Atom

An atom (from the Greek Greek atomos for "uncut") is the smallest, irreducible constituent of a system. Most often the term refers to chemical atoms, which are the constituents of molecules, are not divisible by chemical reactions but are now known to be composed of even smaller particles. This article discusses these chemical atoms. You are gay!! Loser Atoms suck

The ordinary variety of matter that is dealt with in everyday experience consists of discrete atoms. The existence of such things was first proposed by Greek philosophers such as Democritus, Leucippus, and the Epicureans, but without any real way to be sure the concept disappeared until it was revived by Dalton. The atomic theory explains why gases always combine in simple ratios. It was with Amedeo Avogadro's work, in the 19th century, that scientists began to distinguish atoms and molecules. In modern times atoms have been observed experimentally.

As it turns out, atoms are themselves made out of smaller particles. In fact, almost all of an atom is empty space. At the center is a tiny positive nucleus composed of nucleons (protons and neutrons), and the rest of the atom contains only the fairly flexible electron shells. Usually atoms are electrically neutral with as many electrons as protons. Atoms are generally classified by the atomic number, which corresponds to the number of protons in the atom. 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. Atoms having the same atomic number, but different atomic masses (due to having different numbers of neutrons) are called isotopes.

The simplest atom is the hydrogen atom, having atomic number 1 and consisting of one proton and one electron. It has been the subject of much interest in science, particularly in the early development of quantum theory.

The chemical behavior of atoms is largely due to interactions between the electrons. In particular 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.

There is a strong tendency for atoms to try to completely fill (or empty) the outer electron shell, which for the case of hydrogen and helium has space for two electrons and has space for eight electrons in all other atoms. This is achieved either by sharing electrons with neighboring atoms or by completely removing electrons from other atoms. When electrons are shared a covalent bond is formed between the two atoms. Covalent bonds are relatively strong.

When one or more electrons are completely removed from one atom by another, ions are formed. Ions are atoms that possess a net charge due to an imbalance in the number of protons and electrons. The ion that stole the electron(s) is called an anion and is negatively charged. The atom that lost the electron(s) is called a cation and is possitively charged. Cations and anions are attracted to each other due to coulombic forces between the positive and negative charges. This attraction is called ionic bonding and is weaker than covalent bonding.

As mentioned above covalent bonding implies a state in which electrons are shared equally between atoms, while in ionic bonding implies that the electrons are completely confined to the anion. Except for a limitted number of extreme cases, neither of these pictures is completely accurate. In most cases of covalent bonding, the electron is unequally shared, spending more time around the more electronegative atom. Resulting in the covalent bond having some ionic character. Similarly, in ionic bonding the electrons often spend a small fraction of time around the more electropositive atom, resulting in some covalent character for the ionic bond.

Models of the atom:

• (what was the name of the model Aristotle used? where atoms where like small pebble with different shapes)
• The plum pudding model
• The Bohr model
• The quantum mechanical model