Chemistry 9701/11 — May/June 2017

Background Concept

A covalent bond is formed when two atoms share a pair of electrons. A single bond therefore contains one shared pair, i.e. 2 bonding electrons. A double bond contains two shared pairs, i.e. 4 bonding electrons. Bonding electrons are the electrons holding atoms together; they exclude lone (non-bonding) pairs. To count the bonding electrons in a molecule, first know its structure: how many single and multiple bonds it contains, then multiply each bond by its electron count.

Understanding the Question

The question asks which of four molecules contains exactly six bonding electrons in total. This is a counting task: for each molecule you need to identify its covalent bonds and add up the electrons in those bonds. The answer is not necessarily the molecule with the most or fewest atoms; you must check every option.

Approach

Draw or visualise the Lewis (dot-and-cross) structure of each molecule. Count the number of single and double bonds. Convert bonds to electrons: single bond = 2; double bond = 4. Sum the electrons for each molecule and compare with 6.

Step-by-Step Reasoning

1. $\text{C}_2\text{H}_4$ (ethene): the carbon atoms are joined by a double bond, and each carbon carries two hydrogen atoms. There is one $\text{C}=\text{C}$ double bond (4 electrons) and four $\text{C}-\text{H}$ single bonds ($4 \times 2 = 8$ electrons). Total = $4 + 8 = 12$ bonding electrons. This is not 6.

2. $\text{H}_2\text{S}$: sulfur forms two single bonds to hydrogen atoms. There are two $\text{S}-\text{H}$ bonds, giving $2 \times 2 = 4$ bonding electrons. Not 6.

3. $\text{NCl}_3$: nitrogen forms three single bonds to three chlorine atoms. There are three $\text{N}-\text{Cl}$ bonds, giving $3 \times 2 = 6$ bonding electrons. The nitrogen atom also has one lone pair, but that is not a bonding pair and is not counted.

4. $\text{SF}_6$: sulfur forms six single bonds to six fluorine atoms. There are six $\text{S}-\text{F}$ bonds, giving $6 \times 2 = 12$ bonding electrons. Not 6.

Only option C, $\text{NCl}_3$, contains six bonding electrons.

Key Takeaways

• A single bond always contributes 2 bonding electrons; a double bond contributes 4; a triple bond contributes 6.
• Lone pairs are not bonding electrons.
• To answer such questions, draw the structure first and count the bonds, not the total valence electrons.

Common Mistakes

• Counting a double bond as 2 electrons rather than 4. This would make ethene appear to have too few electrons.
• Including lone pairs, such as nitrogen's lone pair in $\text{NCl}_3$, in the bonding-electron count.
• Assuming that more atoms means more bonding electrons without checking the actual bonds; $\text{SF}_6$ has six bonds and hence 12 bonding electrons, not 6.

Things to Be Careful About

• The phrase "bonding electrons" means electrons shared in bonds, not valence electrons or the total number of electrons in the molecule.
• Count the bonds in the whole molecule, not just around one atom.
• Use known structures (or Lewis structures) rather than just the chemical formula, because isomers and multiple bonds change the count.