Factors affecting the size of ionisation energy Ionisation energy is a measure of the energy needed to pull a particular electron away from the attraction of the nucleus. A high value of ionisation energy shows a high attraction between the electron and the nucleus. The charge on the nucleus. The more protons there are in the nucleus, the more positively charged the nucleus is, and the more strongly electrons are attracted to it. The distance of the electron from the nucleus.
Attraction falls off very rapidly with distance.
An electron close to the nucleus will be much more strongly attracted than one further away. The number of electrons between the outer electrons and the nucleus. Consider a sodium atom, with the electronic structure 2,8,1. If the outer electron looks in towards the nucleus, it doesn't see the nucleus sharply. Between it and the nucleus there are the two layers of electrons in the first and second levels.