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A-Level Chemistry > 6 Shapes of molecules and intermolecular forces > Flashcards

6 Shapes of molecules and intermolecular forces Flashcards

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1
Q

What is the electron pair repulsion theory?

A
  • The electron pairs surrounding the central atom repel one another so that they are arranged as far apart as possible.
  • Lone pairs repel more strongly than bonded pairs.
  • Lone pairs repel bonded pairs slightly closer together decreasing the bond angle, by about 2.5 degrees for each lone pair.
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2
Q

What do the wedges mean?

A
  • A solid line represents a bond in the plane of the paper.
  • A solid wedge comes out of the plane of the paper.
  • A dotted wedge goes into the plane of the paper.
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3
Q

Which pair angles are the biggest?

A

1) Lone pair-lone pair angles are the biggest.
2) Lone pair-bonding pair angles are the second biggest.
3) Bonding pair-bonding pair bond angles are the smallest.

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4
Q

What is the shape of 2 electron pairs around the central atom?

A
  • Linear

- 180 degrees

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5
Q

What is the shape of 3 electron pairs around the central atom? (0 LP & 3 BP)

A
  • Trigonal planar

- 120 degrees

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6
Q

What is the shape of 4 electron pairs around the central atom? (0 LP & 4 BP)

A
  • Tetrahedral

- 109.5 degrees

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7
Q

What is the shape of 4 electron pairs around the central atom? (1 LP & 3 BP)

A
  • Trigonal pyramidal

- 107 degrees

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8
Q

What is the shape of 4 electron pairs around the central atom? (2 LP & 2 BP)

A
  • Non-linear

- 104.5 degrees

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9
Q

What is the shape of 5 electron pairs around the central atom? (0 LP & 5 BP)

A
  • Trigonal bipyramidal
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10
Q

What is the shape of 6 electron pairs around the central atom? (0 LP & 6 BP)

A
  • Octahedral

- 90 degrees

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11
Q

What is electronegativity?

A

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

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12
Q

Which element is the most electronegative element?

A

Fluorine.

F>O>Cl>N>C>H

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13
Q

What are non-polar bonds?

A
  • In a non-polar bond, the bonded electron pair is shared equally between the bonded atoms.
  • A bond will be non-polar when:
    1) The bonded atoms are the same.
    2) The bonded atoms have the same or similar electronegativity.
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14
Q

What are polar bonds?

A
  • In a polar bond, the bonded electron pair is shared unequally between the bonded atoms.
  • A bond will be polar when:
    1) The bonded atoms are different.
    2) The bonded atoms have different electronegativity values.
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15
Q

How are covalent bonds polarised?

A
  • In a covalent bond between two atoms of different electronegativities, the bonding electrons are pulled towards the more electronegative atom. This makes the bond polar.
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16
Q

What does the difference in electronegativity between two atoms cause?

A

Permanent dipoles.

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

17
Q

What determines the overall dipole?

A
  • The arrangement of polar bonds in a molecule determines whether or not the molecule will have an overall dipole.
  • If the polar bonds are arranged symmetrically so that the dipoles cancel each other out, then the molecule has no overall dipole and is non-polar.
  • But if the polar bonds are arranged so that they don’t cancel each other out, then charge is distributed unevenly across the whole molecule , and it will have an overall dipole and be polar.
18
Q

What are intermolecular forces?

A

Weak interactions between dipoles of different molecules.

19
Q

What are the three types of intermolecular forces?

A
  • Induced dipole-dipole interactions (London forces)
  • Permanent dipole-dipole interactions
  • Hydrogen bonding
20
Q

What are induced dipole-dipole interactions? (London forces)

A

London forces are weak intermolecular forces that exist between all molecules.

1) Movement of electrons produces a changing dipole in a molecule.
2) An instantaneous dipole is constantly changing.
3) The instantaneous dipole induces a dipole on a neighbouring molecule.
4) The induced dipole induces further dipoles on neighbouring molecules, which then attracts one another.

21
Q

What do induced dipoles result from?

A

1) Induced dipoles result from interactions of electrons between molecules.
2) The more electrons in each molecule:
- The larger the instantaneous and induced dipoles.
- The greater the induced dipole-dipole interactions.
- The stronger the attractive forces between molecules.
- The higher the boiling point as more energy is needed to overcome the intermolecular forces.

22
Q

Do polar or non-polar molecules have a higher boiling point?

A
  • Non-polar molecules only have London forces between them.
  • Polar molecules have London forces AND permanent dipole-dipole interactions between them.
  • Extra energy is needed to break the additional permanent dipole-dipole interactions in polar molecules.
  • Therefore the boiling point of polar molecules is higher.
23
Q

What is a simple molecular substance?

A

A simple molecular substance is made up of simple molecules.

24
Q

What are the molecules in a simple molecular lattice held in place by?

A

Weak intermolecular forces

25
Q

What are the atoms within each molecule in a simple molecular lattice bonded together by?

A

Covalent bonds

26
Q

What are the properties of simple molecular substances?

A

1) Low melting point and boiling point
2) Polar molecules are soluble in water
3) Simple molecular structures are non-conductors of electricity.

27
Q

What is a hydrogen bond?

A
  • The strongest type of intermolecular attractions.
  • Hydrogen bonding can only happen when hydrogen is covalently bonded to fluorine, nitrogen or oxygen.
  • It then forms a bond between an electronegative atom with a lone pair of electrons.
28
Q

Why does water have a relatively high melting point and boiling point?

A
  • Water has London forces between molecules.
  • Hydrogen bonds are extra forces, over and above the London forces.
  • An appreciable quantity of energy is needed to break the hydrogen bonds in water, so water has much higher melting and boiling points than would be expected from just London forces.
29
Q

Why is ice less dense than water?

A
  • Hydrogen bonds hold water molecules apart in an open lattice structure.
  • Each water molecule can form four hydrogen bonds.
  • The water molecules in ice are further apart than in water.
  • Therefore solid ice is less dense than liquid water and floats.

A-Level Chemistry (27 decks)

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