Chapter 6 - Shapes of Molecules and Intermolecular Forces Flashcards

Shapes of molecules and ions, Electronegativity and polarity, Intermolecular forces and Hydrogen bonding.

1
Q

What does molecular shape depend on?

A

Electron pairs around the central atom.

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2
Q

In what way do electron pairs interact?

A

They repel each other.

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3
Q

Which type of pair repels the most?

A

Lone pairs

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4
Q

What is the order of pair repulsion, from strongest to weakest?

A

Lone pair/lone pair
Lone pair/bonding pair
Bonding pair/bonding pair

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5
Q

What is the name of the way to predict a molecule’s shape?

A

Electron pair repulsion theory

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6
Q

What shape name is given to a molecule with no lone pairs?

A

Tetrahedral

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7
Q

What is the bond angle in a tetrahedral molecule?

A

109.5

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8
Q

What shape name is given to a molecule with one lone pair?

A

Trigonal pyramidal

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9
Q

What is the bond angle in a trigonal pyramidal molecule?

A

107

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10
Q

What shape name is given to a molecule with two lone pairs?

A

Non-linear

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11
Q

What is the bond angle in a non-linear molecule?

A

104.5

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12
Q

What shape name is given to a molecule with 2 electron pairs around the central atom?

A

Linear

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13
Q

What is the bond angle in a linear molecule?

A

180

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14
Q

What shape name is given to a molecule with 3 electron pairs around the central atom?

A

Trigonal planar

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15
Q

What is the bond angle in a trigonal planar molecule?

A

120

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16
Q

What shape name is given to a molecule with 5 electron pairs around the central atom?

A

Trigonal bipyramidal

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17
Q

What is the bond angle in a trigonal bipyramidal molecule?

A

There are actually two, 120 in one plane and 90 in the other plane.

18
Q

What shape name is given to a molecule with 6 electron pairs around the central atom?

A

Octahedral

19
Q

What is the bond angle in an octahedral molecule?

A

90

20
Q

What is electronegativity?

A

An atom’s ability to attract the electron pair in a covalent bond.

21
Q

How is electronegativity measured?

A

On the Pauling scale.

22
Q

What does a higher Pauling value mean?

A

A higher electronegativity and thus a greater attraction for an electron pair in a covalent bond.

23
Q

What makes a bond polar?

A

Two atoms with electronegativities differing by at least 0.5, the bonding electrons are pulled towards the more electronegative atom, making it polar.

24
Q

What does a polar bond form?

A

A permanent dipole

25
Q

How is a dipole formed?

A

A difference in charge between the two atoms, caused by a shift in electron density in the bond.

26
Q

Are diatomic gases polar or non-polar, why?

A

Non-polar

Because the atoms have equal electronegativities and so the electrons are equally attracted to both nuclei.

27
Q

What happens if the polar bonds are arranged symmetrically in a molecule?

A

The dipoles cancel each other out, so the molecule has no overall dipole and is non-polar.

28
Q

What happens if the polar bonds are arranged unsymmetrically in a molecule?

A

Uneven distribution of charge and the molecule will have an overall dipole.

29
Q

What can electronegativity be used to predict about bonding?

A

Whether it will be covalent or ionic bonding.

30
Q

What is the general strength of intermolecular forces?

A

Very weak

31
Q

What is the order of the intermolecular forces from strongest to weakest?

A

Hydrogen bonding
Permanent dipole-dipole interactions
Induced dipole-dipole

32
Q

What are induced dipole-dipole forces?

A

Fluctuation in the electron density around a molecule creates an instantaneous dipole in a molecule. The instantaneous dipole induces a dipole in a neighbouring molecule.

33
Q

What is the effect of stronger induced dipole-dipole forces?

A

Higher boiling points

34
Q

Why are higher boiling points brought about?

A

Larger molecules have larger electron clouds so stronger induced dipole-dipole forces.
Boiling point is overcoming the intermolecular forces so stronger ones means a higher boiling point.

35
Q

How are permanent dipole-dipole interactions labelled?

A

With charges of delta+ and delta- on the atoms.

36
Q

When can hydrogen bonding occur?

A

When hydrogen is covalently bonded to fluorine, nitrogen or oxygen.

37
Q

Why are hydrogen bonds brought about?

A

Hydrogen has a high charge densiy and F, N and O are very electronegative. The bond is so polarised that a weak bond forms between the hydrogen of one molecule and a lone pair on a neighbouring molecule’s F, N or O.

38
Q

State and explain two anomalous properties of water.

A
  • Ice is less dense than liquid water because hydrogen bonds hold molecules apart in open lattice structure
  • Higher melting and boiling points than expected because a lot of energy is needed to overcome hydrogen bonds
39
Q

Explain why simple molecular compounds have low melting and boiling points.

A

Weak intermolecular forces are broken by the energy present at low temperatures

40
Q

Explain why simple molecular compounds do not usually dissolve readily in water.

A

There is little interaction between the molecules in the lattice and the polar solvent molecules

41
Q

How do intermolecular forces explain simple covalent compounds not conducting electricity?

A

Overall covalent molecules are uncharged, permanent dipoles are not strong enough.

42
Q

What happens to nuclear charge and atomic radius across the periodic table?

A
  • The nuclear charge increases

- The atomic radius decreases