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An atom is the smallest particle of an element. An element is a substance made up of only one kind of atom.
An atom is made up of three main subparticles called the proton, neutron and electron. The center of the atom is called the nucleus and contains the proton and neutron. The nucleus contains most of the mass of the atom, although its volume is much greater. There are usually the same number of protons and neutrons. When there are more neutrons than protons, the atom is called an isotope. The proton has a positive charge, the neutron no charge and the electron a negative charge.
Atoms have an atomic number. This is the number of protons in the atom. The atomic weight of the atom is the sum of the protons and neutrons in the atom's nucleus. An atom is named by the element it represents. For example, an atom of oxygen would be represented by the symbol O, the hydrogen atom by the symbol H, etc..
The electron orbits around the outside of the nucleus. In a neutral atom the number of electrons is the same as the atomic number, or the same as the number of protons. However, the outermost electrons of an atom can be attracted to those of other types of elements and can combine by sharing, giving or receiving electrons. Such elements as metals give up their electrons more easily and are better conductors of heat and electricity.
The chemical combination of two or more atoms in fixed amounts is called a compound. These compounds can be atoms of the same element or those of different elements. The smallest particle of a compound would be a molecule of that compound. An atom that is not neutral and has an electrical charge is called an ion.