Two atoms of the second period come together, and each brings its 1s, 2s and three 2p orbitals.

Atomic orbitals of two atoms combine into the same number of molecular orbitals: bonding ones, lower in energy, and antibonding ones (starred), higher in energy. They must have similar energies and the same symmetry. Up to $\text{N}_2$ the order is $\sigma 1s$, $\sigma^* 1s$, $\sigma 2s$, $\sigma^* 2s$, $\pi 2p$, $\sigma 2p$, $\pi^* 2p$, $\sigma^* 2p$; for $\text{O}_2$ and $\text{F}_2$, $\sigma 2p$ comes before $\pi 2p$. Electrons fill them by the Aufbau, Pauli and Hund rules.