All matter in the universe is composed of atoms. There are 92 naturally occurring types of atoms, representing the basic chemical elements. In addition, there are 26 elements predicted to exist as of March 2008. Of these, 25 have actually been synthesized through successive bombardment of Uranium isotopes in atom smashing machines like Fermi-Lab and also as a result of thermonuclear atomic explosions. Some of these elements may be synthesized in supernova explosions, but all of these dense atoms have relatively short half lives and would not be around for long.
All atoms are made up of smaller subatomic particles, the three major ones being protons, neutrons and electrons. These subatomic particles are comprised of yet smaller bits of matter or energy, including photons, bosons, leptons, gluons, and quarks. Since these smallest units of matter are quite stealthy and short lived in free space, we know them only by how they act, the trajectory they follow when liberated during decomposition of atoms as a result of high energy particle collisions.
There are four known forces which define matter and energy in the observable universe. Three of these forces, the nuclear strong force, nuclear week force and electromagnetic force, are all involved in binding subatomic particles together to form atoms. In 1925, based on Einstein's theories of special and general relativity, physicist George Lamatre proposed a theory that all of the matter in the universe precipitated from an event he called the primeval atom. Another physicist and friend of Einstein's, Max Planck, developed a method of scientific and mathematical analysis called quantum mechanics to describe the forces and interactions of subatomic particles to form atoms. Through the application of quantum physics, scientists over the past seventy years or so have been able to defined a model of Lamatre's primeval atom and a period of time amounting to a trillionth of a trillionth of a trillionth (1X10[-43]) of a second, called the Planck Epoch, when all the atoms in the visible universe materialized. During the Plank Epoch there was no gravity and the universe experienced a rapid expansion, but thereafter the fourth force gravity came into effect, and has slowed the explanation of the universe ever since. While there are still some holes that need to be filled in with respect to the big bang theory, the evidence for it us substantial, and most scientist except its premise today.
All of the characteristics of atoms stem from the four universal forces. The strong force holds quarks and gluons together to form protons and neutrons. The week force involves high energy particles called bosons and is associated with the radioactive decay of atoms. The electromagnetic force holds electrons in obit around the nucleus of the atom. Gravity is a force in essence exerted by atoms proportional to mass and mutually expressed by all matter in the universe.
Atoms are classified by atomic number, a term which relates to the number of protons in the nucleus of an atom. If an atom gains or looses protons its atomic number changes, and the atoms basic elemental properties change as well. The total number of protons, neutrons, and electrons in a given atom, establish its "atomic mass unit" (amu) an older term that has been replaced by the "unified mass atomic unit" (u) or more simply the Dalton (Da), named after John Dalton - the father of modern Chemistry. Actually, before Dalton himself came up with the idea of atoms as the smallest units of matter, the ancient Greek fellow Democritus, around 450 BCE, proposed a hypothesis that all matter was composed of small particles which he called atoms (atomos in Greek, meaning indivisible).
Atoms combine with other atoms of the same and different atomic numbers to form molecules and compounds. Such combinations occur through chemical reactions in which atoms share or lend electrons to each other. The propensity of one atom to react with another is determined by the number of electrons an atom will share (covalent bonding), lend, or borrow (Ionic bonding). The molecule or compound created in chemical reactions between atoms may exhibit properties similar to its constituent atoms or radically different ones.
Electrons encircling the nucleus of an atom follow a prescribed configuration protocol. There are seven shells or orbits represented by the letters K through Q, the K shell being the closest to the nucleus and Q the furthest out. The number of electrons in the outermost shell, which can never exceed 8, or just 2 in the case of hydrogen and helium, represent the atoms electronegative or electropositive status; an attribute called valance. Valance, determines whether an atom will react with another, and when it does, the amount of energy that will be liberated in the form of photon discharge, or in rare cases absorbed, during the reaction. Atoms with 8 electrons in their outer shell, or 2 in the case of helium, have a valance of zero and therefore do not react with other atoms. Atoms exhibiting this status are referred to as inert. Atoms with four electrons in their outer orbit can either give up their four or gain four to achieve an octet in their outermost shell. Atoms with 1-3 valance elections are considered electropositive, in that they give up electrons in chemical reactions, and atoms with 5-7 valance electrons are considered electronegative in that they gain electrons in chemical reactions.
Dmitri Mendeleev devised the Periodic Table of Elements to explain the characteristics and chemical behavior of atoms. Basically the table organizes the different types of atoms by atomic number and weight, and arranges them in groups corresponding to their valance. It is a great tool for chemists and also a bane to every college chemistry student who must commit it to memory.
Well there you have it, an overview of the fundamental characteristics of atoms as we know them to be today. But it is an incomplete story, there are questions yet to be answered, perhaps even some we don't yet know how to ask. The sciences of chemistry and physics continue to probe the nano-universe of the atom, with hope of unlocking its secrets so that humans may one day fully understand its characteristics.