Electronegativity and bonding

AS Level Chemistry · Chemical bonding. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Electronegativity is the power of an atom to attract electrons to itself. It matters when two atoms are bonded, because the more electronegative atom pulls the bonding electrons closer.

Three things control it: the nuclear charge, the atomic radius and the shielding by inner shells and sub-shells. A small atom with a high nuclear charge and little shielding attracts electrons strongly. Fluorine is the most electronegative element.

The difference in electronegativity between two atoms predicts the bond. No difference gives a non-polar covalent bond. A small difference gives a polar covalent bond. A large difference gives an ionic bond, because one atom takes the electrons completely.

Rules to remember

Common mistake

Students say electronegativity falls down a group because the nuclear charge falls. The nuclear charge rises; the greater distance and shielding are what weaken the attraction.

Worked example

Pauling electronegativities: Mg 1.2, H 2.1, C 2.5, Cl 3.0, O 3.5. Which pair is most likely to bond ionically: Mg and O, C and Cl, or H and Cl?

  1. Find each difference: Mg and O = 3.5 − 1.2 = 2.3.
  2. C and Cl = 3.0 − 2.5 = 0.5; H and Cl = 3.0 − 2.1 = 0.9.
  3. The largest difference means the electrons are transferred, not shared.

Answer: Mg and O (difference 2.3) form an ionic bond; the other two pairs form polar covalent bonds.

Practice questions

Try each one, then open the answer.

1. Which element has the highest electronegativity?

  1. A
    caesium
  2. B
    fluorine
  3. C
    oxygen
  4. D
    chlorine
Show answer

Answer: B. Electronegativity increases across a period and up a group, so fluorine (top right, excluding noble gases) is highest at 4.0. Caesium (bottom left) is among the lowest.

2. Why does electronegativity increase across Period 3 from sodium to chlorine?

  1. A
    nuclear charge increases and atomic radius decreases, while shielding stays about the same
  2. B
    the number of inner shells decreases across the period
  3. C
    the atoms have more electrons, which increases shielding
  4. D
    the atoms get larger, so there is more room for bonding electrons
Show answer

Answer: A. Across the period protons are added but electrons enter the same shell, so shielding hardly changes. The greater nuclear charge and smaller radius let the nucleus attract bonding electrons more strongly.

3. Which list shows the elements in order of increasing electronegativity?

  1. A
    O, N, C
  2. B
    Na, Si, O
  3. C
    Cl, S, P
  4. D
    F, Cl, Br
Show answer

Answer: B. Electronegativity increases across a period and decreases down a group. Na < Si < O follows this; the other three lists are in decreasing order.

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