1.5 Solid Structures

Question 1

Ionic solids are made up of oppositely charged ions packed around each other.

why

Answer 1

This maximises electrostatic attraction between the oppositely charged ions and minimises repulsion between ions with the same charge, ensuring maximum bond energy.

Question 2

sodium chloride

giant ionic lattice

Answer 2

In sodium chloride, each Na+ cation is surrounded by six Cl- anions and each Cl- anion is surrounded by six Na+ cations. For this reason, we say the crystal coordination number for sodium chloride is 6:6.

Question 3

caesium chloride

Answer 3

In caesium chloride, the number of oppositely charged ions surrounding each ion is eight. For this reason, we say the crystal coordination number for caesium chloride is 8:8. The difference in the crystal coordination numbers between the two structures is because the Cs+ cation is larger than that of Na+, so has more room to accommodate a greater number of anions.

Question 4

Answer 4

Question 5

The properties of ionic compounds are determined by

Answer 5

the strength of the electrostatic forces between the ions.

Question 6

General properties of ionic compounds

Answer 6

high melting points and boiling points due to the strong electrostatic attraction between the oppositely charged ions, which requires a lot of energy to overcome.

electrical insulators as solids, but when molten or dissolved in water they conduct due to the ions being free to move and carry a charge when voltage is applied.

brittle due to repulsion between like charged ions when the layers of ions are shifted by a large enough force.

Certain ionic compounds are soluble in water (for example, all Group 1 compounds, all nitrate compounds and most halide compounds). When the solid dissolves, the ions are surrounded by water molecules, which are attracted to the ions due to the water molecules having a permanent dipole.

Question 7

Answer 7

Question 8

diamond and graphite are

Answer 8

allotropes

Question 9

define allotrope

Answer 9

Allotropes are different molecular or crystalline forms of the same element, resulting in different physical properties.

Question 10

diamond

Answer 10

giant covalent structure

In diamond, each carbon bonds strongly to four other carbon atoms in a tetrahedral arrangement to form a giant 3D structure.

Question 11

graphite

Answer 11

giant covalent structure

in graphite only three bonds are made by each carbon atom. Hexagonal layers (only one atom thick) are formed, which are held together by weak van der Waals forces.

Question 12

Answer 12

Question 13

Properties shared by both diamond and graphite

Answer 13

high melting points and boiling points as each carbon atom has three or four strong covalent bonds, which require a lot of heat energy to overcome.

insoluble in water as there are no charged particles capable of interacting with the permanent dipole of water molecules.
Properties specific to dia

Question 14

Properties specific to diamond

Answer 14

hard (i.e., difficult to scratch) due to each carbon atom being bonded to four others, with strong covalent bonds. This holds the atoms together in a rigid 3-dimensional structure.

electrical insulator, due to there being no delocalised electrons within the structure.

Question 15

Properties specific to graphite

Answer 15

soft and slippery due to the layers of carbon atoms (which are only attracted to each other with weak intermolecular forces) being able to slide over each other easily.

electrical conductor as there is one non-bonding electron in the valence shell of each carbon atom and these become delocalised between the layers.

Question 16

Answer 16

Question 17

Simple covalent structures

Answer 17

consist of simple covalent molecules held together in a lattice structure by weak intermolecular forces. See Topic 4 for more details on the different types of intermolecular forces.

Question 18

In iodine, the iodine molecules (I2) are held together by van der Waals forces. VS Cl2

Answer 18

The van der Waals forces between I2 molecules are relatively strong compared to those between Cl2 molecules. This is because the strength of the van der Waals force increases with molecular size.

Question 19

why is ice less dense than water

Answer 19

These strong intermolecular forces mean that the H2O molecules in ice form a tetrahedral structure that is both rigid and spaced out. Due to this unusual solid structure, ice is less dense and takes up a greater volume than liquid water.

Question 20

Answer 20

Question 21

Despite being very different substances, both iodine and ice have certain properties in common due the fact that they both have simple covalent structures. These properties include the following:

Answer 21

low melting and boiling points due to the weak intermolecular forces. These forces require very little heat energy to overcome.

soft due to the weak intermolecular forces, which require only a small force to break.

electrical insulators due to the lack of any mobile charged particles.

Question 22

solubility of iodine

Answer 22

iodine is only sparingly soluble in water as the van der Waals forces between I2 molecules are not strong enough to break up the hydrogen bonding between water molecules.

Question 23

Answer 23

Question 24

Metallic structures

Answer 24

Structure of Metals:

Metals consist of a regular lattice of metal cations (positive ions).

These cations are surrounded by a ‘sea’ of delocalised electrons.

Formation of Delocalised Electrons:

Valence (outer) electrons from metal atoms become delocalised, meaning they are not bound to any specific atom and can move freely throughout the structure.

Bonding:

Strong electrostatic forces exist between the positively charged metal cations and the negatively charged delocalised electrons.

This is known as metallic bonding.

Question 25

Due to the similar structure shared by all metals, all metals have similar properties. These properties include the following:

Answer 25

**good conductors** of heat and electricity, as the delocalised electrons can carry energy (either thermal or electrical) through the lattice of metal ions. **malleable** (able to change shape permanently without breaking), as the layers of metal cations can easily slide over each other and the delocalised electrons move with the ions to maintain the metallic bonding.

Question 26

metals bp, mp and hardness depends on

Answer 26

The melting and boiling points of metals depend on the number of delocalised electrons per atom. Generally, the more delocalised electrons per atom, the higher the melting and boiling points. This also applies to the hardness of the metal. Generally, the more delocalised electrons per atom, the harder the metal.

Question 27

Answer 27