Answer:
a) Ionic bonding
Covalent bonding
Metallic bonding
b) Electrons with the same state must have opposite spins.
Electron state can hold no more than two electrons.
Explanation:
a) When an atom loses electrons it became a cation, with a positive charge, and when an atom gain electrons it became an anion, with a negative charge. Because of the attraction of the opposite charges, the ions form a bond, which is called ionic bonding.
When atoms share electrons to complete the octet rule, they form a molecule by covalent bonding.
When a metal loses its valence electrons, they form a "sea of electrons" and the cations are "glued" by them, which is called metallic bonding.
b) The Pauli exclusion principle determinants that two electrons can not occupy the same state at the same time, it means that two electrons can't have the same four quantic number equal. If they are at the same shell, they must have different spins. The electron can have only two spin (-1/2 or +1/2) so the electron state can hold no more than two electrons, and they have to have opposite spins.