In general, exceptions arise when new subshells are being filled/half-filled, or in cases where the atom is too small. In the first case, Be and Mg are interesting examples: they have a positive electron affinity (just like N, in fact) because of the energy difference between the s and p subshells.
Lesson Summary. The electron affinity is the energy change when an atom gains electrons. The convention is that the higher or more positive the electron affinity value, the more readily the atom accepts an electron.
When electrons are added to an atom, the increased negative charge puts stress on the electrons already there, causing energy to be released. When electrons are removed from an atom, that process requires energy to pull the electron away from the nucleus. Addition of an electron releases energy from the process.
The high electron affinities of the halogens are a result of their small size, high effective nuclear charge and having an almost complete outer shell of electrons. High energy is released when an electron is added to an halogens showing very high electron affinity.
Large atoms have low ionization energy and low electron affinity. Therefore, they tend to lose electrons and do not tend to gain electrons. Any electrons added to a noble gas would have to be the first electron in a new (larger) energy level. This causes the noble gases to have essentially zero electron affinity.
Chlorine and Electron AffinityFluorine is a small atom with a small amount of space available in its 2p orbital. Therefore, chlorine has a higher electron affinity than fluorine, and this orbital structure causes it to have the highest electron affinity of all of the elements.
Electron Affinity of Oxygen is 141 kJ/mol. Electronegativity of Oxygen is 3.44. An atom of Oxygen in the gas phase, for example, gives off energy when it gains an electron to form an ion of Oxygen.
This happens because effective nuclear charge, which is a measure of what the net positive charge felt by the electrons is, increases. This implies that the atomic size of carbon will be a little bigger than that of nitrogen, which in turn will be a little bigger than that of oxygen.
Notice that the Group 2 elements have much lower electron affinities than the Group 1 elements, with beryllium and magnesium even having positive electron affinities. Because of electron-electron repulsions, this is energetically unfavorable, making the electron affinity more positive.
Why does nitrogen have no electron affinity? Nitrogen has a half-filled 2p subshell, so that there is one electron in each orbital. This creates an unusually stable atom because of half-shell stability. Because nitrogen is relatively stable on its own, it has a relatively low electron affinity.
Fluorine has a negative value as energy is released when the electron is gained. This is because there is an attraction between the protons in the nucleus and the added electrons.
b)Is the trend in electron affinities repetitive for Periods 2 and 3? Cite examples. Yes, it is, for example: the last element of both periods has the lowest electron affinities and their second last elements have the highest electron affinities.
1st Electron Affinity is usually exothermic as the energy released when the nucleus attracts the the additional electron is larger than the energy absorbed to overcome inter-electronic repulsion. 2nd Electron Affinity is always endothermic since an additional electron is added to a negative ion.
Electron affinity increases from left to right within a period. This is caused by the decrease in atomic radius. Electron affinity decreases from top to bottom within a group. This is caused by the increase in atomic radius.
Due to small size and high electron density of oxygen compared to sulphur interelectronic repulsion is higher in oxygen resulting in less energy being released when an electron is added to oxygen due to lesser stability after electron is added which is due to the interelectronic repulsion in the small oxygen atom.
