Covalent bond: sharing electrons, and Lewis dot structures
Two non-metal atoms share electron pairs so each completes its octet. One shared pair is a single bond, two a double bond, three a triple bond.
How a covalent bond forms
When two hydrogen atoms come close, each nucleus attracts the other atom's electron. But the two electrons repel each other, and so do the two nuclei. At one distance, attraction and repulsion balance and the energy is lowest. The atoms stay at this distance, sharing the two electrons. This shared pair is a covalent bond, and the distance is the bond length. Each H now has 2 electrons, like helium: H• + •H → H:H, written H-H.
Single, double and triple bonds
| Molecule | Configuration | Electrons each atom shares | Pairs shared | Bond |
|---|---|---|---|---|
| Hydrogen, H₂ | 1 | 1 | 1 | Single: H-H |
| Chlorine, Cl₂ | 2, 8, 7 | 1 | 1 | Single: Cl-Cl |
| Hydrogen chloride, HCl | H: 1; Cl: 2, 8, 7 | 1 each | 1 | Single: H-Cl |
| Oxygen, O₂ | 2, 6 | 2 | 2 | Double: O=O |
| Nitrogen, N₂ | 2, 5 | 3 | 3 | Triple: N≡N |
| Methane, CH₄ | C: 2, 4; H: 1 | C shares 4, each H shares 1 | 4 | Four single C-H bonds |
| Water, H₂O | O: 2, 6; H: 1 | O shares 2, each H shares 1 | 2 | Two single O-H bonds |
How to draw Lewis dot structures
- Write the symbols side by side. Show only the outer-shell electrons.
- Use dots for one atom and crosses for the other, so you can see where each shared electron came from.
- Put each shared pair between the two atoms. Put the other electrons (lone pairs) around the outside.
- Count: each atom must now have 8 electrons around it; hydrogen must have 2.
- Then draw the line structure: one line for each shared pair.
Lewis structures, one by one
- 1Cl₂: each Cl brings 7 electrons; one pair sits between themEach Cl has 6 unshared and 2 shared electrons: 8. Written Cl-Cl
- 2O₂: each O brings 6; two pairs sit between themEach O has 4 unshared and 4 shared electrons: 8. Written O=O
- 3★N₂: each N brings 5; three pairs sit between themEach N has 2 unshared and 6 shared electrons: 8. Written N≡N
- 4CH₄: C brings 4 crosses, each H brings 1 dot; four pairs surround CC has 8 electrons, each H has 2. Four C-H single bonds
- 5HCl: H brings 1, Cl brings 7; one pair sits between themH has 2, Cl has 8. The shared pair sits nearer Cl, which pulls electrons more strongly
★ key step
Ionic bond vs covalent bond
Counting covalent bonds: H₂, Cl₂ and HCl have 1; O₂ and H₂O have 2; N₂ has 3; CH₄ has 4.
The LPG and CNG used in Indian kitchens and autos are mainly butane and methane, made of covalent molecules. The oxygen we breathe has a double bond, and the nitrogen filled into chips packets has a triple bond.
1 marks: “What type of bond holds the two atoms together in a chlorine molecule, Cl₂?” A single covalent bond: one shared pair of electrons.
2 marks: “Draw the Lewis dot structure of methane, CH₄, and say how many electrons the carbon atom has around it after bonding.” C (2, 4) shares one electron pair with each of four H atoms, so four shared pairs surround C. Carbon then has 8 electrons and each H has 2. Line structure: four C-H single bonds.
3 marks: “Oxygen (2, 6) forms a double bond and nitrogen (2, 5) a triple bond. Explain why, with Lewis dot structures.” Each O needs 2 more electrons, so two O atoms share 2 pairs: O=O, each O with 8. Each N needs 3, so two N atoms share 3 pairs: N≡N, each N with 8. Draw dots and crosses with the shared pairs between the atoms.
One pair single, two pairs double, three pairs triple. Count to 8 (hydrogen counts to 2).