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State the main principle of electron-pair repulsion theory.
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electron pairs around a central atom repel each other; they arrange themselves as far apart as possible to minimise repulsion
Explain why a lone pair causes greater repulsion than a bonding pair.
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a lone pair is held closer to the central atom / not shared between two nuclei; therefore its electron density is more concentrated and repels other pairs more strongly
Define the term bond angle.
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the angle between two covalent bonds sharing a common atom
State and explain how bond length relates to bond strength in C–C and C=C bonds.
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C=C is shorter than C–C; the shorter bond is stronger because the nuclei are closer to the shared electrons, increasing electrostatic attraction
State the shape and bond angle of NH₃ and explain why the bond angle is smaller than that of CH₄.
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trigonal pyramidal, 107°; NH₃ has one lone pair on N which repels more strongly than a bonding pair; this compresses the H–N–H angle below the tetrahedral 109.5° in CH₄
Describe the shape of PCl₅ in the gas phase and state both bond angles.
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trigonal bipyramidal; 120° between equatorial Cl–P–Cl bonds; 90° between axial and equatorial Cl–P–Cl bonds
Explain why CO₂ is linear despite having four bonding electron pairs in total.
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each C=O double bond counts as one region of electron density; with two regions, they arrange 180° apart to minimise repulsion, giving a linear shape
Discuss how electron-pair repulsion theory accounts for the shapes and bond angles of CH₄, NH₃ and H₂O. In your answer you should: state the shape and bond angle of each molecule; identify the number of bonding pairs and lone pairs on the central atom; explain the trend in bond angles across the three molecules.
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CH₄ is tetrahedral with bond angle 109.5°; four bonding pairs and zero lone pairs around C. NH₃ is trigonal pyramidal with bond angle 107°; three bonding pairs and one lone pair around N. H₂O is bent / non-linear with bond angle 104.5°; two bonding pairs and two lone pairs around O. Electron pairs repel and arrange as far apart as possible; lone pairs repel more strongly than bonding pairs. Each lone pair compresses the bond angle by approximately 2.5° from the tetrahedral value. The trend 109.5° → 107° → 104.5° reflects increasing number of lone pairs and therefore increasing lone pair–bond pair repulsion.
Predict the shape and bond angle of H₃O⁺ and justify your answer.
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trigonal pyramidal; 107°; O has three bonding pairs and one lone pair, so the lone pair compresses the H–O–H angle below the tetrahedral 109.5°
Predict the shape of BF₄⁻ and state the bond angle.
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tetrahedral; 109.5°; B has four bonding pairs and no lone pairs
Suggest the shape of SiCl₄ and explain your reasoning.
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tetrahedral with bond angle 109.5°; Si has four bonding pairs to four Cl atoms / no lone pairs on Si; four regions of electron density arrange to maximise separation