Regarding the first ionisation energies, why do the values shown on the graph go down from magnesium to aluminium and then rise again from aluminium to silicon

The first ionisation energy of an element is the energy required in order to remove the outermost electron from the element to make it an ion with the charge +1. From magnesium to aluminium the values shown on the graph go down from magnesium to aluminium because from aluminium, the electron is removed from the 3p energy level which is a higher energy level than the 3s level of the magnesium, hence the electron is removed with greater ease. It is harder to remove the electron from Silicon because silicon has 1 more proton than aluminium so has stronger proton charge but the electrons are in 3p level so have the same amount of electron shielding, therefore the charge holding the electron is greater in silicon so the energy is higher. So because the outermost electron in Si is closes to the nucleus, the energy to remove the electron is greatest out of the 3 elements.