Facts About the Atom
Defining the basic building block of macroscopic matter
Atoms are an intermediary step between fundamental particles and the material objects that make up our large scale world. This is a brief primer on the atom.
The word atom means "indivisible" in Greek implying that an atom is the fundamental unit of matter. In the science of chemistry, for most practical purposes, this is held to be true. There are currently 117 discovered or manufactured elements, each defined by an atom. Each element has it's own unique properties because each atom of each type of element is different. An atom is actually a class of object that is composed of smaller particles, so an atom is in fact divisible.
The particular properties of an atom are defined by how the smaller particles are arranged in the atom. These arrangements are governed by a set of rules based on the fundamental forces - gravity doesn't play a significant role, but the strong and weak nuclear forces, and electromagnetic force do. The smallest atom is composed of a single proton. This is hydrogen, the lightest element. Usually the proton is accompanied by an electron, but if the proton has a lot of energy it may lose it's electron. Hydrogen in this state is a plasma - the primary ingredient of stars.
The number of protons a particular atom defines it as a particular element. So, helium has two protons, lithium has 3 protons, and ununoctium has 118 protons (ununseptium, element 117, hasn't been "discovered" yet).
Protons and electrons are two of the three particles that make up atoms. The third is the neutron. Elements beyond hydrogen have neutrons. A hydrogen atom can also have a neutron - this configuration doesn't change it to a different element since the atom's properties aren't significantly different. It does change the weight of the atom, and this type of atom is an isotope. Some elements can have several isotopes, or variations in the number of neutrons. Generally, the number of neutrons in an atom match the number of protons, but bigger atoms tend to have more neutrons than protons.
Very large atoms with many protons and neutrons can be unstable. Protons have a positive charge (the electromagnetic force), so protons tend to repel each other. Neutrons have no charge. The nuclear forces in the neutrons and protons are mutually attractive, and they work against the natural tendencies of the protons to repel from one another, binding the atom together. However, the nuclear forces are no match for electromagnetism over longer distances. In large atoms, the nuclear force from one particle may not reach completely through the atom, and the atom may decay to two or more less unstable atoms.
