Electron affinity

Key point: Electron affinities are highest for elements near fluorine in the periodic table. The electron-gain enthalpy, Δ_eg H ° , is the change in standard molar enthalpy when a gaseous atom gains an electron:

Electron gain may be either exothermic or endothermic. Although the electron-gain enthalpy is the thermodynamically appropriate term, much of inorganic chemistry is discussed in terms of a closely related property, the electron affinity, Ea, of an element (Table 1.6), which is the difference in energy between the gaseous atoms and the gaseous ions at T=0.

Althoughthe precise relation is ∆eg a H E °= 5/2 RT, the contribution 5/2 RT is commonly ignored. A positive electron affinity indicates that the ion A has a lower, more negative energy than the neutral atom, A. The second electron-gain enthalpy, the enthalpy change for the attachment of a second electron to an initially neutral atom, is invariably positive because the electron repulsion outweighs the nuclear attraction. The electron affinity of an element is largely determined by the energy of the lowest unfilled (or half-filled) orbital of the ground-state atom. This orbital is one of the two frontier orbitals of an atom, the other one being the highest filled atomic orbital. The frontier orbitals are the sites of many of the changes in electron distributions when bonds form,

Table 1.6 First electron affinities of the main-group elements, Ea /(kJ mol^-1)

and we shall see more of their importance as the text progresses. An element has a high electron affinity if the additional electron can enter a shell where it experiences a strong effective nuclear charge. This is the case for elements towards the top right of the periodic table, as we have already explained. Therefore, elements close to fluorine (specifically O and Cl, but not the noble gases) can be expected to have the highest electron affinities as their Z_eff is large and it is possible to add electrons to the valence shell. Nitrogen has very low electron affinity because there is a high electron repulsion when the incoming electron enters an orbital that is already half full.

A note on good practice Be alert to the fact that some people use the terms ‘electron affinity’ and ‘electron-gain enthalpy’ interchangeably. In such cases, a positive electron affinity could indicate that A^- has a higher energy than A.

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مواضيع ذات صلة

Polyatomic molecules

Homonuclear diatomic molecules

The hydrogen molecule

The basic shapes

Resonance

Polarizability

Electronegativity

Atomic and ionic radii

The format of the periodic table

The periodic table

Penetration and shielding

The angular variation of atomic orbitals