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Chem 1: Elements of Life > EL5: Shapes of Molecules > Flashcards

EL5: Shapes of Molecules Flashcards

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1
Q

What does VSEPR stand for?

A

Valence Shell Electron Pair Repulsion Theory

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2
Q

Define VSEPR

A

The shape adopted by a simple molecule/ion to keep its repulsive forces at a minimum

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3
Q

How are molecules held together?

A

Covalent bonds

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4
Q

Why does each covalent bond repel the other?

A

Covalent bonds consist of pairs of electrons

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5
Q

Why do bonds push each other as far apart as possible?

A

To reduce the repulsive forces between their pairs of electrons

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6
Q

Why are bonds equally spaced?

A

Because repulsions between each bond are equal

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7
Q

What bond angle does a linear molecule have?

A

180 degrees

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8
Q

How many bond pairs does a linear molecule have?

A

2

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9
Q

Define a linear molecule

A

An atom/ion where two groups of electrons surround its centre

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10
Q

Give 3 examples of linear molecules

A
  • BeCl2
  • CO2
  • Ethyne
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11
Q

What bond angle does a trigonal planar/planar triangular molecule have?

A

120 degrees

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12
Q

How many bond pairs does a trigonal planar/planar triangular molecule have?

A

3

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13
Q

Define a trigonal planar/planar triangular molecule

A

An atom/ion where three groups of electrons surround its centre

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14
Q

Give three examples of trigonal planar/planar triangular molecules

A
  • Ethene
  • Methanol
  • BF3
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15
Q

What bond angle does a tetrahedral molecule have?

A

109.5 degrees

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16
Q

How many bond pairs does a tetrahedral molecule have?

A

4

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17
Q

Define a tetrahedral molecule

A

An atom or ion where four groups of electrons surround its centre

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18
Q

Give three examples of tetrahedral molecules

A
  • Methane
  • Ethane
  • Ammonium ion
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19
Q

What bond angles does a trigonal bipyramidal molecule have?

A

120 or 90 degrees (depending on the position in the molecule)

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20
Q

How many bond pairs does a trigonal bipyramidal molecule have?

A

5

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21
Q

Define a trigonal bipyramidal molecule

A

An atom/ion where five groups of electrons surround its centre

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22
Q

Give an example of a trigonal bipyramidal molecule

A

PF5

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23
Q

What bond angle does an octahedral molecule have?

A

90 degrees

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24
Q

How many bond pairs does an octahedral molecule have?

A

6

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25
Q

Define an octahedral molecule

A

An atom/ion where six groups of electrons surround its centre

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26
Q

Give one less common example of an octahedral molecule

A
  • SF6
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27
Q

Where are octahedral shapes usually found?

A
  • In the shapes of complexes of metal ions with six ligands e.g Ni(NH3)6
28
Q

Why are shapes sometimes slightly distorted away from their regular shapes in a molecule/ion?

A

If the molecule/ion has lone pairs on the central atom, extra repulsion caused by them results in distortion of the regular shape

29
Q

Order the following from the causing the most repulsion to causing the least

  • Lone Pair to Bond Pair
  • Bond Pair to Bond Pair
  • Lone Pair to Lone Pair
A

Lone Pair to Lone Pair > Lone Pair to Bond Pair > Bond Pair to Bond Pair

30
Q

How much does each lone pair reduce the bond angle by?

A

2.5 degrees

31
Q

How many groups of electrons does a trigonal based pyramid molecule have?

A

4

32
Q

What types of electron groups does a trigonal based pyramid molecule have?

A
  • 3x single covalent bonds

- 1x lone pair of electrons

33
Q

What bond angle does the trigonal based pyramid shaped molecule have?

A

107 degrees

34
Q

What is the main example of a trigonal based pyramid molecular shape?

A

Ammonia

35
Q

What causes ammonia to have a trigonal based pyramid molecular shape?

A

The lone pair in nitrogen repulses the hydrogen-nitrogen bond pairs, pushing them closer together

36
Q

What shape is the trigonal based pyramid and bent/v-shaped shape based on?

A

Tetrahedral shape

37
Q

How many groups of electrons does a bent/v-shaped molecule have?

A

4

38
Q

What types of groups of electrons does a bent/v-shaped molecule have?

A
  • 2x single covalent bonds

- 2x lone pair of electrons

39
Q

What is the main example of a bent/v-shaped molecule?

A

Water

40
Q

Why does water have a bent/v-shaped molecular shape?

A

The lone pair to lone pair repulsions of the two lone pairs in the oxygen pushes the Oxygen-Hydrogen bonds together

41
Q

Define “covalent bond”

A

A very strong attraction between the bonding pair of electrons and the nucleui of the atoms involved in the bond

42
Q

Define a “bonding pair”

A

An electron pair that forms a bond in a covalent structure

43
Q

Define a “lone pair”

A

Pairs of electrons uninvolved in the bonding of a covalent structure

44
Q

Define “bond”

A

The electrostatic attraction between atoms/ions in a structure

45
Q

Define “electrostatic attraction”

A

A strong electrical force of attraction

46
Q

What does bonding involve?

A

The sharing or transferring of electrons in the highest occupied electron shells

47
Q

What do all substances “want” to become?

A

Ions with a full outer shell

48
Q

Why do substances want a full outer shell?

A

As this is the most energetically stable arrangement

49
Q

What group of elements have the most energetically stable arrangements naturally and why?

A

The noble gases, by the balancing of electrical forces in the atom due to the equal number of protons and electrons

50
Q

Define “electronegativity”

A

The ability of an atom to attract the electron pair in a covalent bond to itself

51
Q

Explain why a non polar bond is non polar.

A

A bond with similar atoms - so they have the same electronegativity so they will both pull on the electrons to the same extent and be equally shared

52
Q

Explain why a polar bond is polar.

A
  • A bond with different atoms - so they have different electronegativity
  • Therefore one will pull the electron pair closer to its end
  • It will be slightly more negative than the overall bond (δ−)
  • The other atom will be slightly less negative - more positive (δ+)
  • A dipole is formed and the bond is said to be polar
53
Q

How does the electronegativity of the atoms in a polar bond relate to its polarity?

A

The greater the difference in electronegativity between the two atoms in the bond, the greater the bond’s polarity

54
Q

What is the Pauling Scale?

A

A scale for measuring electronegativity

55
Q

What is a non-polar covalent bond between?

A

The same/very similar non metals e.g Cl2

56
Q

What is a polar covalent bond between?

A

Different non-metals e.g H2O

57
Q

Order the following in order of increasing polarisation:

  • Polar covalent
  • Metallic
  • Non - polar covalent
  • Ionic
A
  • Non - polar covalent
  • Polar covalent
  • Ionic
  • Metallic
58
Q

What is a dot and cross diagram used to show?

A

The bonding in covalent structures and represent the way they bond together

59
Q

What do line diagrams show?

A

They show a pair of electrons being shared between two atoms for every line in the diagram

60
Q

Define a “co-ordinative dative bond”

A

Both electrons of a shared pair are provided by one species (element)

61
Q

Describe the bonding of a CO molecule

A
  • Involves a triple bond
  • 2 of the pairs of electrons are formed by the carbon and oxygen atoms each contributing one electron to the pair as in a normal covalent bond
  • Both electrons in the third pair come from the oxygen atom
62
Q

Describe the bonding of an ammonium ion (NH4+)

A
  • The lone pair N is used to share with the hydrogen ion which needs two electrons to fill its outer shell
  • The N now has a positive charge as its now sharing rather than owning two electrons
63
Q

Describe the bonding in boron trifluoride - ammonia (NH3BF3)

A
  • Boron has an incomplete shell in BF3 and can accept a share of a pair of electrons donated by ammonia
  • The B becomes negative as it now shares a pair of electrons that it didn’t have before
64
Q

What are the electrical properties of covalent substances and why?

A

They do not conduct electricity because they have no mobile ions/electrons

65
Q

Are covalent substances soluble in water and why?

A
  • No they tend to be more so in organic solvents

- The polar water molecules are more attracted to each other than the molecular substance and so do not react with it

66
Q

Why do covalent bonds have a low melting point?

A

Because they have weak intermolecular forces which means little energy is needed to separate molecules from each other

67
Q

Why does CH4 have a lower melting point than C2H6?

A

Because the molecule has a smaller surface area

Chem 1: Elements of Life (5 decks)

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