second period elements have no d orbitals and so can not have more than an octet of electrons, that is, only the third period and greater can form sp3d  and sp3d2  orbitals.

C2H2

same spin

5 basic types of hybrid orbitals

fig. 9.2.5: Boron ground state electron configuration on left, and the hybrid orbital configuation on the right that is involved with the three bonds in boron trifluoride. 

fig. 9.2.4:  Beryllium is also sp hybridized in beryllium chloride. What is the orbital of the chlorine in BeCl2? it is a 3p orbital as chlorine has one free-radical orbital in it's valence state, and so there is no need to hybridize it.

Using VSEPR, one can derive that IBr5 has an octahedral parent shape. Now, use Valence Bond Theory to account f

Each molecular orbital can only have 2 electrons, each with an opposite spin.

Electrons in bonding molecular orbitals help stabilize a system of atoms since less energy is associated with bonded atoms as opposed to a system of unbound atoms.  Bonding orbitals are formed by in-phase combinations of atomic orbitals and increase the electron density between the atoms (see figure 2 below)   Electrons in antibonding molecular orbitals cause a system to be destabilized since more energy is associated with bonded atoms than that of a system of unbound atoms. Antibonding orbitals are formed by out-of-phase combinations of atomic orbitals and decrease the electron density between atoms (see figure 2 below). 

The two 2py atomic orbitals overlap in parallel to form two ππ\pi molecular orbitals which are asymmetrical about the axis of the bond.