Electron Gain Enthalpy

Electron Gain Enthalpy is one of the most basic and important phenomena when studying different elements. This is used in determining the reactivity, nature and type of bond they can form and some other factors of an element. It is also called as electron affinity, but they are slightly different.

What is Electron Gain Enthalpy?

The energy released when an electron is added to the valence shell of an isolated gaseous atom is called the electron gain enthalpy of the atom.

The electron gain enthalpy of an atom is negative when energy is released on the addition of an electron. It can be used to estimate the stability of an atom as, more the negative value of enthalpy, the more stable the ion.

It is often represented as \(\Delta H_{eg}\) and is measured in the unit of electron volts per atom or kJ per mole (kJ/mol).

The reaction involved in the gain enthalpy of an atom is,

\(X(g) + e^{-} \rightarrow X^{-}(g) + E\)

Different Electron Gain Enthalpies

There are various electron gain enthalpies for a single atom. These are first electron gain enthalpy, second electron gain enthalpy, and so on.

When an electron is added to a neutral isolated gaseous atom, the energy which is released is the first electron gain enthalpy of that element. These are often exothermic reactions.

\(X(g) + e^{-} \rightarrow X^{-}(g) + E\)

When an electron is added to an isolated gaseous anion with charge -1, then the released energy is called the second electron gain enthalpy of the element.

Endothermic reactions are more common in these reactions as some energy must be supplied in order to pass the repulsion between the already existing electron and the new one.

\(X^{-}(g) + e^{-} \rightarrow X^{2-}(g) + E\)

One can find out things like the valence electrons and stable state of an element if they have sufficient data on its electron gain enthalpies.

Learn about a related concept Ionization Enthalpy

Periodic Table and Electron Gain Enthalpy

Non-metals usually have a more negative electron gain enthalpy than metals, as they are smaller in size than the metals in the same period.

The negative value of enthalpy increases from left to right in a period, this is mainly due to the decrease in atomic size and increase in nuclear charge.

The magnitude of enthalpy decreases from top to bottom in a group, this is due to the increase in atomic size down the group.

The group of elements with the highest electron gain enthalpies are halogens, and chlorine has the highest enthalpies among halogens.

The above-mentioned orders are not always regular due to factors like stability of the atom, shielding effects, effective nuclear charge, and electronic repulsions.

Factors Affecting Electron Gain Enthalpy

There are various factors which affects \(\Delta H_{eg}\) of an element. Let’s discuss some of the prominent one.

Atomic Size

The nuclear force on an approaching electron decreases with increasing atomic size. So, electrons with a large atomic size usually have low enthalpy values as the nuclear force on the last electron is less.

Effective Nuclear Charge

The net nuclear charge on an electron of an atom is called the effective nuclear charge on that electron. The enthalpy of an atom depends on the effective nuclear charge on the incoming electron.

When the effective nuclear charge on the electron is high it is more strongly attracted to the atom and the enthalpy will be high.

Stability

The atoms which are already stable with their current number of electrons have low electron gain enthalpy. This is because they have to lose their stability in order to gain the electron. The stability of an atom in this case can be its half-filled or fully filled valence shell.

On the other hand, if an atom upon gaining an electron reaches a stable state then its enthalpy is high.

Shielding Effect

If the shielding effect is low in an atom then its effective nuclear charge on all the atoms is high, hence the electron gain enthalpy of the atom is also high. In some cases, the shielding effect is so low that it also affects the size of an atom.

This effect is mostly limited to only the elements which have electrons in either d or f sub-shells or both, that is because these sub-shells have considerably poor shielding effect.

Some Common Anomalies

  1. Fluorine & Chlorine
    The general expectation is that fluorine has more negative enthalpy than chlorine due to its small size and high electronegativity. But in reality it is the opposite, chlorine has more negative enthalpy than fluorine.
    This is because fluorine has a small size, due to which its electron density is high, which leads to repulsion between electrons making it difficult for an electron to bond with the atom.
  1. Carbon & Nitrogen
    The electron gain enthalpy of carbon is more negative than that of nitrogen. This is because nitrogen has a half-filled p-subshell, which makes it more stable than carbon.
    So, as nitrogen loses its stability on gaining an electron, it has a lower enthalpy than carbon, which becomes more stable on gaining an electron.

Applications

  1. The enthalpy can be used in comparing elements based on their stability.
  2. It can also be used in finding the valency of an atom.
  3. Also used to find the approximate position of an element in the periodic table.
  4. It can be used in finding the possibility and type of bond formation between two elements.
  5. It was used by Robert S. Mullican in making the electronegativity scale for atoms.

Example Problems

Question 1. Let us consider a hypothetical atom with \(\Delta H_{eg}\) as -349 KJ/mol. What type of bond does this atom form with a metallic atom?

Answer. The given hypothetical atom has a high enthalpy which means that it is a non-metal, and due to its high enthalpy, the atom does not form covalent bonds, also as the atom is reacting with metal there is a high chance of ionic bonding between them.

Question 2. Why do atoms of metals usually have less negative electron gain enthalpies than those of a non-metal?

Answer. Atoms of metals usually have a large atomic size compared to those of non-metals of the same period. The nuclear charge of atoms of metals is usually low compared to the atoms of non-metals of the same period.

FAQs

What is electron gain enthalpy?

The energy released when an electron is added to an isolated gaseous atom is called electron gain enthalpy.
The energy released when the electron is added to a neutral isolated gaseous atom is first electron gain enthalpy, and when the electron is added to an isolated gaseous anion with -1 charge it is called second electron gain enthalpy and so on.

Why is the second electron gain enthalpy of most atoms positive?

The second electron gain enthalpy of most of the atoms is positive due to electron-electron repulsions between already present electrons and the new one.

Why are the electron gain enthalpies of all the noble gases positive?

Noble gases have completely filled shells and are the most stable elements, due to this if an electron is added to their atoms they lose their stability, as the stability is lost we need to provide energy in order to add an electron to the noble gases.

What is the difference between electron gain enthalpy and electron affinity of an atom?

Electron gain enthalpy of an element is the energy released when an electron is added to an isolated gaseous atom, but electron affinity of an element is the energy released when an electron is added to a neutral isolated gaseous atom.

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