Shapes of molecules and intermolecular forces

Question 1

What is electron pair repulsion theory?

Answer 1

Electron pairs repel each other and take up space around the molecule as far apart as possible.
Lone pairs repel more than bonding pairs.
Bond angles between bonding pairs are often reduced because they are pushed together by lone pair repulsion.

Question 2

What is linear shape?

Answer 2

Molecules with two electron pairs have a bond angle of 180° and is linear.

Question 3

What is trigonal planar shape?

Answer 3

Molecules with 3 electron pairs around the central atom have a 120° bond angle, and is trigonal planar.

Question 4

What is non-linear?

Answer 4

If there are two bonding pairs and one lone pair, the shape is non-linear.
The bond angle will be 117.5°.

Question 5

What is tetrahedral shape?

Answer 5

If there are 4 bonding electrons, all the bond angles are 109.5°, and is tetrahedral.

Question 6

What is pyramidal?

Answer 6

If there are 3 bonding pairs and one lone pair, the bond angle is 107°, and trigonal pyramidal.

Question 7

What is trigonal bipyramidal?

Answer 7

Some central atoms expand the octet, and so have more than 8 bonding electrons.
5 bonding pairs is trigonal bipyramidal, and 3 atoms have 120°, and 2 have 90° between them.

Question 8

What is octahedral?

Answer 8

A molecule with 6 bonding pairs are 90° apart and octahedral.

Question 9

What is electronegativity?

Answer 9

The ability of an atom to attract the bonding electrons in a covalent bond.

Question 10

What is the Pauling Scale?

Answer 10

The measure of electronegativity.
A higher number means an element is better able to attract the bonding electrons.

Question 11

What is the trend of electronegativity?

Answer 11

Increases across periods and decreases down groups.
Fluorine is the most electronegative.
Oxygen, nitrogen and chlorine are also very strongly electronegative.

Question 12

Why does electronegativity increase across the periods?

Answer 12

The number of protons (nuclear charge) increases so attraction between the nucleus and shared electrons increases.
The atomic radius decreases.
Shielding stays the same.
So electronegativity increases.

Question 13

Why does electronegativity decrease down the groups?

Answer 13

Atomic radius increases.
Shielding increases.
The nuclear charge increases but the effect is shadowed.
So electronegativity decreases.

Question 14

What is shielding of electrons?

Answer 14

The nuclear charge is shielded by electrons in the inner shells.
This weakens attraction.

Question 15

What are polar bonds?

Answer 15

The electrons are shared unequally.
An electronegative atom is bonded to a less electronegative atom.

Question 16

What are non-polar bonds?

Answer 16

Electrons are shared equally.
The covalent bonds in diatomic gases are non-polar as the atoms have equal electronegativities so the electrons are equally attracted to both nuclei.
E.g. H, Cl ,O
H - C bonds have very similar electronegativities so are non-polar.

Question 17

What are permanent dipoles?

Answer 17

In a polar bond, the difference in electronegativity between the two atoms causes a permanent dipole.
A dipole is a difference in charge between the two atoms caused by a shift in electron density in the bond.
The greater the difference in electronegativity, the more polar the bond.

Question 18

What are polar molecules?

Answer 18

Polar molecules have an overall dipole.
In simple molecules, the one polar bond causes a single permanent dipole, which gives the molecule an overall dipole.

Question 19

What are non-polar molecules?

Answer 19

If the polar bonds are arranged symmetrically, so that the dipoles cancel each other out e.g. in CO2, the molecule has no overall dipole and is non-polar.

Question 20

How can bond types be predicted?

Answer 20

The greater the difference in electronegativities between the two elements the less covalent and more ionic the bond will be.
Bonds where the electronegativity difference is less than 0.4 will be non-polar covalent bonds.
The electronegativity difference within 0.4-2.0 means the bonds will be polar but mainly covalent.
A difference more than 2 means the bonding is mainly ionic.

Question 21

What are intermolecular forces?

Answer 21

Forces between molecules, they’re weaker than covalent, ionic or metallic bonds.
Are induced dipole-dipole interactions, permanent dipole-dipole interactions, and hydrogen bonding.

Question 22

What are induced dipole-dipole interactions?

Answer 22

London forces are weak intermolecular forces between all molecules:
Movement of electrons produces a changing dipole in a molecule.
At any time, an instantaneous dipole will exist, but its position is constantly shifting.
This instantaneous dipole induces a dipole on a neighbouring molecule.
The induced dipole induces further dipoles on neighbouring molecules, which then attract one another.

Question 23

How does the strength of induced dipole interactions vary?

Answer 23

The greater the number of electrons, the stronger the attraction, and so the greater the energy needed to separate the particles.
Molecules with a greater surface area have more strength, as they have a more exposed electron cloud.

Question 24

What are permanent dipole-dipole interactions?

Answer 24

The small S+ and S- charges on polar molecules cause weak electrostatic forces of attraction between molecules.
They happen in addition to induced dipole interactions.

Question 25

What is the strength of permanent dipole interactions?

Answer 25

They are stronger than induced interactions, so require more energy to overcome - higher boiling and melting points.

Question 26

What is hydrogen bonding?

Answer 26

When hydrogen is covalently bonded to fluorine, nitrogen or oxygen.
Hydrogen has a high charge density due to being small, and F, N and O are very electronegative.
The bond is polarised so a weak bond forms between the hydrogen of one molecule and a lone pair on the other element.

Question 27

What effect does hydrogen bonding have on substances?

Answer 27

They are soluble in water and have higher boiling and melting points than molecules of the same size that do not form hydrogen bonds.
Water and Ammonia have very high boiling points when comparing them with other hydrides.

Question 28

How does hydrogen bonding affect ice?

Answer 28

In ice, H2O molecules are held together in a lattice by hydrogen bonds.
When ice melts, hydrogen bonds are broken, so ice has more hydrogen bonds than liquid water.
Hydrogen bonds are long, so ice molecules will be further apart than in liquid water.
This makes ice less dense than water.

Question 29

What are the anomalous propertys of ice?

Answer 29

Ice has a relatively high melting point - because hydrogen bonds are relatively strong (stronger than London forces).
Ice is less dense than water - it has an open structure / the molecules in ice are held apart by hydrogen bonds.

Question 30

What is metallic bonding?

Answer 30

The electrostatic attraction between positive ions and delocalised electrons.

Question 31

What is the solubility of simple covalent compounds?

Answer 31

Water is a polar molecule, so only tends to dissolve other polar substances well.
Compounds with hydrogen bonds, e.g. ammonia, can form hydrogen bonds with water molecules, so are soluble.
Non-polar molecules, e.g. methane, are insoluble.

Question 32

What are simple molecular lattices?

Answer 32

Simple molecules - small units containing a definite number of atoms with a definite molecular formula - such as Ne, H2, H2O, CO2.
The molecules are held in place by weak intermolecular forces.
The atoms within each molecule are bonded together strongly by covalent bonds.

Question 33

What are the melting and boiling points of simple molecular lattices?

Answer 33

The weak intermolecular forces can be broken easily, so have low melting and boiling points.
When it is broken apart during melting, only the weak intermolecular forces break, the covalent bonds are strong and do not break.

Question 34

How do you identify the strongest intermolecular force?

Answer 34

Is there a polar bond?
- no - induced dipole interactions
- yes - do the dipoles cancel/ symmetrical?
- yes - induced dipole interactions
- no - is there a H directly bonded to N, O, or F?
- yes - hydrogen bonding
- no - permanent dipole-dipole interactions.