CHAPTER 6: SHAPES OF MOLECULES AND INTERMOLECULAR FORCES

Question 1

What is the electron-pair repulsion theory?

Answer 1

The electron pairs surrounding a central atom determine the shape of the molecule or ion. The electron pairs repel one another so that they are arranged as far apart as possible. The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape.

Question 2

What do lone pairs do more than bonded pairs?

Answer 2

Lone pairs repel more strongly than bonded pairs, as they lie slightly closer to the atom and occupy more space. Lone pairs reduce the bond angle by 2.5 degrees each.

Question 3

What are the features of a tetrahedral shape?

Answer 3

4 bonded pairs, no lone pairs, 109.5 degree bond angle. Ex. CH4

Question 4

What are the features of a pyramidal shape?

Answer 4

3 bonded pairs, 1 lone pairs, 107 degree bond angle. Ex. NH3

Question 5

What are the features of a non-linear shape?

Answer 5

2 bonded pairs, 2 lone pairs, 104.5 degree bond angle. Ex. H2O

Question 6

What are the features of a linear shape?

Answer 6

2 bonded pairs, 0 lone pairs, 180 degree bond angle. Ex. CO2

Question 7

What are the features of a trigonal planar shape?

Answer 7

3 bonded pairs, 0 lone pairs, 120 degree bond angle. Ex. BF3

Question 8

What are the features of an octahedral shape?

Answer 8

6 bonded pairs, 0 lone pairs, 90 degree bond angle. Ex. SF6

Question 9

What is the definition of electronegativity?

Answer 9

Electronegativity is an element’s ability to attract electrons in a covalent bond.

Question 10

Why might electronegativity may be different?

Answer 10

The electrons are shared unevenly. This may be because the nuclear charges are different, the atoms may be different sizes or the shared pair of electrons may be closer to one nucleus than the other.

Question 11

How is electronegativity measured?

Answer 11

The Pauling scale.

Question 12

Why is fluorine the most electronegative element?

Answer 12

Because across the periodic table, the nuclear charge increases, and upwards the atomic radius decreases.

Question 13

How does electronegativity determine if a bond is ionic or covalent?

Answer 13

If the electronegativity difference is large, one bonded atom will have
a much greater attraction for the shared pair than the other bonded
atom. The more electronegative atom will have gained control of the
electrons and the bond will now be ionic rather covalent.

Question 14

What is a non-polar bond?

Answer 14

When the bonded electron pair is shared equally. It will be non-polar if the bonded atoms are the same, or have the same/similar electronegativity.

Question 15

What is a polar bond?

Answer 15

In a polar bond, the bonded electron pair is shared unequally between the bonded atoms. This causes a small partial positive bond, and a small partial negative bond. This causes a permanent dipole. A bond will be polar when the bonded atoms are different and have different electronegativity values.

Question 16

What are intermolecular forces?

Answer 16

Weak interactions between dipoles of different molecules.

Question 17

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

Answer 17

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

The movement of electrons produces a changing dipole in a molecule.

At any instant, an instantaneous dipole will exist, but its position is constantly shifting.

The instantaneous dipole induces a dipole on a neighbouring molecule.

The induced dipole induces further dipoles on neighbouring molecules, which then attract one another. These are only temporary.

Question 18

What are simple molecular substances?

Answer 18

A substance made out of simple molecules (covalent bonds). In a solid state the molecules are held in place by weak intermolecular forces and the atoms within each molecule are bonded together strongly by covalent bonds.

Question 19

Why do simple molecular substance have a low melting and boiling points?

Answer 19

Simple molecular substances can be solidified into simple molecular lattices by reducing the temperature. In a simple molecular lattice, the weak intermolecular forces can be broken even by the energy present at low temperatures so simple molecular substances have low melting and
boiling points.

When a simple molecular lattice is broken apart during melting, only the weak intermolecular forces break and the covalent bonds are strong and do not break.

Question 20

Are non-polar simple molecular substances soluble?

Answer 20

When a simple molecular compound is added to a non-polar solvent, intermolecular forces form between the molecules and the solvent. The interactions weaken the intermolecular forces in the simple molecular lattice. The intermolecular forces break and the compound dissolves. Therefore, non-polar simple molecular substances tend to be soluble in non-polar solvents.

When a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules.
The intermolecular bonding within the polar solvent is too strong to be broken. Therefore simple molecular substances tend to be insoluble in polar solvents.

Question 21

Are polar simple molecular substances soluble?

Answer 21

Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other. The solubility depends on the strength of the dipole and can be hard to predict.

Question 22

Can simple molecular substances conduct electricity?

Answer 22

There are no mobile charged particles in simple molecular structures. With no charged particles that can move, there is nothing to complete an electrical circuit. Therefore simple molecular structures are non-conductors of electricity.

Question 23

What is a hydrogen bond?

Answer 23

A special type of permanent dipole-dipole interaction found between molecules containing: an electronegative atom with a lone pair of electrons, and a hydrogen atom attached to an electronegative atom, for example, HO, H N, or H-F.

The hydrogen bond acts between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule. Hydrogen bonds are the strongest type of intermolecular attractions.

Question 24

Why is ice less dense than water?

Answer 24

The hydrogen bonds extend outwards, holding water molecules slightly apart and forming an open tetrahedral lattice full of holes. The bond angle is close to 180°. The holes in the open lattice structure decrease the density of water on freezing, When ice melts, the ice lattice collapses and the molecules move closer together. So liquid water is denser than solid ice.

Question 25

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

Answer 25

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. When the ice lattice breaks, the rigid arrangement of hydrogen bonds in ice is broken. When water boils, the hydrogen bonds break completely.