Electron affinity is defined as the change in energy (in kJ/mole) the a neutral atom (in the gaseous phase) when an electron is included to the atom to form a an adverse ion. In other words, the neutral atom"s likelihood of obtaining an electron.

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Introduction

Energy of an atom is identified when the atom loses or gains power through chathamtownfc.netistry reactions that cause the ns or obtain of electrons. A chathamtownfc.netistry reaction the releases energy is dubbed an exothermic reaction and also a chathamtownfc.netical reaction that absorbs power is called an endothermic reaction. Energy from one exothermic reaction is negative, thus energy is provided a an adverse sign; whereas, energy from one endothermic reaction is positive and also energy is provided a positive sign. An instance that displayed both processes is as soon as a human being drops a book. Once he or she lifts a book, that or she offers potential power to the publication (energy absorbed). However, as soon as the he or she autumn the book, the potential energy converts itself to kinetic energy and comes in the form of sound when it access time the floor (energy released).

When an electron is added to a neutral atom (i.e., very first electron affinity) power is released; thus, the an initial electron affinities room negative. However, much more energy is compelled to add an electron come a an adverse ion (i.e., second electron affinity) i beg your pardon overwhelms any type of the relax of power from the electron attachment procedure and hence, 2nd electron affinities room positive.

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

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

second Electron Affinity (positive energy since energy needed is more than gained):

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


First Electron Affinity

Ionization energies are always concerned with the development of optimistic ions. Electron affinities are the an adverse ion equivalent, and their usage is almost always limit to facets in groups 16 and also 17 of the regular Table. The very first electron affinity is the energy released as soon as 1 mole that gaseous atom each gain an electron to type 1 mole of gas -1 ions. That is the power released (per mole that X) when this adjust happens. First electron six have an adverse values. For example, the very first electron affinity of chlorine is -349 kJ mol-1. By convention, the an unfavorable sign mirrors a release of energy.

When an electron is included to a metal element, power is needed to acquire that electron (endothermic reaction). Metals have a less likely chance to get electrons due to the fact that it is less complicated to lose their valance electrons and form cations. It is less complicated to shed their valence electrons because metals" nuclei do not have actually a strong pull on your valence electrons. Thus, steels are recognized to have actually lower electron affinities.


Example (PageIndex1): team 1 Electron Affinities

This tendency of reduced electron six for metals is described 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 gain electrons, the energy adjust is usually negative because they provide off energy to type an anion (exothermic process); thus, the electron affinity will be negative. Nonmetals have a better electron affinity than metals because of their atomic structures: first, nonmetals have an ext valence electron than metals do, thus it is simpler for the nonmetals to get electrons to accomplish a steady octet and secondly, the valence electron covering is closer come the nucleus, hence it is harder to remove an electron and also it much easier to entice electrons from other facets (especially metals). Thus, nonmetals have actually a higher electron affinity than metals, an interpretation they are an ext likely to acquire electrons than atoms v a lower electron affinity.


Example (PageIndex2): team 17 Electron Affinities

For example, nonmetals prefer the facets in the halogens collection in group 17 have a greater electron affinity than the metals. This tendency is defined as below. An alert the an adverse sign for the electron affinity which shows 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 capacity of one atom to expropriate an electron. Unequal electronegativity, electron affinity is a quantitative measure up of the energy change that occurs as soon as an electron is added to a neutral gas atom. The an ext 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 periodic Variation of Electron Affinity through Atomic Number for the an initial Six Rows the the periodic Table. Notification that electron affinities can be both negative and positive. From Robert J. Lancashire (University that the West Indies).


Why is Fluorine an Anomaly?

The incoming electron is walk to it is in closer to the nucleus in fluorine than in any other of these elements, so girlfriend would expect a high value of electron affinity. However, since fluorine is such a little atom, girlfriend are placing the brand-new electron right into a region of an are already crowded with electrons and also there is a far-ranging amount the repulsion. This repulsion lessens the attraction the just arrive electron feels and also so lessens the electron affinity. A comparable reversal that the expected trend happens in between oxygen and sulfur in team 16. The an initial electron affinity the oxygen (-142 kJ mol-1) is smaller sized than the of sulfur (-200 kJ mol-1) for exactly the same factor that fluorine"s is smaller sized than chlorine"s.


Comparing team 16 and also Group 17 values

As you could have noticed, the first electron affinity that oxygen ((-142; kJ; mol^-1)) is much less than that of fluorine ((-328; kJ; mol^-1)). Similarly sulfur"s ((-200; kJ; mol^-1)) is less than chlorine"s ((-349; kJ; mol^-1)). Why? It"s merely that the group 16 element has 1 much less proton in the nucleus than its next door ar in team 17. The quantity of screening is the same in both. That way that the network pull from the nucleus is less in group 16 보다 in team 17, and so the electron affinities are less.

The reactivity that the aspects in group 17 falls as girlfriend go down the team - fluorine is the many reactive and also iodine the least. Often in their reactions this elements kind their an adverse ions. The first impression the is sometimes given that the autumn in reactivity is since the just arrive electron is organized less strongly together you go under the group and so the an unfavorable ion is less likely come form. The explanation looks reasonable till you encompass fluorine!

An as whole reaction will certainly be consisted of of lots of different steps every involving energy changes, and you can not safely shot to describe a trend in terms of simply one that those steps. Fluorine is much much more reactive 보다 chlorine (despite the reduced electron affinity) since the energy released in other procedures in the reactions much more than provides up because that the lower amount of energy released together electron affinity.


Second Electron Affinity

You space only ever before likely to meet this through respect come the team 16 aspects oxygen and sulfur which both form -2 ions. The second electron affinity is the energy required to include an electron to every ion in 1 mole of gaseous 1- ions to develop 1 mole of gaseous 2- ions. This is more easily seen in symbol terms.

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

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

Why is power needed to do this? You room forcing one electron into an already an adverse ion. It"s not going to go 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 reflects that you have to put in power to execute this change. The second electron affinity of oxygen is particularly high since the electron is being forced into a small, very electron-dense space.

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Practice Problems

once an electron is included to a nonmetal atom, is power released or absorbed? Why execute nonmetal atoms have actually a higher electron affinity than metal atoms? Why are atoms with a low electron affinity much more likely to shed electrons than gain electrons? as you relocate down a team of the routine table, go electron affinity boost or decrease, if so, why? Why execute nonmetals want to acquire electrons? Why do metals have a short electron affinity?

Answers

power is released once a electron is added to a nonmetal. Nonmetals have a greater electron affinity 보다 metals since their atom structure enables them to gain electrons rather than shed them. Atoms through a short electron affinity want to provide up your valence electrons since they are further from the nucleus; together a result, they execute not have actually a solid pull ~ above the valence electrons. As you relocate down a team on the regular table, electron affinity decreases. First, the electrons are put in energy levels more away indigenous the nucleus, which results in electrons not having a solid attraction come the nucleus; secondly, the atom does no want gain electrons due to the fact that there is minimal fee on the outer energy levels native the nucleus; and also lastly, the shielding impact increases, bring about repulsion in between the electrons, for this reason they move more from each other and also the cell core itself. Nonmetals desire to acquire electrons because they have much more valence electrons than metals, so the is simpler for them to acquire electrons than shed the valance electron to satisfy a steady octet. In addition, nonmetals" valance electrons are closer come the nucleus, therefore allowing much more attraction between the two. Metals have a low electron affinity (a less likely opportunity to get electrons) because they want to offer up their valence electrons quite than get electrons, which require more energy 보다 necessary. In addition, they carry out not have a solid pull on the valance electrons due to the fact that they are far away indigenous the nucleus, for this reason they have actually less power for one attraction.