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Electron Affinity

Ionization energy: The energy required to

remove an electron from a gaseous atom orion, where the atom or ion is assumed to be in

its ground state. (p91, 3, 5)

Electron affinity (EA): The energy changeassociated with the addition of an electron to

a gaseous atom. (p94, 2)

X(g) + e

X

(g)

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Electron Affinity

M

M

+ e

,I

0always positive

M + e

M

,EA = ??generally negative

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Definition of Electron Affinity

We define electron affinity as achange in

energy, which means that if the addition ofthe electron results in a lower energy, then the

corresponding value for electron affinity will

carry a negative sign. (p94, 4)

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EA Values for the First 20 Elements

p94, Figure 2.34

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Trends in Electron Affinities

The more negative the energy, the greater the

quantity of energy released. (p94, r3)

Although electron affinities generally

become more negative from left to right

across a period, there are several exceptionsto this rule in each period. (p94, r3)

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Electron Affinity: Anion Stable or Not

H (1s1) + e

H

(1s2) + 72.8 kJ / mol

EA = 72.8 kJ / mol stable H

He (1s2) + e

+ energyHe

(1s2 2s1)

EA > 0 unstable He

(p195, 3)

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Electron Affinity

The value of an elements electron affinity is

due to an interplay of several offsettingfactors.

Attraction between the additional electron

and the nucleus favors a negative EA, butthe increase in electron-electron repulsions

that results from addition of the extra

electron favors a positive EA.

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Trends in EAs for the Same Period

Electron affinities generally become more

negative from left to right across a period.(p94, r3)

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8A

Ar (...... 3s2

3p6) + e

Ar

(...... 3s2

3p6

4s1)

p2A

Mg (...... 3s2) + e

Mg

(...... 3s2

3p1)

p5A

N (...... 2p3) + e

N

(...... 2p4)

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Second Electron Affinity

O

(g) + e

O2

(g)

A second electron cannotbe added to an

oxygen atom. (p95, 1)

No element has a negative second electronaffinity. X

(g) + e

X2

(g) is

unfavorable for every element.

Why??

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Trends in EAs for the Same Group

p95, Table 2.7

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Electron Affinity for Fluorine

The energy released when an electron is

added to fluorine is smaller than might beexpected. (p95, 2)

This smaller energy release has been

attributed to the small size of the 2p orbitals.

Because the electrons must be very close

together in these orbitals, there are unusually

large electron-electron repulsions. (p95, 2)