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Shapes of Molecules

Key Concepts

A molecule consists of 2 or more atoms joined by covalent bonds.
The shape of a molecule is a description of the way the atoms in the molecule occupy space.
A diatomic molecule, a molecule composed of only 2 atoms, must always be linear in shape as the centers of the 2 atoms will always be in a straight line.
'Electron Cloud' Repulsion Theory (Valence Shell Electron Pair Repulsion, VSEPR) is used to predict shapes and bond angles of simple molecules

an 'electron cloud' may be a single, double or triple bond, or a lone pair of electrons
a lone pair of electrons is a non-bonding pair of electrons
'electron clouds' are negatively charged since the electrons are negatively charged, so electron clouds repel one another and try to get as far away from each other as possible
lone pairs of electrons exert a greater repelling effect than bonding pairs do
lone pair-bonding pair repulsion is greater than bonding pair-bonding pair repulsion
lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion

Total Number of electron pairs	Arrangement of electron pairs	Number of bonding pairs of electrons	Number of lone pairs of electrons	Name of Shape	Bond Angle	Examples
not applicable	linear	1	not applicable	linear	180^o	H₂, HCl
2	linear	2	0	linear	180^o	CO₂, HCN
3	trigonal planar	3	0	triganol planar	120^o	BCl₃, AlCl₃
4	tetrahedral	4	0	tetrahedral	109.5^o	CH₄, SiF₄
		3	1	trigonal pyramidal	<109.5^o (bond angles in ammonia, NH₃, are 107^o)	NH₃, PCl₃
		2	2	bent	<109.5^o (bond angles in water, H₂O, are 105^o)	H₂O, SCl₂
5	trigonal bipyramidal	5	0	trigonal bipyramidal	120^o in the trigonal planar part of the molecule, 90^o for the others	PCl₅
6	octahedral	6	0	octahedral	90^o	SF₆

Examples

hydrogen chloride, HCl
HCl is composed of only 2 atoms, 1 atom of hydrogen and 1 atom of chlorine covalently bonded.
HCl is, therefore, diatomic.
All diatomic molecules are linear in shape.
H-Cl is linear in shape
hydrogen cyanide, HCN
HCN is composed of 3 atoms, 1 atom of hydrogen, 1 atom of carbon and 1 atom of nitrogen covalently bonded.
Carbon, C, is the central atom in the molecule.
Carbon has 4 valence electrons (electrons that can be used in bonding).
1 of carbon's valence electrons will be used to form a covalent bond with hydrogen.
3 of carbon's valence electrons will be used to form 3 covalent bonds with nitrogen ( a triple bond).
The central carbon atom therefore has no lone pairs of electrons.
The bonding pairs will repel each other as much as possible, so the molecule will be linear.
HCN is linear in shape.
aluminium chloride, AlCl₃
AlCl₃ is composed of 4 atoms, 1 atom of aluminium and 3 atoms of chlorine.
Aluminium, Al, is the central atom in the molecule.
Aluminium has 3 valence electrons (electrons that can be used in bonding).
Each of the aluminium's valence electrons will be used to form a covalent bond with each chlorine atom.
The central aluminium atom will therefore have no lone pairs of electrons and 3 bonding pairs of electrons.
The bonding pairs of electrons will repel each other as much as possible, so the molecule will be trigonal planar.
AlCl₃ is trigonal planar in shape.
methane, CH₄
CH₄ is composed of 5 atoms, 1 atom of carbon and 4 atoms of hydrogen covalently bonded.
Carbon, C, is the central atom in the molecule.
Carbon has 4 valence electrons (electrons that can be used in bonding).
Each of carbon's 4 valence electrons will form a bonding pair with 1 of hydrogen's electrons.
The central carbon atom will therefore have no lone pairs of electrons and 4 bonding pairs of electrons,
The bonding pairs of electrons will repel each other as much as possible, so the molecule will be tetrahedral.
CH₄ is tetrahedral in shape.
ammonia, NH₃
NH₃ is composed of 4 atoms, 1 nitrogen atom and 3 hydrogen atoms covalently bonded.
Nitrogen, N, is the central atom in the molecule.
Nitrogen has 5 valence electrons.
3 of nitrogen's valence electrons will be used to form bonding pairs of electrons with hydrogen (3 covalent bonds).
These bonding pairs repel each other equally and will try to get as far away from each other as possible.
2 of nitrogen's valence electrons will be unused for bonding, these are a lone pair of electrons.
lone pair-lone pair repulsion is greater than bonding pair-bonding pair repulsion, so the lone pair pushes the bonding pairs closer together than in a tetrahedral arrangement of the 'electron clouds'.
This distorted tetrahedral arrangement is called trigonal pyramidal.
NH₃ is trigonal pyramidal in shape.

In this representation, the solid triangles represent bonds coming out of the plane of the screen, the broken lines represented a bond going behind the plane of the screen.
water, H₂O
Water is composed of 3 atoms, 1 atom of oxygen and 2 atoms of hydrogen covalently bonded.
Oxygen, O, is the central atom.
Oxygen has 6 valence electrons.
2 of oxygen's electrons will be used to form bonding pairs of electrons with hydrogen (2 covalent bonds).
These bonding pairs repel each other equally and will try to get as far away from each other as possible.
4 of oxygen's valence electrons will not be used for bonding, these will remain as 2 lone pairs of electrons.
These lone pairs of electrons repel each other equally and will try to get as far away from each as possible.
lone pair-lone pair repulsion is greater than lone pair-bonding pair or bonding pair-bonding pair repulsion, so the lone pairs of electrons push the bonding pairs of electrons closer together than in a tetrahedral arrangement of the 'electron clouds'.
This distorted tetrahedral arrangement is call bent.
H₂O is bent in shape.

In this representation, solid lines represent bonds that are in the same plane as the screen.