If the outer electron looks in towards the nucleus, it doesn't see the nucleus sharply. (There's no reason why you can't use this notation if it's useful!) The number of electrons between the outer electrons and the nucleus.Ĭonsider a sodium atom, with the electronic structure 2,8,1. An electron close to the nucleus will be much more strongly attracted than one further away. The distance of the electron from the nucleus.Īttraction falls off very rapidly with distance. 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 size of that attraction will be governed by: A high value of ionisation energy shows a high attraction between the electron and the nucleus. Ionisation energy is a measure of the energy needed to pull a particular electron away from the attraction of the nucleus. These variations in first ionisation energy can all be explained in terms of the structures of the atoms involved.įactors affecting the size of ionisation energy For example, look at the pattern from Li to Ne, and then compare it with the identical pattern from Na to Ar. That means that it varies in a repetitive way as you move through the Periodic Table. Patterns of first ionisation energies in the Periodic Tableįirst ionisation energy shows periodicity. = 2370 kJ mol -1) doesn't normally form a positive ion is because of the huge amount of energy that would be needed to remove one of its electrons. They vary in size from 381 (which you would consider very low) up to 2370 (which is very high).Īll elements have a first ionisation energy - even atoms which don't form positive ions in test tubes. Ionisation energies are measured in kJ mol -1 (kilojoules per mole). When you are talking about ionisation energies, everything must be present in the gas state. It isn't worth worrying about at the moment. Worried about moles? Don't be! For now, just take it as a measure of a particular amount of a substance. It is the energy needed to carry out this change per mole of X. This is more easily seen in symbol terms. The first ionisation energy is the energy required to remove one mole of the most loosely held electrons from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+. Important! If you aren't reasonable happy about atomic orbitals and electronic structures you should follow these links before you go any further. You will find a link at the bottom of the page to a similar description of successive ionisation energies (second, third and so on). It assumes that you know about simple atomic orbitals, and can write electronic structures for simple atoms. This page explains what first ionisation energy is, and then looks at the way it varies around the Periodic Table - across periods and down groups.