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Covalent Bonding

Principles of Chemistry · 5 question types

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This topic accounts for approximately 8% of your exam marks.

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Dot-and-cross diagrams and comparison of giant covalent vs simple molecular structures appear regularly.

A dot-and-cross diagram shows which atom each shared electron originally came from
Conventions to follow:
- Use dots for one element's electrons and crosses for the other element's
- Show only the outer-shell electrons (Edexcel does not require inner shells)
- The two electrons of a shared pair sit in the overlap region between the atoms — one dot + one cross for each shared pair
- Lone pairs sit on the outside of the atom, away from the bond
Edexcel Section 1.7 expects dot-and-cross diagrams for: H₂, Cl₂, HCl, O₂, N₂, H₂O, NH₃, CH₄ and CO₂

Diatomic molecules

Molecule	Bond type	Shared electrons	Outer-shell arrangement
H₂	single	2 (1 pair)	each H has 2 (full 1st shell)
Cl₂	single	2 (1 pair)	each Cl has 8; 3 lone pairs each
HCl	single	2 (1 pair)	H has 2; Cl has 8 with 3 lone pairs
O₂	double	4 (2 pairs)	each O has 8; 2 lone pairs each
N₂	triple	6 (3 pairs)	each N has 8; 1 lone pair each

Polyatomic molecules

Molecule	Central atom	Bonds	Shape	Lone pairs
H₂O	O	2 single (O–H)	bent	2 on O
NH₃	N	3 single (N–H)	trigonal pyramidal	1 on N
CH₄	C	4 single (C–H)	tetrahedral	0
CO₂	C	2 double (O=C=O)	linear	2 on each O

Dot-and-cross diagram for water H₂O, showing two O–H bonding pairs and two lone pairs on the oxygen, with the H–O–H structural formula — Source: Covalent Bonds: Dot & Cross Diagrams by Save My Exams

Dot-and-cross diagram for ammonia NH₃, showing three N–H bonding pairs and one lone pair on the nitrogen, with the structural formula — Source: Covalent Bonds: Dot & Cross Diagrams by Save My Exams

Dot-and-cross diagram for methane CH₄, showing four C–H bonding pairs and no lone pairs on the carbon, with the structural formula — Source: Covalent Bonds: Dot & Cross Diagrams by Save My Exams

Dot-and-cross diagram for carbon dioxide CO₂, showing two C=O double bonds and two lone pairs on each oxygen, with the O=C=O structural formula — Source: Covalent Bonds: Dot & Cross Diagrams by Save My Exams

Counting electrons in a bond

One covalent bond = one shared pair = 2 electrons
One double bond = 4 electrons shared (two pairs)
One triple bond = 6 electrons shared (three pairs)
e.g. in O₂, the two oxygen atoms share 4 electrons in total through a double bond

Exam tip

Common: complete a dot-and-cross diagram

What comes up: you're given a small molecule (a different one most years, such as NH₃, N₂ or H₂O) and asked to complete its dot-and-cross diagram.

Write: one shared pair in each overlap, then add lone pairs until every atom's outer shell is full.

Watch out: hydrogen needs only 2 electrons, never 8; don't forget the lone pairs on N (1) and O (2).