The discussion centers on the concept of electron affinity (E.A.) for elements in periods 2 and 3 of the periodic table. It is established that the first electron affinity is always negative, indicating an exothermic reaction when a gaseous atom accepts an electron. While E.A. generally decreases down a group due to increased electron shielding, period 3 elements exhibit smaller electron affinities than period 2 elements, with the notable exception of fluorine (F) having a lower E.A. than chlorine (Cl) despite its higher electronegativity. This is attributed to the small atomic size of F, which results in greater electron density and requires more energy to accommodate an additional electron.
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Kushal said:period 3 elements have SMALLER electron affinities than period 2 elements for the reasons above. But an exception is F(period 2) and Cl(period 3). This is due to the fact that F has a very small size...
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ritwik06 said:Why is electron affinity negative?
In my book its written, that on moving down a group, tit decreases. But why is it that period 3 elements have greater E.A. than period 2 elements?
Kushal said:you should precise which E.A.! The first E.A. is the amount of energy released when 1 mol of a gaseous atom accepts 1 mol electrons. the first E.A. is always negative. E.A. is in fact an energy change; when an energy change is -ve, it implies that the reaction is exothermic, i.e energy is liberated...and when energy change is +ve, it implies that the reaction is endothermic, i.e energy is absorbed. right
the attraction between the nucleus and the first electron to be accepted releases energy. the second E.A. and succeeding ones are all endothermic. this is because energy is needed to force the electron into the atom. energy is now required as the second incoming electron is being at the same time attracted by the nucleus and repelled(to a larger extent) by the other electrons of the atom. right
E.A. decreases down a group. this is because, down a group the atoms become shielded by the larger numbers of electrons. (e.g. Li has 3 e-, Na has 11e-, K has 19e-,...) the shielding decreases the attractive force of the nucleus on the incoming electron and furthermore, the incoming electron is being repelled at a greater extent by the larger number of electrons. i.e more energy is required to force that electron into the atom.
period 3 elements have SMALLER electron affinities than period 2 elements for the reasons above. But an exception is F(period 2) and Cl(period 3). This is due to the fact that F has a very small size...
Hope i helped you
TripleS said:how is that an exception?...doesnt F have a greater E.A than Cl?
Nope. Though F is more electronegative than Cl, E.A of F is less than Cl. It is because F is comparatively very small atom which give it higher electron density than it's neighbours. If you want to place an electron in it's (F)outermost shell you have to do some extra work (for the extra density of electrons it has). So overall it effects the net realease of energy.