Why does electron affinity increase from left to right

This repulsion lessens the attraction the incoming electron feels and so lessens the electron affinity.

A similar reversal of the expected trend happens between oxygen and sulfur in Group The first electron affinity of oxygen kJ mol -1 is smaller than that of sulfur kJ mol-1 for exactly the same reason that fluorine's is smaller than chlorine's. It's simply that the Group 16 element has 1 less proton in the nucleus than its next door neighbor in Group The amount of screening is the same in both.

That means that the net pull from the nucleus is less in Group 16 than in Group 17, and so the electron affinities are less. The reactivity of the elements in group 17 falls as you go down the group - fluorine is the most reactive and iodine the least. Often in their reactions these elements form their negative ions.

The first impression that is sometimes given that the fall in reactivity is because the incoming electron is held less strongly as you go down the group and so the negative ion is less likely to form.

That explanation looks reasonable until you include fluorine! An overall reaction will be made up of lots of different steps all involving energy changes, and you cannot safely try to explain a trend in terms of just one of those steps. Fluorine is much more reactive than chlorine despite the lower electron affinity because the energy released in other steps in its reactions more than makes up for the lower amount of energy released as electron affinity.

You are only ever likely to meet this with respect to the group 16 elements oxygen and sulfur which both form -2 ions. The second electron affinity 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. This is more easily seen in symbol terms. Why is energy needed to do this? You are forcing an electron into an already negative ion. It's not going to go in willingly! The positive sign shows that you have to put in energy to perform this change.

The second electron affinity of oxygen is particularly high because the electron is being forced into a small, very electron-dense space. Jim Clark Chemguide. Introduction Energy of an atom is defined when the atom loses or gains energy through chemical reactions that cause the loss or gain of electrons. First Electron Affinity Ionization energies are always concerned with the formation of positive ions.

Nonmetals vs. They absorb energy endothermic to lose electrons. The electron affinity of metals is lower than that of nonmetals. Nonmetals: Nonmetals like to gain electrons to form anions to have a fully stable octet.

They release energy exothermic to gain electrons to form an anion; thus, electron affinity of nonmetals is higher than that of metals. Notice that electron affinities can be both negative and positive. Lancashire University of the West Indies. Patterns in Electron Affinity Electron affinity increases upward for the groups and from left to right across periods of a periodic table because the electrons added to energy levels become closer to the nucleus, thus a stronger attraction between the nucleus and its electrons.

Chlorine A fluorine atom has an electronic structure of 1s 2 2s 2 2px 2 2py 2 2pz 1. Why is Fluorine an Anomaly? Second Electron Affinity You are only ever likely to meet this with respect to the group 16 elements oxygen and sulfur which both form -2 ions. Practice Problems When an electron is added to a nonmetal atom, is energy released or absorbed? Why do nonmetal atoms have a greater electron affinity than metal atoms?

Why are atoms with a low electron affinity more likely to lose electrons than gain electrons? Related questions What units is electron affinity measured in? How are electron affinity and ionization energy related? How does electron affinity affect reactivity? What does electron affinity determine? How can electron affinity be negative? What is electron affinity? Why is the electron affinity for nitrogen positive? Question The energy change that occurs when a neutral atom gains an electron is called its electron affinity.

When energy is released in a chemical reaction or process, that energy is expressed as a negative number. Electron affinities are measured on atoms in the gaseous state and are very difficult to measure accurately. Figure 1. Electron affinities are negative numbers because energy is released.

The elements of the halogen group Group 17 gain electrons most readily, as can be seen from their large negative electron affinities. This means that more energy is released in the formation of a halide ion than for the anions of any other elements. Considering electron configuration, it is easy to see why.

The outer configuration of all halogens is ns 2 np 5. The addition of one more electron gives the halide ions the same electron configuration as a noble gas, which we have seen is particularly stable.

Period and group trends for electron affinities are not nearly as regular as for ionization energy. In general, electron affinities increase become more negative from left to right across a period and decrease become less negative from top to bottom down a group. However, there are many exceptions, owing in part to inherent difficulties in accurately measuring electron affinities.