Dot-and-cross diagrams
Introduced| English | Meaning |
|---|---|
| dot-and-cross diagram/dɒt ænd krɒs ˈdaɪəɡræm/ | A diagram using different electron symbols to show outer-shell electrons and their origins in bonding. |
| expanded octet/ekˈspændɪd ɒkˈtet/ | expanded octet |
| bonding pair/ˈbɒndɪŋ peə/ | A pair of electrons shared between atoms in a covalent bond. |
| lone pair/ləʊn peə/ | A pair of outer-shell electrons on one atom that is not shared in a covalent bond. |
| triple bond/ˈtrɪpl bɒnd/ | A covalent bond involving three shared pairs of electrons. |
Drawing the electrons
- A dot-and-cross diagram 点叉图 shows the outer electrons of each atom.
- Dots for one atom, crosses for the other.
- This makes it clear where each bonding electron came from.
A dot-and-cross diagram uses dots and crosses to:
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Different symbols (dots/crosses) track which atom each outer electron came from.
A dot-and-cross diagram shows the outer electrons of each atom in a bond.
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Dots and crosses distinguish the atoms' electrons.
What they can show
- You can draw them for ionic, covalent and coordinate bonding.
- They also handle molecules with an expanded octet 扩展八隅 or an odd number of electrons.
Dot-and-cross diagrams can be drawn for:
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They work for all three, including expanded-octet and odd-electron species.
Match each dot-and-cross term to its meaning.
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Dots and crosses track which atom each electron came from; lone pairs shape the molecule.
Worked examples
- Water: oxygen has two bonding pairs 成键电子对 and two lone pairs 孤对电子.
- Nitrogen ($\text{N}_2$): the atoms share three pairs (a triple bond 三键), with a lone pair on each.
Water (two bonding + two lone pairs on O) and nitrogen (a triple bond) drawn with dots and crosses.
Shape from bonding and lone pairs
Count the bonding pairs and lone pairs around the central atom; they repel into the shape with least strain. Three bonds and one lone pair give a pyramid, like ammonia (NH3).
In the dot-and-cross diagram of water, the oxygen atom has:
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Oxygen forms two O–H bonds (two bonding pairs) and keeps two lone pairs.
The dot-and-cross diagram of N₂ shows:
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Nitrogen molecules share three pairs of electrons (a triple bond) plus a lone pair on each atom.
Common dot-and-cross slips
- Show only the outer-shell electrons, and make sure each atom reaches a full shell.
- For ions, draw square brackets and the charge outside.
You've got it
- dot-and-cross = outer electrons, dots for one atom and crosses for the other
- shows where each bonding electron came from; works for ionic, covalent and coordinate bonds
- water = 2 bonding + 2 lone pairs on O; $\text{N}_2$ = 3 shared pairs (triple bond)