Explain why there is a general increase in the first ionisation energy across the third period.

The first ionisation energy depends on:the charge on the nucleus, the distance of the outer electron from the nucleus, the amount of shielding by inner electrons and whether the electron is alone in an orbital or part of a pair of electrons. In the whole of Period 3 the outer electrons are in the third shell so they are at similar distances from the nucleus and receive approximately the same amount of shielding from the first and second shells. The main difference is the increase in atomic number (number of protons) across the period. This causes greater attraction between the nucleus and the outer electrons, thus increasing the first ionisation energy.