Periodic Trends — Electron Affinity

1A 2A 3A 4A 5A 6A 7A 8A
(1) (2) (13) (14) (15) (16) (17) (18)
3B 4B 5B 6B 7B 8B 1B 2B
(3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
1

H

-73

He

0

2

Li

-60

Be

18

B

-27

C

-122

N

7

O

-141

F

-328

Ne

29

3

Na

-53

Mg

21

Al

-44

Si

-134

P

-44

S

-200

Cl

-349

Ar

35

4

K

-48

Ca

-2

Sc

xx

Ti

xx

V

xx

Cr

xx

Mn

xx

Fe

xx

Co

xx

Ni

xx

Cu

xx

Zn

xx

Ga

-30

Ge

-116

As

-78

Se

-195

Br

-325

Kr

39

5

Rb

-47

Sr

-5

Y

xx

Zr

xx

Nb

xx

Mo

xx

Tc

xx

Ru

xx

Rh

xx

Pd

xx

Ag

xx

Cd

xx

In

-30

Sn

-116

Sb

-101

Te

-190

I

-295

Xe

41

6

Cs

-46

Ba

46

La

xx

 

Hf

xx

Ta

xx

W

xx

Re

xx

Os

xx

Ir

xx

Pt

xx

Au

xx

Hg

xx

Tl

-20

Pb

-35

Bi

-91

Po

-183

At

-270

Rn

41

7

Fr

xx

Ra

xx

Ac

xx

 

Rf

xx

Db

xx

Sg

xx

Bh

xx

Hs

xx

Mt

xx

Ds

xx

Rg

xx

Uub

xx

Uuq

xx

6

 

Ce

xx

Pr

xx

Nd

xx

Pm

xx

Sm

xx

Eu

xx

Gd

xx

Tb

xx

Dy

xx

Ho

xx

Er

xx

Tm

xx

Yb

xx

Lu

xx

7

 

Th

xx

Pa

xx

U

xx

Np

xx

Pu

xx

Am

xx

Cm

xx

Bk

xx

Cf

xx

Es

xx

Fm

xx

Md

xx

No

xx

Lr

xx

Electron Affinities reported in unites of kilojoules per mole (kJ/mol).

Data taken from John Emsley, The Elements, 3rd edition.  Oxford:  Clarendon Press, 1998.

 

The electron affinity of an element is the energy change which accompanies the addition of an electron to an atom in the gas phase to produce a negatively charged anion:

X(g)  +  e-    X-(g)

Electron affinities are usually negative values, since energy is usually released (an exothermic energy change) when an electron is added to a neutral atom.  If the resulting anion is stable, the value for the electron affinity will be negative; the more stable the anion is, the larger the negative number will be.  If the resulting anion is unstable, the value for the electron affinity will be positive.

Electron affinity generally increases from bottom to top within a group (that is, it goes to larger negative numbers), and increases from left to right within a period.

The halogens in Group 7A all have large negative electron affinities, since they are only one electron away from having a noble gas configuration, they easily accept another electron to generate stable halide anions.  The noble gases already have a complete set of electrons, and an additional electron must go into the next highest shell, which will cost energy to start populating.

The trends for electron affinity are not as smooth as those for atomic radius, ionization energy, and electronegativity, as can be seen on the following graphs.