Electron affinity: why the second electron costs energy

The problem

An oxygen atom happily takes an electron: energy comes out, 141 kJ for every mole. So it should happily take a second one too, right? It doesn't. The second electron needs 780 kJ mol⁻¹ pushed in.

Think of a crowded rickshaw. With one empty seat, the passenger is welcome. Once O⁻ is already carrying a negative charge, the next electron gets pushed away by it, and it takes energy to force it in.

Electron affinity is the energy released or absorbed when an electron is added to a gaseous atom to form a negative ion. Released energy is written as negative; absorbed energy as positive.

Key terms

First electron affinity

Electronegative elements such as the halogens release energy when they take their first electron, so the value is negative.

Second electron affinity

Adding an electron to a uninegative ion means pushing it against the negative charge already there. Energy is absorbed, so the value is positive.

Metallic character

Metals tend to lose electrons and form positive ions. They conduct heat and electricity, and form basic oxides. Metallic character increases down a group and decreases across a period.

Non-metallic character

Non-metals gain electrons and form negative ions. They conduct poorly and form acidic oxides. Fluorine, at the top of the halogens, is the most non-metallic element.

Short questions with model answers

1. Q1. Why is the second value of electron affinity usually shown with a positive sign? Use oxygen.

   • O(g) + e⁻ → O⁻(g) E.A₁ = −141 kJ mol⁻¹
   • O⁻(g) + e⁻ → O²⁻(g) E.A₂ = +780 kJ mol⁻¹
   • −141 + 780 = +639 kJ mol⁻¹

   The second electron is repelled by the negative ion, so energy is absorbed: +780 kJ mol⁻¹.

2. Q2. Fluorine releases 337 kJ mol⁻¹ on gaining an electron; oxygen releases 141. Both are in period 2. Why does fluorine release more?

   • 337 − 141 = 196 kJ mol⁻¹
   • Fluorine: more protons, smaller atom, one vacancy.

   Its greater nuclear charge and smaller size pull the new electron in more strongly.

3. Q3. Why does metallic character increase from top to bottom in a group of metals?

   • Down a group, atoms get bigger.
   • The outer electron is easier to remove.

   Bigger atoms lose electrons more easily, and losing electrons is what makes an element metallic.

Common mistakes

• ✗ Reading a negative electron affinity as “the atom does not want the electron”.

  ✓ Negative means energy is released: the atom welcomes the electron. Positive means energy must be absorbed.

• ✗ Copying “SO₃ + H₂O → 2H₂SO₄” as printed.

  ✓ Count sulphur: one on the left, two on the right. The balanced equation is SO₃ + H₂O → H₂SO₄.

• ✗ Writing that metallic character decreases down a group.

  ✓ It increases down a group. It is non-metallic character that decreases: nitrogen and oxygen are pure non-metals, while bismuth and polonium are fairly metallic.

MCQs

1. 1. When a second electron is added to a uninegative ion, the incoming electron is ______ by the negative charge already present.

   1. (a) attracted
   2. (b) repelled
   3. (c) neutralised
   4. (d) ignored
   Show answer

   (b) Repelled: like charges push apart, so energy is absorbed and the second value is positive.

2. 2. The electron affinity of fluorine given in the book is:

   1. (a) −141 kJ mol⁻¹
   2. (b) +780 kJ mol⁻¹
   3. (c) −337 kJ mol⁻¹
   4. (d) +337 kJ mol⁻¹
   Show answer

   (c) −337 kJ mol⁻¹: energy is released as fluorine gains its first electron. −141 is oxygen's first value.

3. 3. Which is the most non-metallic element of the periodic table?

   1. (a) oxygen
   2. (b) chlorine
   3. (c) fluorine
   4. (d) nitrogen
   Show answer

   (c) Fluorine, the top member of the halogens. Non-metallic character decreases down a group.

4. 4. Which oxide gives a base when dissolved in water?

   1. (a) SO₃
   2. (b) Na₂O
   3. (c) CO₂
   4. (d) P₄O₁₀
   Show answer

   (b) Na₂O is a metal oxide: Na₂O + H₂O → 2NaOH. The others are non-metal oxides, which give acids.

5. 5. Across a period from left to right, metallic character:

   1. (a) increases
   2. (b) decreases
   3. (c) stays the same
   4. (d) first falls, then rises
   Show answer

   (b) It decreases: atoms get smaller and hold their electrons more tightly. The halogens are the least metallic.

Quick revision

• First electron affinity is usually negative (energy released); the second is positive, because the negative ion repels the new electron.
• Electron affinity generally increases across a period and decreases down a group; small atoms with one or two vacancies have large values.
• Metals lose electrons and form basic oxides; non-metals gain electrons and form acidic oxides. Metallic character rises down a group and falls across a period.