Periodicity of properties is a result of the distribution of electrons. The total mass of the atom comes from the atom s protons and neutrons. Not all atoms of a particular element have the same number of neutrons. When the number of neutrons varies, the mass varies. Atoms are bound together to form molecules.
Molecular structure depends on the type and arrangement of bonding.
Covalent bonds are formed when electrons are shared between two atoms. Lewis structures represent valence electrons around an atom with dots. The Octet Rule states that the most stable arrangement of atoms in a molecule will allow all of the atoms to share in eight electons. When one pair of electrons is shared a single bond is formed. When two pairs are shared a double bond is created.