Electron affinity is defined as the readjust in energy (in kJ/mole) that a neutral atom (in the gas phase) when an electron is added to the atom to type a negative ion. In various other words, the neutral atom"s likelihood of getting an electron.
You are watching: Why does electron affinity increase across a period
Energy of one atom is characterized when the atom loses or gains energy through jajalger2018.orgical reactions that cause the lose or obtain of electrons. A jajalger2018.orgical reaction that releases power is called an exothermic reaction and also a jajalger2018.orgical reaction that absorbs energy is called an endothermic reaction. Power from an exothermic reaction is negative, thus energy is provided a an adverse sign; whereas, power from an endothermic reaction is positive and energy is provided a optimistic sign. An instance that demonstrates both processes is once a human being drops a book. Once he or she lifts a book, that or she gives potential energy to the book (energy absorbed). However, as soon as the he or she autumn the book, the potential energy converts chin to kinetic energy and comes in the type of sound once it access time the ground (energy released).
When one electron is added to a neutral atom (i.e., an initial electron affinity) energy is released; thus, the first electron affinities are negative. However, an ext energy is forced to include an electron to a negative ion (i.e., second electron affinity) which overwhelms any kind of the relax of power from the electron attachment process and hence, 2nd electron affinities are positive.an initial Electron Affinity (negative energy since energy released):
< ceX (g) + e^- ightarrow X^- (g) label1>second Electron Affinity (positive energy due to the fact that energy necessary is more than gained):
< ceX^- (g) + e^- ightarrow X^2- (g) label2>
First Electron Affinity
Ionization energies are constantly concerned with the development of hopeful ions. Electron affinities are the an unfavorable ion equivalent, and their use is virtually always confined to elements in teams 16 and 17 of the periodic Table. The very first electron affinity is the energy released as soon as 1 mole the gaseous atoms each acquire an electron to kind 1 mole of gaseous -1 ions. The is the energy released (per mole the X) as soon as this readjust happens. An initial electron affinities have negative values. Because that example, the an initial electron affinity that chlorine is -349 kJ mol-1. By convention, the an adverse sign mirrors a relax of energy.
When an electron is added to a steel element, energy is essential to acquire that electron (endothermic reaction). Metals have actually a much less likely opportunity to acquire electrons due to the fact that it is simpler to shed their valance electron and type cations. The is easier to shed their valence electrons because metals" nuclei perform not have actually a strong pull on their valence electrons. Thus, steels are well-known to have actually lower electron affinities.
Example (PageIndex1): team 1 Electron Affinities
This tendency of lower electron six for steels is explained by the team 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
Notice that electron affinity decreases down the group.
When nonmetals gain electrons, the energy readjust is usually an unfavorable because they provide off energy to form an anion (exothermic process); thus, the electron affinity will certainly be negative. Nonmetals have a greater electron affinity than metals because of their atomic structures: first, nonmetals have much more valence electron than steels do, thus it is less complicated for the nonmetals to acquire electrons to fulfill a stable octet and secondly, the valence electron covering is closer come the nucleus, thus it is harder to eliminate an electron and it easier to lure electrons native other elements (especially metals). Thus, nonmetals have actually a higher electron affinity 보다 metals, meaning they are much more likely to acquire electrons 보다 atoms v a lower electron affinity.
Example (PageIndex2): team 17 Electron Affinities
For example, nonmetals favor the aspects in the halogens collection in group 17 have actually a greater electron affinity than the metals. This tendency is explained as below. Notice the an adverse sign for the electron affinity which reflects 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 that electron affinity decreases down the group, however increases up with the period.
As the name suggests, electron affinity is the ability of one atom to accept an electron. Uneven electronegativity, electron affinity is a quantitative measure of the energy adjust 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 because that electrons.
Why is Fluorine an Anomaly?
The incoming electron is going to it is in closer to the cell nucleus in fluorine than in any kind of other of these elements, so you would expect a high value of electron affinity. However, since fluorine is such a little atom, friend are placing the brand-new electron into a an ar of room already crowded through electrons and also there is a far-ranging amount the repulsion. This repulsion lessens the attraction the just arrive electron feels and so lessens the electron affinity. A similar reversal of the expected trend happens between oxygen and sulfur in group 16. The first electron affinity of 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 than chlorine"s.
Comparing group 16 and also Group 17 values
As you might have noticed, the first electron affinity of oxygen ((-142; kJ; mol^-1)) is much less than the of fluorine ((-328; kJ; mol^-1)). Likewise sulfur"s ((-200; kJ; mol^-1)) is much less than chlorine"s ((-349; kJ; mol^-1)). Why? It"s simply that the group 16 facet has 1 much less proton in the nucleus 보다 its next door neighbor in group 17. The quantity of screening is the exact same in both. That way that the network pull indigenous the nucleus is less in team 16 than in team 17, and also so the electron affinities are less.
The reactivity the the aspects in group 17 falls as you go under the team - fluorine is the most reactive and iodine the least. Often in their reactions these elements kind their negative ions. The very first impression the is sometimes offered that the loss in reactivity is due to the fact that the incoming electron is held less strongly together you go under the group and also so the an adverse ion is much less likely to form. That explanation watch reasonable till you include fluorine!
An as whole reaction will be made up of too many of various steps every involving energy changes, and you can not safely shot to define a trend in state of simply one the those steps. Fluorine is much much more reactive than chlorine (despite the reduced electron affinity) since the energy released in other procedures in the reactions much more than makes up because that the reduced amount of energy released as electron affinity.
Second Electron Affinity
You space only ever likely to fulfill this with respect come the group 16 aspects oxygen and also sulfur which both type -2 ions. The 2nd electron affinity is the power required to add an electron to every ion in 1 mole of gas 1- ion to develop 1 mole of gaseous 2- ions. This is an ext easily viewed in prize terms.
< X^- (g) + e^- ightarrow X^-2 (g) label3>
It is the energy needed to carry out this readjust per mole that (X^-).
Why is energy needed to carry out this? You room forcing one electron right into an already an adverse ion. It"s no 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 optimistic sign shows that you need to put in energy to perform this change. The second electron affinity of oxygen is particularly high due to the fact that the electron is being forced into a small, very electron-dense space.
See more: How Long Does It Take To Boil Water In A Microwave, How Long Does It Take To Boil Water