VSEPR theory states that electron pairs around a central atom repel each other and arrange themselves as far apart as possible to minimise repulsion .

True or False? A lone pair – lone pair repulsion is weaker than a bond pair – bond pair repulsion.

False. A lone pair – lone pair repulsion is the strongest type, because lone pairs occupy more space and lie closer to the nucleus than bonding pairs.

What is the bond angle and shape of methane, CH 4 ?

Methane is tetrahedral with a bond angle of 109.5° , as it has four bonding pairs and no lone pairs around the central carbon atom.

In ammonia, NH 3 , nitrogen has three bonding pairs and .......... lone pair, giving a .......... shape with a bond angle of .......... .

In ammonia, NH 3 , nitrogen has three bonding pairs and one lone pair, giving a pyramidal shape with a bond angle of 107° .

True or False? Water has a bond angle of 109.5° because oxygen has four electron pairs.

False. Water has a bond angle of 104.5° . The two lone pairs on oxygen repel the bonding pairs more strongly, compressing the angle below the tetrahedral value.

Why does the bond angle in NH 3 (107°) differ from the bond angle in CH 4 (109.5°)?

In NH 3 , the lone pair on nitrogen exerts greater repulsion than a bonding pair, pushing the three bonding pairs closer together and reducing the bond angle to 107° .

Define trigonal planar

Trigonal planar describes a molecular shape with three bonding pairs and no lone pairs arranged in a plane, giving bond angles of 120° (e.g. BF 3 ).

Define electronegativity

Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself.

How does electronegativity change across a period and down a group?

Electronegativity increases across a period (increasing nuclear charge, constant shielding) and decreases down a group (increased shielding and atomic radius).

True or False? Fluorine is the most electronegative element, with a Pauling scale value of 4.0.

True. Fluorine has the highest electronegativity of all elements because it has a high nuclear charge , small atomic radius and minimal shielding .

In a polar covalent bond, the more electronegative atom carries a partial charge of .......... and the less electronegative atom carries a partial charge of .......... .

In a polar covalent bond, the more electronegative atom carries a partial charge of δ− and the less electronegative atom carries a partial charge of δ+ .

Why is CCl 4 a non-polar molecule despite having four polar C–Cl bonds?

The four polar bonds are arranged symmetrically in a tetrahedral shape, so the individual dipole moments cancel each other out, giving an overall dipole of zero.

True or False? A molecule with polar bonds is always a polar molecule.

False. If the polar bonds are arranged symmetrically , their dipole moments cancel and the molecule is non-polar overall (e.g. CCl 4 ).

A dipole moment is a measure of bond polarity , showing the direction and magnitude of charge separation in a polar bond, with the arrow pointing towards the δ− end.

Define induced dipole-dipole forces

Induced dipole-dipole forces (also called London dispersion forces ) are temporary attractive forces that exist between all atoms and molecules, arising from a momentary uneven electron distribution.

What conditions are needed for hydrogen bonding to occur between molecules?

A molecule must contain a hydrogen atom covalently bonded to a highly electronegative atom ( O , N or F ), creating a sufficiently δ+ hydrogen to attract a lone pair on an O, N or F in a neighbouring molecule.

True or False? Induced dipole-dipole forces increase in strength as the number of electrons in a molecule increases.

True. More electrons mean a greater electron cloud that is more easily distorted, producing stronger temporary dipoles and therefore stronger induced dipole-dipole forces.

Water has anomalously high melting and boiling points because its molecules are held together by .......... bonds, which require a lot of energy to .......... .

Water has anomalously high melting and boiling points because its molecules are held together by hydrogen bonds, which require a lot of energy to break .

Why does ice have a lower density than liquid water?

In ice, water molecules form a 3D hydrogen-bonded network in a rigid lattice, spacing them slightly further apart than in liquid water and giving ice a lower density .

True or False? Permanent dipole-dipole forces are always stronger than induced dipole-dipole forces.

False. Permanent dipole-dipole forces are stronger only in small molecules with similar electron counts. In larger molecules, induced dipole-dipole forces can dominate due to greater electron cloud size.

Why does hexanol dissolve poorly in water compared to ethanol?

As the carbon chain grows, the non-polar part of the molecule becomes larger relative to the polar OH group, reducing the ability to form hydrogen bonds with water.

Define hydrogen bonding

Hydrogen bonding is the strongest form of intermolecular force , occurring when a δ+ hydrogen bonded to O, N or F interacts with a lone pair on an O, N or F atom in another molecule.

A LevelChemistryOCRFlashcards2. Foundations in ChemistryThe Shapes of Simple Molecules & Ions

The Shapes of Simple Molecules & Ions (OCR A Level Chemistry A): Flashcards

Exam code: H432

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FrontThe Shapes of Simple Molecules & Ions
Define VSEPR theory

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Cards in this collection (22)

Define VSEPR theory
VSEPR theory states that electron pairs around a central atom repel each other and arrange themselves as far apart as possible to minimise repulsion.
True or False?
A lone pair – lone pair repulsion is weaker than a bond pair – bond pair repulsion.
False.
A lone pair – lone pair repulsion is the strongest type, because lone pairs occupy more space and lie closer to the nucleus than bonding pairs.
What is the bond angle and shape of methane, CH₄?
Methane is tetrahedral with a bond angle of 109.5°, as it has four bonding pairs and no lone pairs around the central carbon atom.
In ammonia, NH₃, nitrogen has three bonding pairs and .......... lone pair, giving a .......... shape with a bond angle of .......... .
In ammonia, NH₃, nitrogen has three bonding pairs and one lone pair, giving a pyramidal shape with a bond angle of 107°.
True or False?
Water has a bond angle of 109.5° because oxygen has four electron pairs.
False.
Water has a bond angle of 104.5°. The two lone pairs on oxygen repel the bonding pairs more strongly, compressing the angle below the tetrahedral value.
Why does the bond angle in NH₃ (107°) differ from the bond angle in CH₄ (109.5°)?
In NH₃, the lone pair on nitrogen exerts greater repulsion than a bonding pair, pushing the three bonding pairs closer together and reducing the bond angle to 107°.
Define trigonal planar
Trigonal planar describes a molecular shape with three bonding pairs and no lone pairs arranged in a plane, giving bond angles of 120° (e.g. BF₃).
Define electronegativity
Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself.
How does electronegativity change across a period and down a group?
Electronegativity increases across a period (increasing nuclear charge, constant shielding) and decreases down a group (increased shielding and atomic radius).
True or False?
Fluorine is the most electronegative element, with a Pauling scale value of 4.0.
True.
Fluorine has the highest electronegativity of all elements because it has a high nuclear charge, small atomic radius and minimal shielding.
In a polar covalent bond, the more electronegative atom carries a partial charge of .......... and the less electronegative atom carries a partial charge of ...........
In a polar covalent bond, the more electronegative atom carries a partial charge of δ− and the less electronegative atom carries a partial charge of δ+.
Why is CCl₄ a non-polar molecule despite having four polar C–Cl bonds?
The four polar bonds are arranged symmetrically in a tetrahedral shape, so the individual dipole moments cancel each other out, giving an overall dipole of zero.
True or False?
A molecule with polar bonds is always a polar molecule.
False.
If the polar bonds are arranged symmetrically, their dipole moments cancel and the molecule is non-polar overall (e.g. CCl₄).
Define dipole moment
A dipole moment is a measure of bond polarity, showing the direction and magnitude of charge separation in a polar bond, with the arrow pointing towards the δ− end.
Define induced dipole-dipole forces
Induced dipole-dipole forces (also called London dispersion forces) are temporary attractive forces that exist between all atoms and molecules, arising from a momentary uneven electron distribution.
What conditions are needed for hydrogen bonding to occur between molecules?
A molecule must contain a hydrogen atom covalently bonded to a highly electronegative atom (O, N or F), creating a sufficiently δ+ hydrogen to attract a lone pair on an O, N or F in a neighbouring molecule.
True or False?
Induced dipole-dipole forces increase in strength as the number of electrons in a molecule increases.
True.
More electrons mean a greater electron cloud that is more easily distorted, producing stronger temporary dipoles and therefore stronger induced dipole-dipole forces.
Water has anomalously high melting and boiling points because its molecules are held together by .......... bonds, which require a lot of energy to .......... .
Water has anomalously high melting and boiling points because its molecules are held together by hydrogen bonds, which require a lot of energy to break.
Why does ice have a lower density than liquid water?
In ice, water molecules form a 3D hydrogen-bonded network in a rigid lattice, spacing them slightly further apart than in liquid water and giving ice a lower density.
True or False?
Permanent dipole-dipole forces are always stronger than induced dipole-dipole forces.
False.
Permanent dipole-dipole forces are stronger only in small molecules with similar electron counts. In larger molecules, induced dipole-dipole forces can dominate due to greater electron cloud size.
Why does hexanol dissolve poorly in water compared to ethanol?
As the carbon chain grows, the non-polar part of the molecule becomes larger relative to the polar OH group, reducing the ability to form hydrogen bonds with water.
Define hydrogen bonding
Hydrogen bonding is the strongest form of intermolecular force, occurring when a δ+ hydrogen bonded to O, N or F interacts with a lone pair on an O, N or F atom in another molecule.

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