Atoms of an element that have different atomic weights are called isotopes . They vary only in the number of neutrons. Atoms of an element may vary in the number of electrons they possess. Ions retain most of the properties of the uncharged atom of the element, but other properties may change. Each element has its own unique chemical symbol , which consists of one or two letters, the first is often the first letter of the name, and is always capitalized.
Atoms react with and join with each other, by sharing or transferring electrons, to form molecules .
A compound is a molecule that is made of two or more atoms of different elements. Water is composed of 2 atoms of hydrogen and one of oxygen, covalently bound together. NaCl represents sodium chloride, or table salt, consisting of one atom of sodium and one of chlorine. There are four important types of chemical bonds . The first is ionic bonding , the bonding that occurs between sodium and chlorine in table salt.