Electron affinity is characterized as the adjust in power (in kJ/mole) of a neutral atom (in the gaseous phase) as soon as an electron is added to the atom to kind a an unfavorable ion. In other words, the neutral atom"s likelihood of acquiring an electron.

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Introduction

Energy of one atom is characterized when the atom loses or gains power through aufdercouch.netical reactions that cause the lose or get of electrons. A aufdercouch.netical reaction that releases power is called an exothermic reaction and a aufdercouch.netical reaction the absorbs energy is referred to as an endothermic reaction. Power from one exothermic reaction is negative, thus energy is offered a negative sign; whereas, power from one endothermic reaction is positive and energy is provided a hopeful sign. An instance that demonstrates both procedures is as soon as a human drops a book. When he or she lifts a book, that or she provides potential energy to the publication (energy absorbed). However, when the that or she drops the book, the potential power converts chin to kinetic energy and also comes in the kind of sound when it access time the floor (energy released).

When one electron is added to a neutral atom (i.e., an initial electron affinity) power is released; thus, the first electron affinities space negative. However, much more energy is forced to add an electron to a negative ion (i.e., 2nd electron affinity) i m sorry overwhelms any type of the release of power from the electron attachment procedure and hence, 2nd electron affinities are positive.

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

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

second Electron Affinity (positive energy due to the fact that energy essential is an ext than gained):

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


First Electron Affinity

Ionization energies are always concerned through the formation of hopeful ions. Electron affinities are the an adverse ion equivalent, and also their usage is almost always border to aspects in groups 16 and also 17 the the periodic Table. The an initial electron affinity is the energy released once 1 mole that gaseous atom each get an electron to kind 1 mole of gas -1 ions. It is the energy released (per mole of X) when this change happens. Very first electron affinities have an adverse values. For example, the very first electron affinity the chlorine is -349 kJ mol-1. By convention, the an adverse sign shows a relax of energy.

When an electron is added to a metal element, power is required to obtain that electron (endothermic reaction). Metals have a less likely possibility to acquire electrons due to the fact that it is simpler to shed their valance electron and type cations. That is easier to shed their valence electrons because metals" nuclei do not have actually a solid pull on your valence electrons. Thus, metals are known to have lower electron affinities.


Example (PageIndex1): team 1 Electron Affinities

This trend of reduced electron six for metals is defined by the group 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 offer off energy to form an anion (exothermic process); thus, the electron affinity will be negative. Nonmetals have a better electron affinity than metals due to the fact that of your atomic structures: first, nonmetals have much more valence electron than metals do, for this reason it is easier for the nonmetals to gain electrons to accomplish a stable octet and also secondly, the valence electron shell is closer to the nucleus, thus it is harder to eliminate an electron and also it much easier to tempt electrons indigenous other elements (especially metals). Thus, nonmetals have a greater electron affinity 보다 metals, an interpretation they are an ext likely to acquire electrons 보다 atoms v a reduced electron affinity.


Example (PageIndex2): team 17 Electron Affinities

For example, nonmetals like the elements in the halogens collection in team 17 have a higher electron affinity than the metals. This tendency is explained as below. Notification the negative sign for the electron affinity which mirrors that energy 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 the electron affinity decreases down the group, however increases up through the period.


As the name suggests, electron affinity is the capacity of an atom to expropriate an electron. Unlike electronegativity, electron affinity is a quantitative measurement of the energy readjust that occurs when an electron is added to a neutral gas atom. The much more negative the electron affinity value, the higher an atom"s affinity for electrons.

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


Why is Fluorine one Anomaly?

The just arrive electron is walking to be closer to the nucleus in fluorine than in any other of this elements, so you would expect a high worth of electron affinity. However, since fluorine is such a small atom, you are placing the new electron right into a region of an are already crowded through electrons and there is a far-ranging amount that repulsion. This repulsion lessens the attraction the incoming electron feels and so lessens the electron affinity. A similar reversal that the expected trend happens in between oxygen and also sulfur in group 16. The very first electron affinity the 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 group 16 and also Group 17 values

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

The reactivity that the facets in team 17 falls as girlfriend go down the team - fluorine is the most reactive and also iodine the least. Regularly in their reactions this elements form their an adverse 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 also so the an adverse ion is less likely come form. That explanation look at reasonable until you incorporate fluorine!

An all at once reaction will certainly be consisted of of lots of various steps every involving power changes, and also you cannot safely try to explain a trend in state of simply one of those steps. Fluorine is much much more reactive than chlorine (despite the lower electron affinity) because the energy released in other steps in that reactions more than makes up because that the reduced amount of power released as electron affinity.


Second Electron Affinity

You are only ever likely to satisfy this v respect come the team 16 facets oxygen and sulfur which both kind -2 ions. The second electron affinity is the power required to include an electron to every ion in 1 mole of gas 1- ion to create 1 mole of gaseous 2- ions. This is more easily viewed in prize terms.

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

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

Why is power needed to carry out this? You room forcing one electron into an already an unfavorable 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 reflects that you have to put in energy to do this change. The 2nd electron affinity of oxygen is particularly high due to the fact that the electron is being required 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 carry out nonmetal atoms have actually a greater electron affinity than metal atoms? Why space atoms with a low electron affinity an ext likely to shed electrons than get electrons? together you relocate down a group of the routine table, go electron affinity rise or decrease, if so, why? Why carry out nonmetals want to get electrons? Why execute metals have a low electron affinity?

Answers

energy is released when a electron is included to a nonmetal. Nonmetals have a greater electron affinity 보다 metals because their atomic structure allows them to acquire electrons quite than lose them. Atoms v a low electron affinity desire to give up your valence electrons due to the fact that they are additional from the nucleus; together a result, they execute not have actually a strong pull on the valence electrons. As you move down a team on the periodic table, electron affinity decreases. First, the electron are put in energy levels further away indigenous the nucleus, which outcomes in electron not having a solid attraction to the nucleus; secondly, the atom does not want gain electrons because there is minimal charge on the outer energy levels native the nucleus; and lastly, the shielding result increases, bring about repulsion between the electrons, hence they move further from each other and the nucleus itself. Nonmetals want to gain electrons because they have much more valence electrons 보다 metals, so it is much easier for lock to get electrons than lose the valance electrons to accomplish a secure octet. In addition, nonmetals" valance electrons room closer come the nucleus, hence allowing more attraction in between the two. Metals have actually a low electron affinity (a much less likely chance to gain electrons) because they want to give up your valence electrons rather than get electrons, which require more energy than necessary. In addition, they do not have actually a solid pull on the valance electrons because they are much away from the nucleus, thus they have less power for one attraction.