The chemical properties of the elements are determined by their outermost, highest energy, valence electrons.
Electrons occupy atomic orbitals which can contain two electrons.
Ionic Bonding - The attractive force between two oppositely charged ions. Covalent bonding - The sharing of one or more pairs of electrons between two atoms. Valence - The number of bonds an atom can form. This is generally equal to the number of electrons needed to fill the atoms valence shell. The hydrogen atom has one electron and therefore needs one more to fill its valence shell.