Electron affinity is defined as the adjust in energy (in kJ/mole) that a neutral atom (in the gas phase) once an electron is added to the atom to kind a an unfavorable ion. In various other words, the neutral atom"s likelihood of acquiring an electron.

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Introduction

Energy of one atom is identified when the atom loser or gains power through hunterriverpei.comistry reactions that reason the loss or gain of electrons. A hunterriverpei.comistry reaction the releases energy is called an exothermic reaction and a hunterriverpei.comistry reaction the absorbs power is referred to as an endothermic reaction. Energy from an exothermic reaction is negative, thus power is given a negative sign; whereas, power from an endothermic reaction is positive and energy is given a positive sign. An instance that demonstrates both processes is once a human being drops a book. Once he or she elevator a book, the or she gives potential energy to the book (energy absorbed). However, when the the or she fall the book, the potential energy converts chin to kinetic energy and also comes in the kind of sound when it hits the ground (energy released).

When one electron is added to a neutral atom (i.e., first electron affinity) energy is released; thus, the first electron affinities room negative. However, much more energy is required to include an electron come a an adverse ion (i.e., 2nd electron affinity) which overwhelms any kind of the relax of power from the electron attachment process and hence, second electron affinities are positive.

very first Electron Affinity (negative energy because energy released):

< ceX (g) + e^- ightarrow X^- (g) label1>

2nd Electron Affinity (positive energy due to the fact that energy needed is much more than gained):

< ceX^- (g) + e^- ightarrow X^2- (g) label2>


First Electron Affinity

Ionization energies are always concerned v the development of optimistic ions. Electron affinities room the negative ion equivalent, and their use is nearly always confined to elements in teams 16 and 17 of the routine Table. The very first electron affinity is the energy released once 1 mole the gaseous atoms each obtain an electron to type 1 mole of gaseous -1 ions. The is the power released (per mole of X) once this adjust happens. First electron six have an adverse values. For example, the very first electron affinity that chlorine is -349 kJ mol-1. Through convention, the an unfavorable sign reflects a relax of energy.

When one electron is included to a metal element, energy is necessary to gain that electron (endothermic reaction). Metals have actually a much less likely chance to gain electrons since it is less complicated to lose their valance electrons and form cations. The is less complicated to lose their valence electrons since metals" nuclei carry out not have a solid pull on your valence electrons. Thus, steels are recognized to have actually lower electron affinities.


Example (PageIndex1): team 1 Electron Affinities

This trend of reduced electron six for steels is explained by the team 1 metals:

Lithium (Li): -60 KJ mol-1 salt (Na): -53 KJ mol-1 Potassium (K): -48 KJ mol-1 Rubidium (Rb): -47 KJ mol-1 Cesium (Cs): -46 KJ mol-1

Notice the electron affinity decreases down the group.


When nonmetals acquire electrons, the energy readjust is usually an adverse because they give off energy to type an anion (exothermic process); thus, the electron affinity will certainly be negative. Nonmetals have a greater electron affinity 보다 metals because of your atomic structures: first, nonmetals have more valence electron than steels do, therefore it is less complicated for the nonmetals to get electrons to accomplish a steady octet and also secondly, the valence electron covering is closer to the nucleus, thus it is harder to remove an electron and also it simpler to lure electrons native other facets (especially metals). Thus, nonmetals have actually a greater electron affinity 보다 metals, an interpretation they are much more likely to gain electrons 보다 atoms v a lower electron affinity.


Example (PageIndex2): team 17 Electron Affinities

For example, nonmetals prefer the elements in the halogens collection in group 17 have actually a higher electron affinity 보다 the metals. This trend is explained as below. Notice the negative sign for the electron affinity which reflects that power is released.

Fluorine (F) -328 kJ mol-1 Chlorine (Cl) -349 kJ mol-1 Bromine (Br) -324 kJ mol-1 Iodine (I) -295 kJ mol-1

Notice that electron affinity decreases under the group, yet increases up with the period.


As the name suggests, electron affinity is the capability of one atom to accept an electron. Unequal electronegativity, electron affinity is a quantitative measure of the energy readjust that occurs as soon as an electron is included to a neutral gas atom. The more negative the electron affinity value, the greater an atom"s affinity for electrons.

*
api/deki/files/55500/ElectronAffinity.png?revision=1&size=bestfit&width=780&height=456" />Figure (PageIndex1): A Plot of routine Variation that Electron Affinity through Atomic Number for the first Six Rows of the periodic Table. An alert that electron affinities have the right to be both negative and positive. Indigenous Robert J. Lancashire (University the the West Indies).


Why is Fluorine an Anomaly?

The just arrive electron is walk to it is in closer to the nucleus in fluorine 보다 in any type of other of these elements, so you would suppose a high value of electron affinity. However, since fluorine is such a small atom, friend are placing the new electron into a an ar of room already crowded v electrons and there is a far-ranging amount that repulsion. This repulsion lessens the attraction the just arrived electron feels and also so lessens the electron affinity. A similar reversal of the meant trend happens in between oxygen and also sulfur in group 16. The first electron affinity of oxygen (-142 kJ mol-1) is smaller than the of sulfur (-200 kJ mol-1) for specifically the same factor that fluorine"s is smaller than chlorine"s.


Comparing team 16 and also Group 17 values

As you might have noticed, the first electron affinity the oxygen ((-142; kJ; mol^-1)) is less than that of fluorine ((-328; kJ; mol^-1)). An in similar way sulfur"s ((-200; kJ; mol^-1)) is much less than chlorine"s ((-349; kJ; mol^-1)). Why? It"s just that the team 16 facet has 1 less proton in the nucleus than its next door ar in group 17. The lot of screening is the very same in both. That means that the net pull native the nucleus is less in team 16 보다 in group 17, and so the electron affinities space less.

The reactivity the the elements in group 17 drops as friend go under the group - fluorine is the many reactive and iodine the least. Often in their reactions this elements form their an adverse ions. The very first impression the is sometimes offered that the fall in reactivity is since the just arrived electron is organized less strongly together you go under the group and also so the negative ion is much less likely come form. The explanation watch reasonable till you encompass fluorine!

An all at once reaction will certainly be made up of too many of different steps every involving power changes, and also you cannot safely try to define a tendency in state of just one of those steps. Fluorine is much much more reactive than chlorine (despite the reduced electron affinity) since the power released in other actions in that reactions more than renders up for the lower amount of energy released as electron affinity.


Second Electron Affinity

You space only ever likely to meet this through respect come the group 16 elements oxygen and sulfur i m sorry both form -2 ions. The second electron affinity is the power required to include an electron to each ion in 1 mole of gas 1- ions to develop 1 mole of gaseous 2- ions. This is much more easily seen in prize terms.

< X^- (g) + e^- ightarrow X^-2 (g) label3>

It is the energy needed to bring out this change per mole of (X^-).

Why is energy needed to execute this? You are forcing one electron right into an already an adverse ion. It"s not going to walk in willingly!

< O_g + e^- ightarrow O^- (g) ;;; ext1st EA = -142 kJ mol^-1 label4>

< O^-_g + e^- ightarrow O^2- (g) ;;; ext2nd EA = +844 kJ mol^-1 label5>

The confident sign shows that you need to put in power to execute this change. The second electron affinity that oxygen is an especially high since the electron is being forced into a small, an extremely electron-dense space.

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Practice Problems

as soon as an electron is included to a nonmetal atom, is power released or absorbed? Why perform nonmetal atoms have actually a greater electron affinity than steel atoms? Why space atoms through a short electron affinity an ext likely to lose electrons than gain electrons? together you move down a team of the periodic table, go electron affinity rise or decrease, if so, why? Why carry out nonmetals want to acquire electrons? Why perform metals have actually a short electron affinity?

Answers

energy is released as soon as a electron is added to a nonmetal. Nonmetals have a higher electron affinity 보다 metals due to the fact that their atomic structure allows them to obtain electrons rather than lose them. Atoms with a short electron affinity desire to give up their valence electrons since they are additional from the nucleus; as a result, they perform not have a solid pull ~ above the valence electrons. Together you relocate down a team on the periodic table, electron affinity decreases. First, the electron are inserted in energy levels additional away indigenous the nucleus, which results in electron not having a strong attraction come the nucleus; secondly, the atom does no want obtain electrons due to the fact that there is minimal fee on the outer energy levels indigenous the nucleus; and also lastly, the shielding effect increases, bring about repulsion between the electrons, for this reason they move further from each other and also the nucleus itself. Nonmetals desire to gain electrons because they have much more valence electrons 보다 metals, so it is simpler for castle to acquire electrons than lose the valance electrons to accomplish a stable octet. In addition, nonmetals" valance electrons space closer to the nucleus, for this reason allowing an ext attraction in between the two. Metals have actually a low electron affinity (a less likely chance to acquire electrons) due to the fact that they want to offer up their valence electrons rather than obtain electrons, i beg your pardon require an ext energy 보다 necessary. In addition, they perform not have actually a solid pull on the valance electrons because they are far away from the nucleus, for this reason they have actually less power for an attraction.