Electron affinity and electronegativity

Taking an electron, holding a pair

Electron affinity is the energy change when an electron is added to the outer shell of an isolated gaseous atom. When fluorine takes an electron, 328 kJ mol⁻¹ of energy is released, so ΔH is −328 kJ mol⁻¹.

Electron affinity also depends on size: a smaller atom pulls the incoming electron more strongly. So the book says it decreases down a group. Table 8.8 shows the group 17 values: F −328, Cl −349, Br −324.6 and I −295.2 kJ mol⁻¹. Notice chlorine releases a little more than fluorine; from chlorine down, the values fall.

Electronegativity is different: it is how strongly an atom pulls the shared pair of electrons towards itself in a bond. It is measured on the Pauling scale, with no units. Across period 2 it rises from Li 1.0 to F 4.0; down group 17 it falls from F 4.0 to I 2.7.

So the most electronegative atoms, F, O, N and Cl, sit at the top right of the table, and the most electropositive elements sit at the bottom left.

Electron affinity is about a lone atom gaining an electron; electronegativity is about a bonded atom pulling a shared pair.

Key terms

Electron affinity

How readily ions like Cl⁻ form.

Electronegativity

Predicting bond type.

Pauling scale

Comparing atoms.

Electropositive

Bottom-left metals.

Short questions with model answers

1. Q1. Choose the element with the lowest ionization energy and the one with the highest electron affinity: Li, K, O, F, Cl. (Exercise 4, p. 115)

   • Lowest ionization energy
   • Highest electron affinity

   Lowest ionization energy: K. Highest electron affinity: Cl.

2. Q2. Why are the most electronegative elements in groups 16 and 17? (Short question vii, p. 117)

   • Across a period
   • Near a full shell

   They are small atoms with a large nuclear charge that are close to a full octet, so they pull shared electrons strongly.

Common mistakes

• ✗ Electron affinity and electronegativity are the same thing.

  ✓ EA is an energy for a lone gaseous atom; EN is a pull on a shared pair in a bond (p. 113).

• ✗ Fluorine has the highest electron affinity in group 17.

  ✓ Table 8.8 shows Cl (−349) above F (−328); F has the highest electronegativity.

• ✗ Electronegativity is measured in kJ mol⁻¹.

  ✓ It is a unitless number on the Pauling scale (Table 8.9).

MCQs

1. 1. Electronegativity is the pull of an atom on:

   1. (a) Its own nucleus
   2. (b) A shared electron pair in a bond
   3. (c) Protons
   4. (d) Neutrons
   Show answer

   (b) p. 113.

2. 2. Which has the highest electronegativity?

   1. (a) Li
   2. (b) O
   3. (c) F
   4. (d) Cl
   Show answer

   (c) F: 4.0 (Tables 8.9, 8.10).

3. 3. In Table 8.8, which halogen releases the most energy on gaining an electron?

   1. (a) F
   2. (b) Cl
   3. (c) Br
   4. (d) I
   Show answer

   (b) Cl: −349 kJ mol⁻¹.

4. 4. An element with low ionization energy and low electron affinity likely belongs to: (Exercise x, p. 117)

   1. (a) Group 1
   2. (b) Group 13
   3. (c) Group 16
   4. (d) Group 17
   Show answer

   (a) Big atoms at the left lose electrons easily and attract them weakly.

5. 5. The most electropositive elements are found at the:

   1. (a) Top right
   2. (b) Bottom left
   3. (c) Top left
   4. (d) Bottom right
   Show answer

   (b) p. 113.

Quick revision

• Electron affinity is the energy change when a gaseous atom gains an electron.
• Electronegativity is an atom's pull on a shared pair, on the unitless Pauling scale.
• Both generally rise across a period and fall down a group; Cl has the highest electron affinity in Table 8.8.
• F, O, N and Cl are the most electronegative atoms.