![]() The first electron affinity of oxygen (-142 kJ mol -1) is smaller than that of sulphur (-200 kJ mol -1) for exactly the same reason that fluorine's is smaller than chlorine's.Īs you might have noticed, the first electron affinity of oxygen (-142 kJ mol -1) is less than that of fluorine (-328 kJ mol -1). This repulsion lessens the attraction the incoming electron feels and so lessens the electron affinity.Ī similar reversal of the expected trend happens between oxygen and sulphur in Group 6. However, because fluorine is such a small atom, you are putting the new electron into a region of space already crowded with electrons and there is a significant amount of repulsion. The incoming electron is going to be closer to the nucleus in fluorine than in any other of these elements, so you would expect a high value of electron affinity. What we have said so far is perfectly true and applies to the fluorine-chlorine case as much as to anything else in the group, but there's another factor which operates as well which we haven't considered yet - and that over-rides the effect of distance in the case of fluorine. However, comparing chlorine and bromine, say, makes things seem more difficult because of the more complicated electronic structures involved. Note: Comparing fluorine and chlorine isn't ideal, because fluorine breaks the trend in the group. The greater the distance, the less the attraction and so the less energy is released as electron affinity. The over-riding factor is therefore the increased distance that the incoming electron finds itself from the nucleus as you go down the group. This will be approximately the same in both these cases and so doesn't affect the argument in any way (apart from complicating it!). Note: If you want to be fussy, there is also a small amount of screening by the 2s electrons in fluorine and by the 3s electrons in chlorine. It has 17 protons in the nucleus.īut again the incoming electron feels a net attraction from the nucleus of 7+ (17 protons less the 10 screening electrons in the first and second levels). It therefore feels a net attraction from the nucleus of 7+ (9 protons less the 2 screening electrons).īy contrast, chlorine has the electronic structure 1s 22s 22p 63s 23p x 23p y 23p z 1. The incoming electron enters the 2-level, and is screened from the nucleus by the two 1s 2 electrons. Each outer electron in effect feels a pull of 7+ from the centre of the atom, irrespective of which element you are talking about.įor example, a fluorine atom has an electronic structure of 1s 22s 22p x 22p y 22p z 1. The increased nuclear charge as you go down the group is offset by extra screening electrons. These factors are discussed in more detail on that page than they are on this one. Note: If you haven't read about ionisation energy recently, it might be a good idea to follow this link before you go on. The factors which affect this attraction are exactly the same as those relating to ionisation energies - nuclear charge, distance and screening. The electron affinity is a measure of the attraction between the incoming electron and the nucleus - the stronger the attraction, the more energy is released. Fluorine breaks that pattern, and will have to be accounted for separately. Yes - as you go down the group, first electron affinities become less (in the sense that less energy is evolved when the negative ions are formed). That doesn't matter - the pattern will still be the same. If you are using a different data source, you may have slightly different numbers. Note: These values are based on the most recent research. The first electron affinities of the group 7 elements F By convention, the negative sign shows a release of energy. For example, the first electron affinity of chlorine is -349 kJ mol -1. It is the energy released (per mole of X) when this change happens.įirst electron affinities have negative values. This is more easily seen in symbol terms. ![]() The first electron affinity is the energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions. Electron affinities are the negative ion equivalent, and their use is almost always confined to elements in groups 6 and 7 of the Periodic Table. Ionisation energies are always concerned with the formation of positive ions. Important! If you aren't reasonable happy about atomic orbitals and electronic structures you should follow these links before you go any further. ![]() It assumes that you know about simple atomic orbitals, and can write electronic structures for simple atoms. This page explains what electron affinity is, and then looks at the factors that affect its size.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |