Edexcel International A Level Chemistry: Shapes of molecules and metallic bonding (YCH11) – Revision Notes

These notes condense Topic 3 parts 3C Shapes of molecules and 3D Metallic bonding (outcomes 3.16 to 3.22) of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017. The content belongs to Unit 1: Structure, Bonding and Introduction to Organic Chemistry, the IAS compulsory unit. For full explanations and worked examples, read the shapes and metallic bonding study guide first.

Test yourself afterwards with the practice questions. Earlier Topic 3 material (ionic and covalent bonding, polarity) is in the bonding revision notes and the ionic and covalent bonding revision notes. Other links: the Edexcel International A Level Chemistry course hub, a printable checklist of outcomes, and free 10-minute diagnostics to show where your gaps are.

Definitions to write word for word

Term	Definition
Electron-pair repulsion theory (3.16)	The electron pairs round a central atom repel one another and settle where repulsion is least; a lone pair repels more than a bonding pair does
Bond length (3.17)	The distance between the nuclei of two covalently bonded atoms
Bond angle (3.17)	The angle between two bonds meeting at the same atom
Metal structure (3.20)	A giant lattice of metal ions in a sea of delocalised electrons
Metallic bonding (3.21)	The strong electrostatic attraction between metal ions and the delocalised electrons

Repulsion order: lone-lone > lone-bond > bond-bond. Each lone pair in place of a bonding pair closes the remaining angles by roughly 2.5°.

Method in steps: predicting any shape

1. Outer electrons on the central atom.
2. Minus electrons it uses in bonds (1 per single, 2 per double, 3 per triple bond).
3. Add 1 per negative charge; subtract 1 per positive charge.
4. Halve the remainder: lone pairs.
5. Regions = bonded atoms + lone pairs. A double or triple bond is one region.
6. Arrangement from regions; shape from where the atoms are; angle from the table, minus about 2.5° per lone pair on a four-region centre.

Shapes and angles at a glance

Regions	Lone pairs	Shape	Angle	Named in 3.18	Analogous examples (3.19)
2	0	linear	180°	BeCl₂ (gas), CO₂	BeF₂ (gas), CS₂
3	0	trigonal planar	120°	BCl₃, each C in C₂H₄	BBr₃, each C in C₂F₄
4	0	tetrahedral	109.5°	CH₄, NH₄⁺	SiH₄, BF₄⁻
4	1	pyramidal	about 107°	NH₃	H₃O⁺
4	2	bent (non-linear)	about 104.5°	H₂O	NH₂⁻
5	0	trigonal bipyramidal	120° (equatorial) and 90° (axial to equatorial)	PCl₅ (gas)	AsF₅
6	0	octahedral	90°	SF₆	SiF₆²⁻, AlF₆³⁻

The angle ladder in one line: 180° > 120° > 109.5° > 107° > 104.5° > 90°.

Small worked reminders

BBr₃. B has 3 outer electrons, uses 3 in three B-Br bonds, so 0 lone pairs. Three regions: trigonal planar, 120°. Boron has only six electrons round it.

AsF₅. As has 5 outer electrons, uses all 5 in five As-F bonds, so 0 lone pairs. Five regions: trigonal bipyramidal, 120° and 90°.

AlF₆³⁻. Al has 3 outer electrons. Six bonds use 6. The 3- charge adds 3. 3 - 6 + 3 = 0 lone pairs. Six regions: octahedral, 90°.

C₂H₄ in words. Each carbon has two C-H bonds and one C=C: three regions, no lone pairs, so trigonal planar at each carbon, about 120°, with all six atoms in one plane.

How to write an “explain the shape” answer

1. Number of bonding pairs round the central atom.
2. Number of lone pairs (say “no lone pairs” if there are none).
3. Pairs repel and get as far apart as possible.
4. Lone pairs repel more than bonding pairs (only if there are lone pairs).
5. Shape name and bond angle with a degree sign.

A sketch helps but does not replace the words. In a 3D sketch, a plain line lies in the plane of the paper, a solid wedge points towards you and a dashed wedge points away.

Must-know distinctions

• Arrangement vs shape. NH₃: tetrahedral arrangement of pairs, pyramidal shape. H₂O: tetrahedral arrangement, bent shape.
• NH₃ vs NH₄⁺. Same four regions round nitrogen. Adding H⁺ turns the lone pair into a dative bond, so the angle opens from about 107° to 109.5°.
• CO₂ vs H₂O. Both have two bonded atoms. CO₂ has no lone pairs on carbon and is linear; H₂O has two lone pairs on oxygen and is bent.
• Gaseous vs solid PCl₅. Only gaseous PCl₅ is made of trigonal bipyramidal molecules; the solid has an ionic structure, which is why 3.18 names the gas. In an answer, write “gaseous PCl₅” or “PCl₅(g)”.
• Bond length vs bond angle. Length is a distance between nuclei (in nm or pm); angle is between bonds (in degrees). More shared pairs between two atoms give a shorter bond.
• Metallic vs ionic lattice. Both contain positive ions. In a metal the negative charge is a mobile electron sea; in an ionic solid it is fixed anions.
• Solid vs molten metal. Both conduct, because the delocalised electrons are present in both states.

Metallic bonding in brief

• Atoms release their outer electrons into the lattice: Na → Na⁺ plus 1 electron, Mg → Mg²⁺ plus 2, Al → Al³⁺ plus 3.
• The attraction is non-directional and acts throughout the giant lattice.

Stronger metallic bonding comes from:

Factor	Effect
Higher charge on the ion	Stronger attraction to the electron sea
More delocalised electrons per atom	More negative charge holding the ions
Smaller ion	Electrons closer to the centre of positive charge

Property explanations to learn:

• Electrical conductivity: when a potential difference is applied, the mobile delocalised electrons drift through the lattice, carrying charge.
• High melting temperature: the ion-electron attraction is strong and extends through the whole giant lattice, so melting takes a lot of energy.
• Malleability: layers of ions slide while the electron sea keeps holding them, since the bonds are not between fixed pairs of atoms.

Quick self-test

1. Define bond angle.
2. Give the shape and bond angle of SiH₄.
3. Explain why BBr₃ has no lone pair on boron.
4. Give the shape and both bond angles in gaseous AsF₅.
5. Show by counting that AlF₆³⁻ has no lone pairs on aluminium, and state its shape.
6. Gaseous BeF₂ is analogous to which named molecule, and what is its shape?
7. Put the H-X-H angles in CH₄, NH₃, H₂O and NH₄⁺ in order from largest to smallest.
8. Why does a C=C double bond count as one region when predicting shape?
9. Define metallic bonding.
10. Why does a molten metal still conduct electricity?
11. Which has the higher melting temperature, sodium or aluminium? Give two reasons.
12. How many 90° F-S-F angles are there in one SF₆ molecule?

Answers

1. The angle between two bonds that meet at the same atom.
2. Si has 4 outer electrons, all used in four Si-H bonds, so 0 lone pairs: tetrahedral, 109.5°.
3. Boron has 3 outer electrons and uses all 3 in its three B-Br bonds, leaving none to form a lone pair.
4. Trigonal bipyramidal: 120° between equatorial bonds and 90° between axial and equatorial bonds.
5. 3 outer electrons - 6 used in bonds + 3 for the charge = 0, so 0 lone pairs; six regions, octahedral, 90°.
6. BeCl₂; it is linear, 180°.
7. CH₄ = NH₄⁺ (109.5°) > NH₃ (about 107°) > H₂O (about 104.5°).
8. Both pairs of the double bond occupy the same region of space between the same two atoms, so they act as one region of electron density.
9. The strong electrostatic attraction between metal ions and the delocalised electrons.
10. The delocalised electrons are still present and still free to move, so they carry charge.
11. Aluminium. Al³⁺ has a higher charge than Na⁺ and Al gives three delocalised electrons per atom rather than one (also Al³⁺ is smaller), so the metallic bonding is stronger.
12. 12. Each of the six bonds is at 90° to four others: 6 × 4 ÷ 2 = 12.

Where marks are usually lost

• Stating a shape with no reasoning when the question says “explain”; the counts of bonding pairs and lone pairs are what earn credit.
• Writing “lone pairs take up more space” without the word repel.
• Naming the shape of NH₃ or H₂O as “tetrahedral” (that is the arrangement of pairs, not the shape).
• Calling H₂O “linear” or giving “bent” with an angle of 109.5°.
• Giving only one angle for PCl₅, or forgetting that 3.18 refers to the gaseous molecule.
• Forgetting the charge when counting electrons on an ion such as NH₄⁺ or AlF₆³⁻.
• Counting a double bond as two regions, which turns linear CO₂ into a wrong shape.
• Saying metals conduct because “ions move” or because of “free electrons” without saying they carry charge through the structure.
• Describing metallic bonding as attraction between “atoms”, or leaving out electrostatic.
• Explaining a melting-temperature difference by charge alone, without linking it to stronger attraction and more energy needed.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, published by Pearson Education Limited. Topic 3: Bonding and Structure, parts 3C Shapes of molecules and 3D Metallic bonding, outcomes 3.16 to 3.22.