Unit 2 · Topic 2.5 Beta

Lewis Diagrams

A Lewis diagram shows a molecule's or ion's valence electrons as bonding pairs (lines) and lone pairs (dots).

Practice 1: Models and RepresentationsPractice 3: Representing Data and Phenomena

Question set for this topic

Part 1 · Hook

Why this matters

A chemist looking at a new formula wants to know, before doing anything else, which atoms are joined, by how many shared pairs, and where the spare electrons sit. A Lewis diagram answers all three in a sketch you can draw in a minute, as long as you count carefully.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. How many valence electrons does a sulfur atom (group 16) have?

  1. 6
  2. 16
  3. 2
  4. 8
Show the answer

For groups 13-18, the number of valence electrons is the group number minus 10: S has 6 (3s² 3p⁴).

  • Correct: 6:
  • 16:
  • 2:
  • 8:

2. A double bond between two atoms is

  1. two shared pairs, four electrons
  2. two shared electrons
  3. one shared pair and one lone pair
  4. two atoms each with an octet
Show the answer

Each shared pair is two electrons; a double bond shares two pairs.

  • Correct: two shared pairs, four electrons:
  • two shared electrons:
  • one shared pair and one lone pair:
  • two atoms each with an octet:

Part 4 · See it

See it first

Four Lewis diagrams. H2O: O bonded to two H atoms with two lone pairs on O, 8 valence electrons. NH3: N bonded to three H atoms with one lone pair on N, 8 valence electrons. CO2: C double-bonded to two O atoms, each O with two lone pairs, 16 valence electrons. NH4+: N bonded to four H atoms with no lone pairs, in brackets with a plus charge, 8 valence electrons.
Lewis diagrams of H₂O, NH₃, CO₂ and NH₄⁺. Each line is a bonding pair; each pair of dots is a lone pair. The ammonium ion is drawn in brackets with its charge. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Only valence electrons take part in bondinga Lewis diagram shows only valence electrons, counted from each atom's group and the ion's charge
  2. Bonding pairs are shared by two atomseach line counts as two electrons for both atoms it joins
  3. Eight valence electrons fill the valence s and p subshellsmost period 2 atoms end with an octet; H ends with two
  4. The central atom may be short of eight after outer atoms are completeouter lone pairs become double or triple bonds
  5. B has only three valence electrons, odd totals leave one unpaired, and period 3 atoms can hold more than eightincomplete octets, free radicals and expanded octets are the exceptions

Part 6 · Key ideas

Key ideas

  • A Lewis diagram shows every valence electron: lines for bonding pairs, dot pairs for lone pairs.
  • Count first: group valence electrons for every atom, plus one per negative charge, minus one per positive charge.
  • Octet rule: most period 2 atoms end with eight electrons; H with two.
  • Put the central atom in the middle (never H), complete the outer atoms, put leftovers on the center, then make multiple bonds if the center is short.
  • Exceptions: incomplete octet (B), odd-electron species (NO), expanded octet from period 3 on (SF₆).

Part 7 · Misconception

A common mistake

The wrong idea: For a positive ion such as NH₄⁺, add an electron for the charge.

What actually happens: A positive charge means the species has lost electrons, so subtract one per + charge: NH₄⁺ has 5 + 4 − 1 = 8 electrons. Add electrons only for negative charges.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Model

Four student-drawn Lewis diagrams

A student drew these Lewis diagrams. Lines are bonding pairs; dot pairs are lone pairs.

HCNDiagram 1HCNCOHHDiagram 2H2CONNDiagram 3N2BFFFDiagram 4BF3

1. Which diagram shows the wrong total number of valence electrons for its formula?

  1. Diagram 1
  2. Diagram 3
  3. Diagram 2
  4. Diagram 4
Show the answer

N₂ has 2 × 5 = 10 valence electrons. Diagram 3 shows a double bond (4) and four lone pairs (8), which is 12. The correct diagram is a triple bond with one lone pair on each N: 6 + 4 = 10.

  • Diagram 1: HCN has 1 + 4 + 5 = 10: one single bond (2), a triple bond (6) and one lone pair (2). It is correct.
  • Correct: Diagram 3: Right: 12 electrons drawn for a 10-electron molecule.
  • Diagram 2: H₂CO has 2 + 4 + 6 = 12: two C–H bonds (4), a C=O bond (4) and two lone pairs (4). It is correct.
  • Diagram 4: BF₃ has 3 + 21 = 24: three bonds (6) and nine lone pairs (18). It is correct, with an incomplete octet on B.

2. In Diagram 4, how many electrons surround the boron atom, and what does this show?

  1. Ten, an expanded octet, because B is in period 2
  2. Eight, because each atom in a molecule has an octet
  3. Six, so the diagram has to be wrong
  4. Six, an incomplete octet, which boron can have
Show the answer

B has three bonds and no lone pairs: 3 × 2 = 6 electrons. Boron has only three valence electrons, so in BF₃ it ends with six, an incomplete octet. This is one of the known exceptions to the octet rule.

  • Ten, an expanded octet, because B is in period 2: Period 2 atoms such as B cannot have more than eight electrons; expanded octets start in period 3.
  • Eight, because each atom in a molecule has an octet: Not every atom has an octet: H has 2, and B commonly has 6.
  • Six, so the diagram has to be wrong: Six around B is allowed; the diagram uses exactly 24 electrons and each F has an octet.
  • Correct: Six, an incomplete octet, which boron can have: Right: three bonding pairs, six electrons.

3. How does the C atom in Diagram 1 (HCN) reach an octet?

  1. A single bond to H and a triple bond to N: 2 + 6 = 8
  2. It has two lone pairs plus two single bonds to H and N
  3. It has one lone pair, a single bond and a double bond
  4. It does not reach an octet; carbon has an incomplete octet in HCN
Show the answer

Count every bonding electron around C: H–C contributes 2 and C≡N contributes 6. 2 + 6 = 8, an octet, with no lone pairs on C.

  • Correct: A single bond to H and a triple bond to N: 2 + 6 = 8: Right: two bonding electrons plus six.
  • It has two lone pairs plus two single bonds to H and N: The diagram shows no lone pairs on C and a triple bond to N.
  • It has one lone pair, a single bond and a double bond: The C–N bond is triple, not double, and C has no lone pair.
  • It does not reach an octet; carbon has an incomplete octet in HCN: C has eight electrons around it; incomplete octets are typical of B, not C.

4. What is the correct Lewis diagram for carbon dioxide, CO₂?

  1. O≡C–O, with one lone pair on the triple-bonded O and two on the other
  2. O–C–O, with three lone pairs on each O and no lone pair on C
  3. O–C–O, with three lone pairs on each O and two on C
  4. O=C=O, with two lone pairs on each O
Show the answer

CO₂ has 4 + 2 × 6 = 16 electrons. Two double bonds (8) and two lone pairs on each O (8) use all 16, and every atom has an octet.

  • O≡C–O, with one lone pair on the triple-bonded O and two on the other: That uses 8 + 2 + 4 = 14 electrons and leaves the single-bonded O short of an octet.
  • O–C–O, with three lone pairs on each O and no lone pair on C: That uses 4 + 12 = 16 electrons but leaves C with only four electrons; lone pairs on O must become double bonds.
  • O–C–O, with three lone pairs on each O and two on C: That uses 4 + 12 + 4 = 20 electrons, four more than CO₂ has.
  • Correct: O=C=O, with two lone pairs on each O: Right: 16 electrons, octets everywhere.

5. A student drew the ammonium ion, NH₄⁺, as N bonded to four H atoms with one lone pair on N. What is wrong?

  1. It has too few electrons; N needs two lone pairs
  2. It shows 10 electrons; NH₄⁺ has 8, so no lone pair
  3. Nothing is wrong; N keeps its one lone pair
  4. H atoms need lone pairs to complete their octets
Show the answer

NH₄⁺ has 5 + 4 − 1 = 8 electrons. Four N–H bonds use all 8, so there is no lone pair. The student added an electron for the charge instead of removing one.

  • It has too few electrons; N needs two lone pairs: Two lone pairs would make the count even larger, 12 electrons.
  • Correct: It shows 10 electrons; NH₄⁺ has 8, so no lone pair: Right: the 1+ charge removes an electron.
  • Nothing is wrong; N keeps its one lone pair: In NH₃ nitrogen has a lone pair, but in NH₄⁺ that pair has become the fourth N–H bond.
  • H atoms need lone pairs to complete their octets: H holds two electrons, one bonding pair; it never needs eight.

6. In a Lewis diagram, what does a lone pair represent?

  1. Two valence electrons on one atom, not shared
  2. Two electrons shared between two atoms in a bond
  3. Two core electrons held close to the nucleus
  4. A single unpaired electron on one atom
Show the answer

A lone pair (nonbonding pair) is a pair of valence electrons that belongs to one atom and is not shared.

  • Correct: Two valence electrons on one atom, not shared: Right: an unshared pair of valence electrons.
  • Two electrons shared between two atoms in a bond: Two shared electrons are a bonding pair, drawn as a line.
  • Two core electrons held close to the nucleus: Lewis diagrams show valence electrons only; core electrons are left out.
  • A single unpaired electron on one atom: An unpaired electron is a single dot, found in odd-electron species.

Part 9 · Summary

Summary

A Lewis diagram shows a molecule's or ion's valence electrons as bonding pairs (lines) and lone pairs (dots). Count the valence electrons, adjusting for charge; connect the outer atoms to the central atom; complete the outer octets; put leftovers on the center; and form double or triple bonds if the center is short. Most period 2 atoms follow the octet rule and H has two electrons, but boron can have an incomplete octet, odd-electron species leave one electron unpaired, and atoms from period 3 on can have expanded octets.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections