Chapter 4 · Question 2

Draw the electron dot structures for the following molecules and indicate the type of covalent bond (single, double, or triple) formed in each: (a) hydrogen molecule ( $\text{H}_2$ ), (b) oxygen molecule ( $\text{O}_2$ ), (c) nitrogen molecule ( $\text{N}_2$ ), and (d) carbon dioxide ( $\text{CO}_2$ ).

Answer

Direct Answer

(a)

\text{H}_2

: Two H atoms share one electron pair → single covalent bond (

\text{H} \!-\! \text{H}

). (b)

\text{O}_2

: Each O atom (atomic number 8, 6 valence electrons) shares two electron pairs with the other → double covalent bond (

\text{O} \!=\! \text{O}

). (c)

\text{N}_2

: Each N atom (atomic number 7, 5 valence electrons) shares three electron pairs → triple covalent bond (

\text{N} \!\equiv\! \text{N}

). (d)

\text{CO}_2

: Carbon shares two electron pairs with each of the two oxygen atoms → two double bonds (

\text{O} \!=\! \text{C} \!=\! \text{O}

).

Simple Explanation

Hydrogen (

\text{H}_2

) has a single bond — a simple handshake sharing one pair of electrons. Oxygen (

\text{O}_2

) needs to share two pairs to fill its outer shell, so it forms a double bond — like holding both hands. Nitrogen (

\text{N}_2

) is even stronger, sharing three pairs in a triple bond. Carbon dioxide has carbon in the middle making double bonds to two oxygen atoms, one on each side:

\text{O} \!=\! \text{C} \!=\! \text{O}

.

Exam-Ready Structure

Electron dot structures (Lewis structures) show how valence electrons are shared in covalent bonding: 1. Single covalent bond —

\text{H}_2

molecule: Each hydrogen atom (atomic number 1) has 1 electron in its K shell and needs 1 more electron to complete its duplet. Two hydrogen atoms share their single electrons to form one shared pair:

\text{H} \!\bullet\!\bullet\! \text{H}

, represented as

\text{H} \!-\! \text{H}

. This is a single covalent bond (one shared electron pair). 2. Double covalent bond —

\text{O}_2

molecule: Oxygen (atomic number 8) has electronic configuration K(2), L(6) — 6 valence electrons. It needs 2 more electrons to complete its octet. Two oxygen atoms share two electron pairs (4 electrons total), forming a double bond:

\ddot{\text{O}} \!=\! \ddot{\text{O}}

. This is represented as

\text{O} \!=\! \text{O}

. 3. Triple covalent bond —

\text{N}_2

molecule: Nitrogen (atomic number 7) has configuration K(2), L(5) — 5 valence electrons. It needs 3 more electrons for an octet. Two nitrogen atoms share three electron pairs (6 electrons total), forming a triple bond:

\ddot{\text{N}} \!\equiv\! \ddot{\text{N}}

, represented as

\text{N} \!\equiv\! \text{N}

. 4. Carbon dioxide —

\text{CO}_2

: Carbon (4 valence electrons) shares two electron pairs with each oxygen atom (6 valence electrons each) to form two carbon–oxygen double bonds:

\text{O} \!=\! \text{C} \!=\! \text{O}

. In the electron dot structure, carbon is at the centre with two double bonds to the two oxygen atoms.

Key Points

Single bond: one shared pair of electrons (e.g., $\text{H}_2$ , $\text{Cl}_2$ , $\text{CH}_4$ )
Double bond: two shared pairs of electrons (e.g., $\text{O}_2$ , $\text{CO}_2$ , $\text{C}_2\text{H}_4$ )
Triple bond: three shared pairs of electrons (e.g., $\text{N}_2$ , $\text{C}_2\text{H}_2$ )
$\text{H}_2$ : $\text{H} \!-\! \text{H}$ ; $\text{O}_2$ : $\text{O} \!=\! \text{O}$ ; $\text{N}_2$ : $\text{N} \!\equiv\! \text{N}$
$\text{CO}_2$ : $\text{O} \!=\! \text{C} \!=\! \text{O}$ (two carbon–oxygen double bonds)

Answer

Direct Answer

Simple Explanation

Exam-Ready Structure

Key Points

Related Questions

Why does carbon not form ionic bonds like sodium or chlorine? Explain why carbon overcomes this limitation by forming covalent bonds instead.

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