Explain the trend in first ionisation energy along period 3

Going across a period there is generally an increase in first ionisation energy. This is because there are more protons in each nucleus leading to a greater positive charge and stronger electrostatic force of attraction between the nucleus and outer electron. Each successive electron enters the same shell so there is no increase in shielding.The two exceptions to the trend are aluminium and and sulphur. For aluminium IE is lower because the electron enters 3p rather than 3s so less energy is required to remove it. For sulphur IE falls because it is 3p4, this means there are two electrons in one p orbital. As a result, IE will be lower as electron repulsion means that the force of attraction between the electron and nucleus is reduced which makes it easier to remove.