How does ionisation energy vary across a period in the periodic table?

Ionisation energy generally increases across a period in the periodic table. This is because across a period, one more proton is added to the nucleus. This increases the quantity of positive charge so there is a greater attractive force for the negatively charged electrons so it becomes more difficult to remove each successive electron.There are dips at boron and oxygen. This is because when going from beryllium to boron, the electrons are being added to a new shell that is further away from the nucleus. At oxygen there is a dip because we have to start pairing the electrons and two electrons that both have negative charge cause repulsion when paired within the same orbital, so it is a little bit easier than expected to remove that electron. This makes the ionisation energy less high.