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 Revealed

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)

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.

Describe the three allotropes of carbon — diamond, graphite, and buckminsterfullerene — with their structures and one key property of each. Why do they have different physical properties despite being made of the same element?