A compound is formed when two or more atoms are joined together. An atom is the smallest particle of an element that still retains the properties of that element. A molecule is the smallest component of a compound that still has the properties of the compound.
Elements have properties that make them different from one another, but when two or more elements join chemically to form a compound, the compound has properties that are different from its constituents. Atoms in a compound can be joined ionically or covalently. An ionic compound forms when electrons are transferred from one atom to another. When electrons are transferred, one atom becomes positively charged and the other takes on a negative charge. The bond that forms between the two atoms arises from the attraction of opposite charges. A covalent bond forms between two atoms that share electrons.