Factors affecting the size of ionisation energy

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 size of that attraction will be governed by: 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. (There's no reason why you can't use this notation if it's useful!) 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. The 11 protons in the sodium's nucleus have their effect cut down by the 10 inner electrons. The outer electron therefore only feels a net pull of approximately 1+ from the centre. This lessening of the pull of the nucleus by inner electrons is known as screening or shielding. Ionisation energies and reactivity The lower the ionisation energy, the more easily this change happens: You can explain the increase in reactivity of the Group 1 metals (Li, Na, K, Rb, Cs) as you go down the group in terms of the fall in ionisation energy. Whatever these metals react with, they have to form positive ions in the process, and so the lower the ionisation energy, the more easily those ions will form. The danger with this approach is that the formation of the positive ion is only one stage in a multi-step process. For example, you wouldn't be starting with gaseous atoms; nor would you end up with gaseous positive ions - you would end up with ions in a solid or in solution. The energy changes in these processes also vary from element to element. Ideally you need to consider the whole picture and not just one small part of it. However, the ionisation energies of the elements are going to be major contributing factors towards the activation energy of the reactions. Remember that activation energy is the minimum energy needed before a reaction will take place. The lower the activation energy, the faster the reaction will be - irrespective of what the overall energy changes in the reaction are. The fall in ionisation energy as you go down a group will lead to lower activation energies and therefore faster reactions