 Let's solve a few questions on Electron Gain enthalpy. Now the first question says, arrange phosphorus, oxygen and sulphur in the correct order of their electron gain enthalpies. To quickly take a recap, electron gain enthalpy refers to the energy change that occurs when a neutral atom in a gaseous state acquires an electron to form a negatively charged ion. Basically it helps us measure how easily an electron can be added to a neutral atom. And when this happens, energy can be released, in which case delta H would be negative, or energy can be absorbed, in which case you need to provide energy for this process to occur. So if you look at the trend of electron gain enthalpy, you will see that it generally increases across a period. This is because across a period the effective nuclear charge increases. As a result, it would be easier to add the electron to these smaller atoms and it would experience greater nuclear attraction. So this is why the trend is that it increases in general across a period and decreases down the group. As we go down the group, the atomic size increases and that means the added electron would be farther and farther away from the nuclear attraction. So based on this, oxygen seems to have the highest electron gain enthalpy, followed by sulphur and the least would be phosphorus. But you know what, this is not the correct order. The correct order is actually sulphur greater than oxygen greater than phosphorus. Now let's pause the video here and try to figure out why this is the correct order. Okay so we know that sulphur belongs to the third period while oxygen belongs to the second period. That means oxygen has smaller size as compared to sulphur. The valence electrons in oxygen atom occupies the 2s and 2p orbitals whereas those in the sulphur atom occupy 3s and 3p orbitals. So that is 2s2, 2p4, 3s2, 3p4. Now because of the small size, the added electron in the oxygen atom experiences very high electron-electron repulsions because you know all the electrons are in the second shell as compared to sulphur where the electron goes to the third energy level. In the case of sulphur, the added electron occupies much larger space and as a result the electrons experience less repulsion from each other. This is why electron-gin enthalpy of sulphur is greater than oxygen. Okay so let's look at the next question. It says, in which of the following processes the electron-gin enthalpy will be positive? Now a positive electron-gin enthalpy would mean that this process of gaining an electron is not particularly favorable. It is endothermic and we need to input a lot of energy for this electron addition to take place. Now this can happen if we are moving from a more stable electronic configuration to a less stable state or if we are adding an electron to a very small atom in which case the added electron will experience a lot of repulsion from its neighbouring electrons. So let's look at the electronic configurations of each of these atoms and see if adding an electron makes it more stable or less stable. Now chlorine has an outer electronic configuration of 3s to 3p5. It has 7 valence electrons and when we add an electron it attains a highly stable noble gas configuration of argon 3s to 3p6. Now it has a completely filled octet. So here addition of an electron results in a highly stable electronic configuration. So in this case electron-gin enthalpy would be negative because energy is released in this process. In fact the first electron-gin enthalpy of chlorine is about minus 349 kJ per mole. Ok so let's look at the second option. Electronic configuration of hydrogen is 1s1 and on adding an electron we get 1s2. Here again addition of an electron results in the formation of a fully filled one-inch subshell. This is also quite stable and therefore the electron-gin enthalpy here is also negative. And if you look at the actual value it's about minus 73 kJ per mole. Alright let's look at the last option. The outer electronic configuration of nitrogen is 2s2 2p3 and on adding an electron we get 2s2 2p4. So here we are going from a stable half filled 2p3 electronic configuration to a less stable state. Not just that nitrogen is actually a small atom and adding an electron is not very welcome here because the added electron goes to the second energy level n is equal to 2 and it experiences strong repulsion from the already existing electrons. Therefore to overcome this repulsion we need to supply energy which is why the electron-gin enthalpy of nitrogen is positive and the energy required to bring about this conversion is about plus 7 kJ per mole. As you can see here electron-gin enthalpy will be positive for nitrogen.