Electron Gain Enthalpy
Now the next property that we are going to discuss is the electron gain enthalpy. Or as some books quote it as, the electron affinity. But I will tell you they both are closely related but they are not exactly the same. Now during the addition of an el there could be two possibilities of energies, either the energy is given out or the energy is absorbed. Now there is a sign convention that we will be following for electron gain enthalpy where if the energy is released by the atom while accepting an el that would be termed as negative and if we have to supply energy to make the electron go inside the atom then we would call it as positive. So there would be four terms which could there, that is, more negative, less negative, more positive or less positive. Now, when and why would we use the negative and the positive we will just take a look.
Now let’s say I have sodium and I have magnesium with me right? Now if I want to add an electron to each one of them, will the electron gain enthalpy have a positive value or a negative value? Think. See, now sodium and magnesium, both are metals. And since they are metals, they would not want to take electrons so they will oppose the addition of electron. So that means we would have to give energy in order to add the electron to either one of them. Right? So that means for metals the electron gain enthalpy can never have a negative value. It would always be talked in terms of positive that is you will have to supply it energy. Now, the question comes out of the two which one would have more positive electron gain enthalpy and which one would have less positive electron gain enthalpy. Right? So here carefully what words am I using? Now out of sodium and magnesium which one is smaller in size? Yes, magnesium is smaller in size. So now what happens is the nuclear force of attraction on the electron via magnesium is more because the size of magnesium is smaller and in the case of sodium since the size of sodium is big, the amount of nuclear force on the same electron is lesser. So that means in a way the nuclear force of magnesium is helping me out in attracting the el but the nucleus of sodium is not helping me out. So in both the cases I am giving energies but because that small amount of help I am getting from the magnesium atom so the amount of energy that I have to supply to magnesium for accepting the el would be less as compared to sodium. So in the cases of metal the electron gain enthalpy would always have a positive value and the one which has a smaller size or a higher nuclear charge would be the one which would have a lesser positive electron gain enthalpy and the other one would have a higher positive electron gain enthalpy. Right? So, two factors which we have taken care of over here on which electron gain enthalpy is depending is the nuclear charge and the atomic size.
Now let’s take another example where we would consider two non-metals. Right? So let’s take Sulphur and let’s take chlorine. So the electronic configuration for Sulphur would have 3p4 and for chlorine it would be 3p5. Now if I put one electron over here, which one do you think would accept the electron first and become more stable? Yes, it would be chlorine because if it takes one el it achieves a noble gas configuration. So that means the stability for chlorine after accepting the electron is increasing so it would remove the maximum amount of energy. So that is why in the chlorine and Sulphur because of the electronic configuration the electron gain enthalpy of chlorine would be more negative as compared to Sulphur.
Now let’s take a look with another property. Now if I have the atomic radius considerations then the atomic radius of chlorine is smaller as compared to Sulphur. Then again the amount of nuclear force or the amount of force that the nucleus would have on this el would be more in the case of chlorine and the el would be more attracted towards chlorine as compared to Sulphur. The second condition also favors chlorine, and this is why chlorine would have a more negative electron gain enthalpy value. Now why negative over here? Because it’s a non-metal and non-metals have a tendency to accept an el, so for accepting an el they would release energy. That is why is why in the cases of metal we were saying that the electron gain enthalpy would always be positive and in the cases of non-metals it would always be negative. Right?
So the third factor on which electron gain enthalpy is depending is the electronic configuration. Right? So, with the electronic configuration, if there is any element which has a half filled or a fully filled subshell that would have a very low electron gain enthalpy. Now depending on if it is a metal or a non-metal we would judge that will it have a positive value or a negative value. But please take into consideration the electronic configuration also.
So the three factors on which electron gain enthalpy is depending is the atomic radius, the nuclear charge and the electronic configuration. So now, if we talk about the periodic trends or the periodicity for electron gain enthalpy, let me tell you for electron gain enthalpy you will not be able to have a proper periodic trend because your value would keep on changing depending on what are you taking. So I cannot say, or I cannot generalize that moving from left to right in a period, it should increase or decrease or moving down the group it should increase or decrease. It all depends on the examples that you are taking.
Now, let me introduce the el affinity that we were discussing. Now affinity is the word that indicates love. That is el loving. So the actual definition for electron affinity is, it is the negative of the electron gain enthalpy. So where the electron gain enthalpy had a positive value electron affinity would become negative and wherever the electron gain enthalpy had a negative value the electron affinity would become positive. So that means if there is an element or an atom which loves electrons or which would want to attract the electron to itself there you would say that the electron affinity is positive and the ones which do not want to take the electrons there the electron affinity would be negative. Now I said that the electron affinity is very closely related to electron gain enthalpy. Now if you talk in terms of thermodynamics, then the enthalpy, that is the electron gain enthalpy and the electron affinity has a relationship where the electron gain enthalpy is equal to the electron affinity minus 5 by 2 times of R T. Now that value R is the universal gas constant and T is the temperature in Kelvin scale. So using this relationship we can find out what is the value of electron affinity if the value of electron gain enthalpy is given to you or what is the value of electron gain enthalpy if the value of electron affinity is given to you.
Now, let us discuss few exceptions which arise in the case of electron gain enthalpy. The first prominent exception that is there for electron gain enthalpy is which will have a higher negative electron gain enthalpy value when you compare fluorine and chlorine. Now the logic says that fluorine has a very small size, so the electron gain enthalpy would be very negative for fluorine because it will absorb the electron very easily. Now for chlorine, chlorine also has the same electronic configuration so it can also absorb the el easily. But the size of fluorine being smaller gives it the upper hand. But that is not the correct answer. The correct answer is the chlorine has a higher negative electron gain enthalpy value. Now why? Because fluorine has a very small size, right? And in the outermost shell there are 7 electrons. So now in a very small space 7 electrons are already present and if I bring in the 8th electron would the 7 electrons allow the 8th electron to come in? No, they would start repelling and they would not let that one electron to come in. So what is actually happening? There is one attractive force from the nucleus to the incoming electron but there are 7 electron-electron repulsions which are forcing the electron not to come in. So there you would have to supply energy in order to make it F minus. So that means a positive electron gain enthalpy is happening. Now in the case of chlorine the outermost shell can easily accommodate 18 electrons and only 7 electrons are present. So the incoming electron would not have a problem while coming in so a higher negative electron gain enthalpy value is for chlorine. So that is why the graph, if we plot for electron gain enthalpy looks something like this. It should have been something like this, but this anomalous behavior makes the graph look like this. Right? So this is about the fluorine and chlorine example.
Now one such exception is also there between oxygen and Sulphur. There also the same thing happens. For oxygen that is the smallest, the second smallest element for us where in the outermost shell there are 6 electrons and for Sulphur in the outermost shell there are again 6 electrons but for oxygen the maximum number of electrons that can be accommodated in the second shell is 8. So similarly if one electron is trying to come in, there are 6 electron-electron repulsions happening. But in the case of Sulphur, that is not true. So the higher negative electron gain enthalpy value is for Sulphur and not for oxygen. So these are the two exceptions that you should have in mind.
Now, which one would you think will have the highest negative electron gain enthalpy values? Yes, it would be for the halogens. The halogens would be the ones which would have the highest negative electron gain enthalpy values. Now there are some examples which have almost zero electron gain enthalpy values like beryllium and magnesium, or, nitrogen and phosphorus. Why? Because for beryllium and magnesium, the electronic configuration is 2s2 and 3s2 where we have a fully filled electronic configuration. So if I am trying to put in an electron what am I doing? I am distorting the electronic configuration and if I am distorting the electronic configuration that would not go well with the atom. So atom would not want to take that electron and that is why the electron gain enthalpy value for beryllium and magnesium is almost zero or it’s a very low value. And similarly in nitrogen and phosphorus, the electronic configurations have 2p3 and 3p3 values, which is a half-filled electronic configuration. And again because of the same reason that if the incoming electron comes in the half-filled subshells would be broken off and hence a decrease in stability will take place that is why the electron gain enthalpy is not favored for that. So it has a very low value.
Now if like in ionization enthalpy, we had the successive ionization enthalpies where I was trying to remove first electron, second electron, third electron, similarly for electron gain enthalpies also we have successive electron gain enthalpy where I add one electron, then I add the second electron and if required I can add the third electron also. But now what happens over here? Suppose if I take the example of oxygen and I put one electron in it, so what do you think the value should be negative or positive? The value would be negative. Because it will be moving closer to the stable noble gas configuration. Right? So the value is somewhere around -140 kJ/mol. Now, if I add one more electron to it, what should happen? Should it be a negative electron gain enthalpy or a positive electron gain enthalpy? Think. The normal logic says that it should be a negative electron gain enthalpy value because it is O- now. So if I bring in one more electron it will reach the noble gas configuration. So it should be easier. But no. At the same time you have to see that when that electron has added, the size of oxygen becomes bigger, and if the size of oxygen becomes bigger the attractive force of the nucleus on the incoming electron decreases and that is why the second electron gain enthalpy for oxygen is having a positive value, and it is close to somewhere around 700 kJ/mol. Now remember this fact that whenever a second electron gain enthalpy is happening or you are trying to add the second electron it would always have a positive value. Right? It will never have a negative value. Because the increase in size would be given more prominence as compared to anything else. So this is about the electron gain enthalpy.
Now if I correlate the electron gain enthalpy with the electron affinity. For this let’s take an example of oxygen and sodium. For oxygen we know that the electron gain enthalpy has a negative value and for sodium it has a positive electron gain enthalpy. Right? Now if I say electron affinity, out of the two which has more electron affinity? It is oxygen. Right? So oxygen would have a positive electron affinity and for sodium, because sodium does not love electrons so it would have a negative electron affinity. So this is how we say that electron affinity is the negative of the electron gain enthalpy. So wherever you have an atom which is a non-metal, the non-metals would always have a higher electron affinity, that is a positive electron affinity and the metals would always have a negative electron affinity. So don’t confuse yourself with electron gain enthalpy and electron affinity. Right?