M Prakash Institute Chemistry STD-VIII
L14- Periodic Properties and Trends of elements
Part-2
Electron Affinity (EA):
It is defined as change in energy (electron volt) of a neutral gaseous atom when an electron is added
to it to form a negative ion. When an electron is added to a neutral atom , energy is released; (i.e.
the first electron affinity). Thus the first electron affinities are negative. However, more energy is
required to add an electron to a negative ion (i.e. second electron affinity) and hence second
electron affinities are positive.
According to the new convention, electron gain enthalpy is the energy associated when one mole of
electron is added to one mole of an element in its isolated gaseous state (ground state).
A( g) + e A− ; ΔHeg = ± x kJ mol−1
According to the old convention, for an element A,
A + e → A− ΔHEA = ( − )x kJ mol−1
The negative sign indicates that energy is released. For those cases in which energy is absorbed, the
EA value is considered as zero.
★ΔHeg may be negative or positive depending upon the circumstances and electron affinity (EA)
is defined as EA = − ΔHeg ( at absolute zero temperature )
★We consider |ΔHeg|≈|EA| for all practical temperatures.
The unit of electron affinity is eV atom −1 or kJmol−1.
★ Unit to express Electron Affinity : 1 ev/atom = 1.6×10-19 Joule, 96 KJ/mole = 1 ev/atom.
Note:
(i) EA1 > EA2, that is for an element, the electron affinity for the first electron is greater than that
of the second electron.
(ii) Successive electron affinity always decreases. For example,
(iii) Heg for all elements is positive.
2
+ e, EA of A
(iv) The electron affinity and ionization energy are related as: A A-
-e; IE of A-
It is clear that
∣ EA of A | = | IE of A− ∣
Accordingly,
∣ EA of A+ | = | IE of A ∣
∣ EA of A2+ | = | IE of A+ ∣
… and so on






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M Prakash Institute Chemistry STD-VIII
Periodic trends of electron affinity:
Variation of electron affinities of the elements with atomic number is shown below. Unfortunately,
the electron affinities of the lanthanides and of a few other elements have not yet been measured.
For representative ( s - and p - block) elements:
a. In a group: There is decrease of electron affinity in general but for p-block elements only the EA1
of second period element is less than EA1 of third period element.
Second period Third period
B < Al
C < Si
N < P
O < S
F < Cl
The third period elements have larger size and vacant 3d orbitals, which can accommodate
incoming electron easily.
b. In a period: The electron affinity increases from left to right as Zeff increases (except inert gases)
in a period. Some exceptions to the trend are:
(i)
EA of alkali metals > EA of alkaline earth metals, depicted as follows:
Alkali metals n s 1 n s 2
Fully-filled electronic configuration is achieved ΔHeg is (-)ve
Alkaline earth metals n s 2 n s 2 np 1
Fully-filled electronic configuration is disturbed ΔHeg is (+)ve
(ii)
EA of C > EA of N
Similarly,
EA of Si > EA of P
EA of Ge > EA of As
EA of Sn > EA of Sb
EA of Pb < EA of Bi


















M Prakash Institute Chemistry STD-VIII
On addition of an electron, the elements C, Si, Ge and Sn achieve half-filled electronic
configuration while in case of N, P, As and Sb do not have the tendency to gain electron and thus
lose their half-filled configuration.
🔘 Electron affinity depends upon a number of factors, that include:
1. It decreases when radius increases.
2. It increases when Zeff increases
3. It decreases when σ increases.
4. EA of half-filled and full-filled electronic configuration is less.
5. EA order for addition of electron in different orbitals having the same principal quantum
number is s > p > d > f
🔘 Note: Some general observations are listed as follows.
1. ΔHeg is negative for most of the elements.
1
2. ΔHeg is positive for:
1
a. inert gas elements.
b. alkaline earth metals ( Be and Mg only).
c. for N only
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Exercise:
1. Electron affinity depends on
(a) Atomic size
(b) Nuclear charge
(c) Atomic number
(d) Atomic size and nuclear charge both
2. Increasing order of electron affinity is
(a) N < O < Cl < Al
(b) O < N < Al < Cl
(c) Al < NO < Cl
(d) Cl < N < O < Al
3. Which one has maximum electron affinity?
(a) N
(b) Be
(c) B
(d) Cl
4. The electron affinity for the inert gases is
(a) Zero
(b) High
(c) Negative
(d) Positive










M Prakash Institute Chemistry STD-VIII
5. The electron affinities of halogens in KJ/mol are are F : 322, Cl: 349, Br : 324, I: 295.
The higher value for Cl as compared to that of F is due to
(a) Weaker electron-electron repulsion in Cl
(b) Higher atomic radius of F
(c) Smaller electronegativity of F
(d) More vacant P - subshell in Cl
6. Electron affinity is the
(a) Energy absorbed when an electron is added to an isolated atom in the gaseous state
(b) Energy released when an electron is added to an isolated atom in the gaseous state
(c) Energy required to take out an electron from an isolated gaseous atom
(d) Power of an atom to attract an electron to itself
7. The electron affinity values for the halogens show the following trend
(a) F < Cl > Br > I
(b) F < Cl < Br < I
(c) F > Cl > Br > I
(d) F < Cl > Br < I
8. Which element has maximum electron affinity?
(a) Na
(b) S
(c) Mg
(d) Al
9. Which of the following has the least electron affinity in Kjmol1
(a) Oxygen
(b) Carbon
(c) Nitrogen
(d) Boron
10. Fluorine has low electron affinity than chlorine because of
(a) Smaller radius of fluorine, high density
(b) Smaller radius of chlorine, high density
(c) Bigger radius of fluorine, less density
(d) Smaller radius of chlorine, less density
11. For electron affinity of halogens which of the following is correct
(a) Br > F
(b) F > Cl
(c) Br < Cl
(d) F > I
12. Ionic compounds are formed most easily with
(a) Low electron affinity, high ionisation energy
(b) High electron affinity, low ionisation energy
(c) Low electron affinity, low ionisation energy
(d) High electron affinity, high ionisation energy
13. In comparison with alkali metals, the E.A. of halogens is
(a) Very high
(b) Very low
(c) Nearly same
(d) Exactly same












M Prakash Institute Chemistry STD-VIII
14. The electron affinity of
(a) Carbon is greater than oxygen
(b) Sulphur is less than oxygen
(c) Iodine is greater than bromine
(d) Bromine is less than chlorine
15. The amount of energy which is released due to addition of extra electron to the outermost
orbit of gaseous atom is called
(a) Electron capacity
(b) Electron affinity
(c) Ionisation potential
(d) Electronegativity
16. Which of the following species has the highest electron affinity
(a) F
(b) O
(c) O−
(d) Na+
17. The electron affinity values (in Kjmol−1 ) of three halogens X, Y and Z are respectively
349,333 and 325 k . Then X, Y and Z are respectively
(a) F2, Cl2 and Br2
(b) Cl2, F2 and Br2
(c) Cl2, Br2 and F2
(d) Br2, Cl2 and F2
18. Electron affinity is the lowest for
(a) Nitrogen
(b) Carbon
(c) Oxygen
(d) Sulphur
19. Which one of the elements has the maximum electron affinity?
(a) F
(b) Cl
(c) Br
(d) I
20. Which of the following pairs show reverse properties on moving along a period from left to
right and from top to down in a group
(a) Nuclear charge and electron affinity
(b) Ionisation energy and electron affinity
(c) Atomic radius and electron affinity
(d) None of these
21. Which of the following properties show gradual decrease with increase in atomic number
across a period in the periodic table
(a) Electron affinity
(b) Ionization potential
(c) Electronegativity
(d) Size of atom















M Prakash Institute Chemistry STD-VIII
22. Order of electron affinity of F, Cl, Br and I is
(a) F < Cl > Br > I
(b) F > Cl > Br > I
(c) F < Cl < Br < I
(d) F > Cl < Br > I
23. Which one of the following arrangements represents the correct order of electron gain
enthalpy (with negative sign) of the given atomic species.
(a) Cl < F < S < O
(b) O < S < F < Cl
(c) S < O < Cl < F
(d) F < Cl < O < S









