What is the trend in ionisation energy across a period?

Ionisation energy generally increases across a period from left to right on the periodic table.

As you move across a period from left to right, the number of protons in the nucleus of each atom increases. This means that the positive charge of the nucleus also increases. At the same time, the number of energy levels (shells) remains the same. Therefore, the electrons are more strongly attracted to the nucleus, making it harder to remove an electron from the atom. This is why the ionisation energy increases.

However, there are some exceptions to this trend. For example, the ionisation energy decreases slightly from beryllium to boron and from nitrogen to oxygen. This is due to the electron configuration of these elements. In the case of boron, the added electron goes into a 2p orbital rather than the 2s orbital of beryllium. The 2p orbital is slightly further from the nucleus and is also shielded by the 2s electrons, so it is easier to remove, resulting in a lower ionisation energy. Similarly, in oxygen, the added electron pairs up in a 2p orbital, which slightly repels the other electron, making it easier to remove and thus lowering the ionisation energy.

In summary, the trend in ionisation energy across a period is generally an increase due to the increasing nuclear charge. However, electron configuration can cause slight decreases in this trend. Understanding these trends and exceptions can help you predict the chemical behaviour of different elements.