A-level Chemistry/OCR (Salters)/Molecular geometry

The shapes of molecules is the title of Section 3.3 in Chemical Ideas and it covers the topic of molecular geometry.

Introductory examples

Methane molecule

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  dot-cross diagram
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  bond-line diagram
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  ball-and-stick model

Ammonia molecule

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  dot-cross diagram
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  bond-line diagram
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  ball-and-stick model with lone pairs displayed
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  ball-and-stick model without lone pairs displayed

Water molecule

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  dot-cross diagram
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  bond-line diagram
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  ball-and-stick model with lone pairs displayed
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  ball-and-stick model without lone pairs displayed

Hydrogen fluoride molecule

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  dot-cross diagram
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  bond-line diagram
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  ball-and-stick model with lone pairs displayed
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  ball-and-stick model without lone pairs displayed

Polyhedra

Tetrahedra

You have probably come across tetrahedra before in maths, although you most likely called them triangle-based pyramids. Tetrahedra have four vertices (corners), four faces and six edges. Each face is an equilateral triangle.

The tetrahedron is one of the most important shapes in chemistry because a very great many molecules contain them. Tetrahedral molecules don't actually contain little pyramids. What they do contain is a central atom bonded to four other atoms. The four atoms surrounding the central atom occupy positions that you can imagine as the vertices of a tetrahedron.

In the image gallery below, the central atom is coloured magenta and the surrounding atoms are coloured white.

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  a tetrahedral molecule
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  to see the tetrahedron, connect the surrounding atoms with lines
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  the lines form the edges of the tetrahedron
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  tetrahedral angle ≈ 109.5°
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  chemists represent tetrahedra using hashed and wedged bonds
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  this is how chemists represent methane
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  this 'flat' representation - GCSE-style - is simpler but less realistic

The angle between any two bonds in a tetrahedral molecule is approximately 109.5°. The tetrahedral angle can be calculated as accurately as required because it is equal to cos⁻¹(–⅓).

Octahedra

You may or may not have met an octahedron before. Octahedra have six vertices (corners), eight faces and twelve edges. Each face is an equilateral triangle.

Octahedra are very important in chemistry because many transition metal-based molecules are octahedral.

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  an octahedral molecule
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  to see the octahedron, connect the surrounding atoms with lines
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  the lines form the edges of the octahedron
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  octahedral angle = 90° exactly
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  chemists represent octahedra using hashed and wedged bonds
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  this is how chemists represent sulfur hexafluoride, SF₆
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  a ball-and-stick model of SF₆

Common molecular geometries

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  linear
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  bent
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  trigonal planar
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  pyramidal
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  square planar
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  tetrahedral
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  trigonal bipyramidal
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  octahedral

Further examples

Example molecules

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  linear: CO₂
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  bent: H₂S
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  trigonal planar: BI₃
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  pyramidal: NH₃
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  square planar: XeF₄
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  tetrahedral: CH₄
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  trigonal bipyramidal: PCl₅
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  octahedral: SF₆

Example ions

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  linear:
  hydroxide, OH⁻
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  trigonal planar:
  carbonate, CO₃²⁻
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  trigonal planar:
  nitrate, NO₃⁻
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  trigonal pyramidal:
  hydronium, H₃O⁺
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  tetrahedral:
  thiosulfate, S₂O₃²⁻
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  tetrahedral:
  sulfate, SO₄²⁻
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  tetrahedral:
  phosphate, PO₄³⁻
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  tetrahedral:
  ammonium, NH₄⁺

Molecular geometry and lone pairs

You can use the so-called AXE method to calculate the shape of a molecule. It is based on molecules that have a central atom, which we label A. Atoms or groups bonded to A are labelled X. Lone pairs are labelled E. A molecule with three lone pairs and two atoms/groups bonded to it would be denoted AX₂E₃. The table below shows how X and E and molecular shape are related.

Valence shell electron pair repulsion theory (VSEPR) is used to predict the shape of a molecule once X and E are known. This sounds more complicated than it is. You consider any X's and E's to be regions of charge that position themselves as far apart from each other as possible, in order to minimize the forces of electrostatic repulsion between each other.

AXE label	X (substituents)	E (lone pairs)	Shape	2D diagram lone pairs shown	2D diagram lone pairs not shown	3D model lone pairs shown	3D model lone pairs not shown	Examples
AX1E0	1	0	Linear					H2
AX1E1	1	1	Linear					CN−
AX1E2	1	2	Linear					O2
AX1E3	1	3	Linear					HCl
AX2E0	2	0	Linear					BeCl2 HgCl2 CO2
AX2E1	2	1	Bent					NO2− SO2 O3
AX2E2	2	2	Bent					H2O H2S OF2
AX2E3	2	3	Linear					XeF2 I3−
AX3E0	3	0	Trigonal planar					BF3 CO32− NO3− SO3
AX3E1	3	1	Trigonal pyramidal					NH3 PCl3
AX3E2	3	2	T-shaped					ClF3 BrF3
AX4E0	4	0	Tetrahedral					CH4 NH4+ PO43− SO42− ClO4−
AX4E1	4	1	Seesaw					SF4
AX4E2	4	2	Square Planar					XeF4
AX5E0	5	0	Trigonal Bipyramidal					PCl5
AX5E1	5	1	Square pyramidal					ClF5 BrF5
AX5E2	5	2	Pentagonal planar					XeF5-
AX6E0	6	0	Octahedral					SF6
AX6E1	6	1	Pentagonal pyramidal					IF6-
AX7E0	7	0	Pentagonal bipyramidal					IF7
AX8E0	8	0	Square antiprismatic					IF8-
AX8E1	8	1	Distorted square antiprismatic					XeF82-
AX9E0	9	0	Tricapped trigonal prismatic OR capped square antiprismatic					ReH92- (tricapped trigonal prismatic)