A Brief Note on Electron Gain Enthalpy

In electron gain enthalpy, the greater or higher the amount of energy released in the whole process, the higher or greater the element’s electron gain enthalpy. We can conclude the measure of the electron gain enthalpy of an element by its particular firmness or strength with which an extra electron is always bound to it. 

The unit by which the electron gains enthalpy of an electron is measured in electron volts per atom or kJ per mole. Either exothermic or endothermic is the usual process of adding an electron to the atom. However, the electron gain enthalpy is negative when energy is released due to the addition of an electron in an atom. 

This is usually the main reason why electron gain enthalpy becomes more negative as we move from left to right in a period. One particular example of this phenomenon is, that the electron gain enthalpy is highly negative for halogens because electron gain enthalpy is capable of acquiring the closest or nearest stable noble gas configuration only after accepting an extra electron. 

Mainly noble gases acquire large positive electron gain enthalpy because of the reason that in noble gases the extra electron has to be set down in the next consecutive higher principal quantum energy levels there only when highly unstable electronic configurations are generated. 

The electron gain enthalpy depends on some factors such as:

Atomic Size

As there is an increase in the size of the atom, there is an increase in the distance present between the nucleus and the last shell which will be receiving the incoming electrons. 

This phenomenon results in electron gain enthalpy becoming less negative due to the force of attraction caused between the nucleus and the incoming electron increases. The atomic radius is varied predictably for the increase or decrease in electron gain enthalpy across the periodic table, for example, the atomic radius tends to decrease for each period in a row of the table from left to right and vice versa (increase down each group in the column). 

With the increase of atomic radius along each period as a row of the periodic table, the electrons that are added move readily to the same outer shell which results in the decrease of the atomic radius due to the varied increase in the nuclear charge. 

As we move down the group of the periodic table, there is an increasing group of the atomic radius in the periodic table, it increases because of the presence of principal energy which is added consecutively on different levels, which are a distance away from the nucleus. This change (increase and decrease) of the atomic size explains why electron gain enthalpy becomes more negative as we move from left to right in a period. 

It was observed and concluded from experiments that in the first case of moving across the periodic table, the increase in nuclear charge defies the repulsion caused due to the additional electrons in the valence level or the outermost shell. Therefore, there is a gradual decrease in the size of atoms as one moves across a period from left to right in the periodic table.

Nuclear Charge

Nuclear charge is mainly the force of attraction between the nucleus and the incoming electron that increases as the nuclear charge increases. This continuous increase of charge and force of attraction results in the electron gain enthalpy becoming more negative. 

The shielding effect of nuclear charge perfectly describes the balance between the proton pull on valence electrons and the repulsion forces created from inner electrons. 

The nuclear charge shielding effect also describes the easy removal of the valence shell or outermost shell electrons from the atom, the atomic size is also explained by this effect. The more the valence shell can spread out and the bigger the atoms will be due to the nuclear charge shielding effect.

Electronic Configuration

As said earlier negative: when energy is released while accepting an electron is explained also by electronic configuration as the elements comprising exactly half-filled or filled orbitals are very stable. With the addition of the electron, the energy has to be supplied, in electron gain enthalpy this stable electronic configuration has large positive values since the easy addition of electrons is not accepted by them. From moving top to bottom the electron gain enthalpy becomes less negative and more negative from moving left to right in a period of the periodic table. 

For example, the electron gain enthalpies of some elements of the 2nd period in the periodic table such as O and F are comparatively less negative than the other corresponding elements present in the third period of the periodic table. 

Now, why is the electron gain enthalpy of fluorine comparatively less negative than that of chlorine? This is because of the small size of fluorine, because of its small size the repulsion between the electron and electron caused in the 2p subshell is larger. However, the incoming electron of fluorine is not accepted as easily as the incoming electron of chlorine.

Conclusion

In conclusion, we can state that negative: is when energy is released while accepting an electron, and positive: is when energy is supplied to an atom while adding an electron. 

After the addition of one electron, the atoms are known to be negatively charged and to the negatively charged ion, the second electron is added. But due to electrostatic repulsion, the consecutive addition of a second electron is added resulting in the produced energy being supplied for the addition of the second electron. 

All of this results in the positive charge of the second electron gain enthalpy. For example, when an electron is added to an oxygen atom to generate the O- negative ion, there is a release of energy. Later when in an O-negative ion another electron is added that O-negative ion turns to form the O2-negative ion.