Assuming our focus is in period 2 elements, the early period may represent elements from Li2 to N2, while late period consists of elements from O2 to F2. The difference that exists between between these two groups is due to the different arrangement in their molecular orbitals.
From L2 to N2, for instance, 2 molecular orbitals are lower in energy than 2 orbital because there is less separation between the 2s and 2p atomic orbitals (the two are similar in energy). However, further across the period, this separation between 2s and 2p becomes greater and thus decreasing the stability of the 2 orbitals increasing their energy relative to 2 orbitals.
According to quantum Mechanics, the more similar are two orbitals, the better they mix resulting in more stability and less energy.