1.7.2Principles of Chemistry
Covalent Bonds: Dot and Cross Diagrams — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 42 of 50
Part 1 of 5Drawing method
1
Work out the number of outer-shell electrons of each atom from its group number: H has 1, C has 4, N has 5, O has 6, Cl has 7.
2
Draw the outer shells overlapping, so the bonding electrons sit in the overlap region.
3
Use dots for electrons from one atom type and crosses for the other, so each shared pair shows one dot and one cross.
4
Add the remaining electrons as non-bonding pairs (lone pairs) on the atoms that have them.
5
Count the electrons round each atom: two for hydrogen, eight for the others, unless the question allows otherwise.
Part 2 of 5Molecules to know
| Molecule | Bonds | Bonding pairs | Lone pairs |
|---|---|---|---|
| H2 | H–H | 1 | 0 |
| Cl2 | Cl–Cl | 1 | 3 on each Cl |
| HCl | H–Cl | 1 | 3 on Cl |
| H2O | H–O–H | 2 | 2 on O |
| NH3 | three N–H | 3 | 1 on N |
| CH4 | four C–H | 4 | 0 |
| O2 | O=O | 2 shared between the atoms | 2 on each O |
| N2 | N≡N | 3 shared between the atoms | 1 on each N |
| CO2 | O=C=O | 4 (two double bonds) | 2 on each O |
Part 3 of 5Worked example: ammonia, NH3
1Count electrons
- Count electrons Nitrogen has five outer electrons and each hydrogen has one.
2Share three pairs
- Share three pairs Nitrogen shares one electron with each of three hydrogens, using three of its five electrons.
3Fill the shell
- Fill the shell The two remaining electrons on nitrogen form one non-bonding (lone) pair.
4Check
- Check Nitrogen has eight outer electrons and each hydrogen has two.
Part 4 of 5Multiple bonds in diagrams
Oxygen, O2
- Oxygen, O2 Each oxygen has six outer electrons and needs two more, so the atoms share two pairs, a double bond, with two lone pairs left on each atom.
Carbon dioxide, CO2
- Carbon dioxide, CO2 Carbon shares two pairs with each oxygen atom, forming two double bonds, so carbon has eight electrons around it.
Nitrogen, N2
- Nitrogen, N2 Three pairs are shared in a triple bond, leaving one lone pair on each nitrogen.
Part 5 of 5Limits of dot and cross diagrams
No shape
- No shape The diagrams are flat and do not show the three-dimensional shape of a molecule such as methane.
No size or strength
- No size or strength They cannot show the relative sizes of atoms or how strong a bond is.
Electrons look different
- Electrons look different Dots and crosses are only labels; all electrons are identical.
Key terms
| Term | Meaning |
|---|---|
| Dot and cross diagram | A diagram that shows the outer electrons of atoms using dots and crosses to tell their origin apart. |
| Bonding pair | A pair of electrons shared between two atoms. |
| Lone pair | A pair of outer electrons that is not involved in bonding. |
| Outer shell | The highest occupied energy level of an atom. |
| Covalent bond | A shared pair of electrons between atoms. |
Avoid these mistakes
1
Leaving out the lone pairs, so the atom appears not to have a full outer shell.
2
Drawing too many electrons round hydrogen. It has only a pair.
3
Using only one shared pair for O2 or CO2. These contain double bonds.
4
Showing electrons as having moved completely to one atom, which is ionic bonding.
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