Electron Affinity

Electron affinity is defined as the quantitative measurement of the energy change that results from adding a new electron to a neutral atom or molecule in the gaseous state. The more negative the electron affinity value, the higher an atom’s affinity for electrons.The energy of an atom is stated when an atom loses or gains energy through chemical reactions that cause the loss or gain of electrons. 

The reaction that releases energy is called an ‘exothermic’ reaction and the reaction in which energy is absorbed is called an ‘endothermic’ reaction. Talking about energies, energy from an endothermic reaction is positive. Hence, given a positive sign whereas energy from an exothermic reaction is negative. It is given a negative sign. The energy is released when an electron is being added to a neutral atom. Thus first electron affinities are always negative whereas second electron affinity ( electron to negative ion ) is positive. The electron affinity is further discussed below:

First Electron Affinity 

Negative energy because energy is released.

X(g) + e- —> X⁻ (g)

Second Electron Affinity: Positive energy because the energy needed is more than gained.

X⁻(g) + e- —> X²⁻ (g)

First Electron Affinity: The energies are always concerned by the formation of positive ions. The first electron affinity is the energy released when 1 mole of gaseous atoms acquire an electron to form 1 mole of gaseous -1 ions. Example: The first electron affinity of chlorine is -349 kJ mol⁻¹ The energy is needed to gain the electron when an electron is added to a metal element. However, metals are less likely to gain electrons as it is easier to lose their valence electrons and form cations. The reason behind losing their valence electrons is that metals’ nuclei do not have a strong pull on their valence electrons. Therefore, metals are said to have lower electron affinities. The Trend of Lower Electron Affinities For Metals is described by Group 1 

Metals: 

Lithium (Li): -60 kJ mol⁻¹ 

Sodium (Na): -53 kJ mol⁻¹

Potassium (K): -48 kJ mol⁻¹

Rubidium (Rb): -47 kJ mol⁻¹

Cesium (Cs): -46 kJ mol⁻¹ 

When nonmetals gain electrons, the energy change noted is negative because they give off energy. Non-metals have greater electron affinity because of their atomic structures. There are two reasons associated with why non-metals have greater electron affinity. Non-metals have more valence electrons than metals have which makes it easy to gain electrons to fulfill a stable octet. The valence electron shell is closer to the nucleus, this makes it harder to remove an electron. It is rather easy to attract an electron from another element. 

Example: Non-metals like the elements in the Halogen series in Group 17 have higher electron affinity. The electron affinity trend is stated below:

Fluorine (F) : -328 kJ mol⁻¹ 

Chlorine (Cl) : -349 kJ mol⁻¹ 

Bromine (Br) : -324 kJ mol⁻¹ 

Iodine (I) : -295 kJ mol⁻¹

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Second Electron Affinity

Second electron affinity is only met concerning the group 16 elements oxygen and sulfur which both form -2 ions. It is the energy required to add an electron to each ion in 1 mole of gaseous 1- ions to produce 1 mole of gaseous 2- ions.

Factors Affecting Electron Affinity 

Electron Affinity is affected by three main factors. These factors are generally related to the structure and configuration of the molecule in question. The three factors affecting the electron affinity of a molecule are Nuclear Charge, Atomic Size, and Electronic Configuration.

• Nuclear Charge: The greater the nuclear charge, the greater will be the attraction of the incoming electron. This will result in a larger value of electron affinity. To explain it in simpler terms, the nuclear charge can be understood as the pull that is exerted by the nucleus on the electrons. Therefore, the greater the pull of the nucleus, the greater the chance of electrons attaching to the atom.

• Atomic Size: The larger the size of an atom, the larger will be the distance between the nucleus and electron. This will result in a smaller force of attraction by electrons. Therefore, the value of electron affinity will be small. In general, too, the electronic affinity increases by going down the group and decreases from left to right across the periods. 

• Electronic Configuration: Stable the configuration of an atom, its tendency will be less to accept the electron. Therefore, it will face a lower value of its electron affinity. Electron affinity is almost zero or low in elements having a stable electronic configuration. This is due to the small tendency to accept another electron. Electron affinities of inert gases are zero. This is due to their atoms having stable ns²np⁶ configuration in their shell. Electron affinity of Beryllium, and calcium is practically zero. If the atom has fully or half-filled orbits, its electron affinity will be less.

Example: Electron affinity of Be and N is almost 0 because they are having filled electrons in their valence shells. Filled orbits are all stable due to symmetry. Therefore, these elements will be having the least tendency to accept any electron. In general, the electron affinity follows the below trend: Halogens > Oxygen family > Carbon family > Nitrogen family > Metals of group 1 and 13 > Metals of group 2.

Studying Electron Affinity with Vedantu

At Vedantu, we are aware that studying can sometimes be a bit stressful, however, with access to the right study material and a good study schedule, anyone can study well and ace their exams. Here is how you can study Electron Affinity and other topics in the Chemistry Syllabus.

First, create a good study schedule for yourself. Spread out your study schedule over a few weeks at least. This will give you enough time to study without trying to cram in a lot of information in a short time. Each day, allot a few hours, at least 2-3, to studying chemistry. 

When you begin studying, you should ideally start with your textbooks. However, once you finish reading up on the topic, you can refer to the resources available on Vedantu, including this page.

To find resources on Vedantu, you can use the search bar and navigation menu to look for specific documents or notes. For electron affinity, you can refer to this page and also the example questions available here. You can also download chemistry past papers or sample papers and practise solving those.