Explain the trend in first ionisation energies across a period.

There is a general increase in the ionisation energy as we move across a period. This is because the the number of shielding electrons is constant and the atomic number increases, which means the effective nuclear charge increases so there is a greater attraction to the valence electrons, which means more energy is required to remove them. Group 3 has a lower ionisation energy than group 2 since the P orbital is at a higher energy than the S, meaning it requires less energy to remove an electron from this orbital. Another exception to this trend is that group 6 has a lower ionisation energy than group 5 since there is electrostatic repulsion between opposing spin electrons in one of the P orbitals.