The word atom comes from the Greek word atomos, meaning uncuttable.
From Wikipedia :
An atom is a microscopic structure found in all ordinary matter around us.
Atoms are composed of 3 types of subatomic particles: Electrons, which have a negative charge; Protons, which have a positive charge; and Neutrons, which have no charge.
Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. An atom is the smallest particle differentiable as a certain chemical element; when an atom of an element is divided, it ceases to be that element. Only 90 elements have been identified as occurring naturally on Earth although some elements (for example, technetium and californium) have been identified in supernovae.
Each element is unique by the number of protons in each atom of that element. Every atom has a number of electrons equal to its number of protons; if there is an imbalance, the atom is called an ion. Atoms of the same element can have different numbers of neutrons, as long as the number of protons or electrons does not change. Atoms with different numbers of neutrons are called isotopes of a chemical element.
Other elements have been artificially created, but they are usually unstable and spontaneously change into stable natural chemical elements by the processes of radioactive decay.
Though only 90 naturally occurring elements exist, atoms of these elements are able to bond into molecules and other types of chemical compounds. Molecules are made up of multiple atoms; For example, a molecule of water is a combination of 2 hydrogen and one oxygen atoms.
Because of their ubiquitous nature, atoms have been an important field of study for many centuries. Current research focus on quantum effects, such as in Bose-Einstein condensate.
The atomic theory is a theory of the nature of matter. It states that all matter is composed of atoms.
The most widely accepted model of an atom is the wave model. It is based on the Bohr model, but takes into account recent developments and discoveries in quantum mechanics. It states that: Atoms are composed of subatomic particles (protons, electrons, and neutrons). However, much of an atom's volume is empty space. At the center of the atom is a tiny, positively charged nucleus composed of protons and neutrons (known as nucleons). The nucleus is more than 100,000 times smaller than the atom. If an atom was expanded to the size of Heathrow Airport, the nucleus would be about the size of a golf ball. The electron orbital wavefunctions of hydrogen: the principal quantum number is at the right of each row and the azimuthal quantum number is denoted by letter at top of each column. Most of the atom's space is taken up by orbitals containing electrons in a certain electron configuration.
Each orbital can hold up to two electrons, and is governed by three quantum numbers: the principal, azimuthal, and magnetic.
Each electron in an orbital has a unique value for the fourth quantum number, spin. Orbitals are not physical contructs, but are actually probability distributions of where two electrons having equal values for the first three quantum numbers might be. The "edge" of an orbital is generally considered to be where the probability of an electron's presence drops below 90%.
As electrons join an atom, they fall into the lowest energy shell; that is, the orbitals closest to the nucleus (the first shell). Only the electrons in the outermost orbital (the valence shell) are available for atomic bonding; see "Valence and bonding" for more information.
ATOM SIZES The size of an atom is not easily defined since the electron orbitals just gradually go to zero as the distance from the nucleus increases. For atoms that can form solid crystals, the distance between adjacent nuclei can give an estimate of the atom size. For atoms that do not form solid crystals other techniques are used, including theoretical calculations. As an example, the size of a Hydrogen atom is estimated to be approximately 1.2 10-10m. Compare this to the size of the Proton which is the only particle in the nucleus of the Hydrogen atom which is approximately 0.87 10-15m. Thus the ratio between the sizes of the Hydrogen atom to its nucleus is about 100,000. Atoms of different elements do vary in size, but the sizes are roughly the same to within a factor of 2 or so. The reason for this is that elements with a large positive charge on the nucleus attract the electrons to the center of the atom more strongly.
ELEMENTS AND ISOTOPES Atoms are generally classified by their atomic number, which corresponds to the number of protons in the atom. The atomic number defines which element the atom is. For example, carbon atoms are those atoms containing 6 protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical behavior. The various kinds of atoms are listed in the periodic table in order of increasing atomic number.
The mass number, atomic mass number, or nucleon number of an element is the total number of protons and neutrons in an atom of that element, because each proton or neutron essentially has a mass of 1 amu. The number of neutrons in an atom has no effect on which element it is. Each element can have numerous different atoms with the same number of protons and electrons, but varying numbers of neutrons. Each has the same atomic number but a different mass number. These are called the isotopes of an element. When writing the name of an isotope, the element name is followed by the mass number. For example, carbon-14 contains 6 protons and 8 neutrons in each atom, for a total mass number of 14.
The simplest atom is the hydrogen atom, which has atomic number 1 and consists of one proton and one electron. The hydrogen isotope which also contains 1 neutron is called deuterium or hydrogen-2; the hydrogen isotope with 2 neutrons is called tritium or hydrogen-3.
The atomic mass listed for each element in the periodic table is an average of the isotope masses found in nature, weighted by their abundance.
VALENCE AND BONDING The chemical behavior of atoms is largely due to interactions between electrons. Electrons of an atom must remain within certain, predictable electron configurations. Electrons fall into shells based on their relative distance from the nucleus (see Atomic structure for more details). The electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus.
Each shell, numbered from the one closest to the nucleus, can hold up to a limited number of electrons due to its differing number and type of orbitals:
Shell 1: 2 electrons Shell 2: 8 electrons Shell 3: 8 or 18 electrons (depending on the element)
Electrons fill orbitals and shells from the inside out, beginning with shell one. Higher numbered shells only exist when made necessary by the number of electrons. Whichever extant shell is currently most outward is the valence shell, even if it only has one electron. The reason why shells fill up in order is that the energy levels of electrons in the innermost shells are significantly lower than the energy levels of electrons in outer shells. So if the inner shells were not completely full, the electron in an outer shell would quickly "fall" into the inner shell (with the emission of a photon that would carry away the difference in the energy levels.
The number of electrons in an atom's outermost valence shell governs its bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the periodic table of the elements. Group (i.e. column) 1 elements contain one electron on their outer shell; Group 2, two electrons; Group 3, three electrons; etc. As a general rule, the fewer electrons in an atom's valence shell, the more reactive it is. Group 1 metals are therefore very reactive, with caesium, rubidium, and francium being the most reactive of all elements.
Every atom is much more stable (i.e. less energetic) with a full valence shell. This can be achieved one of two ways: an atom can either share electrons with neighboring atoms (a covalent bond), or it can remove electrons from other atoms (an ionic bond). Another form of ionic bonding involves an atom giving some of its electrons to an other atom; this also works because it can end up with a full valence by giving up its entire outer shell. By moving electrons, the two atoms become linked. This is known as chemical bonding and serves to build atoms into molecules. Five major types of bonds exist: ionic bonds; Covalent bonds; Coordinate covalent bonds; Hydrogen bonds; and Metallic bonds.
ATOMS IN THE UNIVERSE AND OUR WORLD Using inflation theory, the number of atoms in the observable universe can be estimated to be between 4x1078 and 6x1079. However, because of the possibly infinite nature of the universe, the total number of atoms in the entire universe may be much larger or even infinite. This does not change the estimated number of atoms in the observable universe since that is the number of atoms within about 14 billion light years of us - which is all that we can observe since the universe is only about 14 billion years old.