Electron Gain Enthalpy

What Is Ionisation Energy?

Ionisation energy is a quantity that represents the amount of energy that is needed for an isolated gaseous atom to lose an electron while in a ground state. The loss of electrons results in particles called cations. For an A atom to become the A+ ion, a certain energy is required (first ionisation energy), the units of which are given in KJ mol-1.

There is also second ionisation energy. This is used to express the energy required to take an electron from its valence shell. The removal of electrons from atoms, of course, requires certain energy. This is why the enthalpies of the elements are always greater than zero. The force exerted from the nucleus to the electrons in the same shell varies, being greater towards the second electron and less towards the first. For this reason, the second ionisation energy is by definition greater than the first. 

Factors Affecting Ionisation Energy

There are certain factors that affect the ionisation energy:

• The attraction is exerted from the nucleus to the electrons and vice versa.
• The repulsion is experienced between the electrons.

As mentioned, the force from the nucleus to the outermost electrons is measured by the force exerted by the innermost electrons, a kind of shield, created by the innermost electrons. For this reason, the effective nuclear charge on the outermost electrons in the outermost shells is less than the actual charge. This effect is known as the shielding effect. To illustrate this, let’s look at a sodium atom. In Na, the electrons are in the electronic configuration the core electrons (1s², 2s² and 2p⁶) shield their outer electrons which are in 3s¹ will be shielded. This effect increases when there are electrons in the inner orbitals that completely fill the subshell.

Electron Gain Enthalpy

The electron gain enthalpy describes the enthalpy change when the atom gains an electron (again, this atom is in special conditions, such as in the gaseous state and in the ground state). This reaction results in the formation of an anion. The reaction can occur as follows:

First gain enthalpy of electrons

S(g) + e- → S- (g)

Second gain enthalpy of electrons

S-(g) + e-(g) → S2- (g)

There is a variation of this enthalpy. This can be explained by the following factors:

• The size of the atoms
• The charge exerted by the nucleus
• The electronic configuration

Enthalpy Symbols

Acceptors and donors have different signs on the enthalpy. The negative sign indicates the atom is an acceptor and the reaction releases energy. Conversely, a positive sign means the atom is a donor and absorbs energy 

When an electron is accepted, the reaction might be exothermic or endothermic, meaning it can either consume or release heat. This reaction is usually exothermic, with a negative enthalpy of electron gain. In the case of halogens, the electron gain enthalpy is extremely negative. Because an electron in a halogen only needs one electron to reach the nearest noble gas configuration, this is the case. In the case of noble gases, on the other hand, this enthalpy is quite positive because it must add an electron at a higher quantum level because all shells are full. This necessitates a tremendous quantity of energy. 

Electronegativity, or Electron Gain Enthalpy?

There is a term that can be confused with gain enthalpy. Electronegativity describes the tendency of an atom to attract a pair of electrons in a covalent bond. There is no specific unit for electronegativity. This measure is dimensionless. Electronegativity measurements are given with respect to different scales. One such scale is the Linus Pauling scale, on which the most electronegative element is fluorine (with an electronegative value of 4.0). On this same scale, the element with the lowest electronegativity has a value of 0.7 and is caesium.

As mentioned in the previous paragraph, covalent bonds are mediated by electronegativity. In fact, it determines the strength of the covalent bond between two atoms. It is the difference between the electronegativities of the different atoms that are most affected. Therefore, when the bond is between two equal mononuclear atoms, the electronegativity is said to be pure, since it is the same between them.  Examples are oxygen or bimolecular hydrogen.

When the two species are different, a polarisation is generated at the bond. This is clearly because one of the atoms is more electronegative than the other, which means that the electrons will be closer to this atom and not to the other. This gives a partial charge on each of the atoms, giving polarities in the chemical bond.