordination number) and a quantum mechanical property known as spin.[[#cite_note-76|[76]]] On the periodic table of the elements, atom size tends to increase when moving down columns, but decrease when moving across rows (left to right).[[#cite_note-77|[77]]] Consequently, the smallest atom is helium with a radius of 32 pm, while one of the largest is caesium at 225 pm.[[#cite_note-78|[78]]] These dimensions are thousands of times smaller than the wavelengths of light (400–700 nm) so they can not be viewed using an optical microscope. However, individual atoms can be observed using a scanning tunneling microscope.
Some examples will demonstrate the minuteness of the atom. A typical human hair is about 1 million carbon atoms in width.[[#cite_note-79|[79]]] A single drop of water contains about 2 sextillion (2 × 1021) atoms of oxygen, and twice the number of hydrogen atoms.[[#cite_note-80|[80]]] A single carat diamond with a mass of 2 × 10-4 kg contains about 10 sextillion (1022) atoms of carbon.[[#cite_note-81|[note 2]]] If an apple were magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple.[[#cite_note-82|[81]]]
​ taken from: http://en.wikipedia.org/wiki/Atom