Shapes of molecules
Cambridge A-Level Chemistry (9701) · Unit 3: Chemical bonding · 9 flashcards
Shapes of molecules is topic 3.5 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 3 — Chemical bonding , alongside Electronegativity and bonding, Ionic bonding and Metallic bonding. In one line: BF3 has a trigonal planar shape due to 3 bonding pairs and no lone pairs around the central Boron atom. The bond angle is 120°.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 9 flashcards — 8 definitions and 1 key concept — covering the precise wording mark schemes reward. Use the 8 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Shape and bond angle of BF3
BF3 has a trigonal planar shape due to 3 bonding pairs and no lone pairs around the central Boron atom. The bond angle is 120°.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- state and explain the shapes of, and bond angles in, molecules by using VSEPR theory, including as simple examples: • BF3 (trigonal planar, 120°) • CO2 (linear, 180°) • CH4 (tetrahedral, 109.5°) • NH3 (pyramidal, 107°) • H2O (non-linear, 104.5°) • SF6 (octahedral, 90°) • PF5 (trigonal bipyramidal, 120° and 90°)
- predict the shapes of, and bond angles in, molecules and ions analogous to those specified in 3.5.1 3.6 Intermolecular forces, electronegativity and bond properties Learning outcomes
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Shapes of molecules
- › Only use 'intermolecular' when referring to simple molecules; use 'electrostatic attraction' for ionic, metallic, or giant covalent lattice forces.
- › Describe hybridisation as the mixing of atomic orbitals of similar energies to form a new set of equivalent hybrid orbitals.
- › Remember that a water molecule can form four hydrogen bonds: two through its lone pairs and two through its hydrogen atoms.
- › Count all single bonds and one bond from every double/triple bond as sigma bonds; ensure every C-H bond is included.
- › Recall that an octahedral structure has both 90 and 180 degree bond angles; visualize trans atoms to identify the 180 degree angles.
State the shape and bond angle of BF3.
BF3 has a trigonal planar shape due to 3 bonding pairs and no lone pairs around the central Boron atom. The bond angle is 120°.
State the shape and bond angle of CO2.
CO2 is a linear molecule. The central Carbon atom has two double bonds to oxygen atoms, resulting in a bond angle of 180°.
State the shape and bond angle of CH4.
CH4 has a tetrahedral shape due to four bonding pairs and no lone pairs around the central Carbon atom. The bond angle is 109.5°.
State the shape and bond angle of NH3.
NH3 has a pyramidal shape due to three bonding pairs and one lone pair around the central Nitrogen atom. The bond angle is approximately 107°.
State the shape and bond angle of H2O.
H2O has a non-linear (bent) shape due to two bonding pairs and two lone pairs around the central Oxygen atom. The bond angle is approximately 104.5°.
State the shape and bond angle of SF6.
SF6 has an octahedral shape due to six bonding pairs and no lone pairs around the central Sulfur atom. The bond angle is 90°.
State the shape and bond angles of PF5.
PF5 has a trigonal bipyramidal shape. There are two bond angles present: 120° between the equatorial Fluorine atoms, and 90° between the axial and equatorial Fluorine atoms.
Explain how VSEPR theory is used to predict molecular shapes.
VSEPR (Valence Shell Electron Pair Repulsion) theory states that electron pairs around a central atom will arrange themselves to minimize repulsion. This arrangement determines the molecule's shape and bond angles. Lone pairs have greater repulsive force than bonding pairs, affecting bond angles.
Predict the shape of the [BeCl2] molecule.
The [BeCl2] molecule is predicted to be linear. There are two bonding pairs of electrons around the central Beryllium atom and no lone pairs, resulting in a bond angle of 180°.
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Shapes of molecules sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Shapes of molecules deck
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