Predicting Physical Properties of Ionic Compounds and Metals

Question 1

Formation of Ionic Bonds

Describe how ionic bonds are formed in terms of cations and anions.

Answer 1

Electrons are transferred from the metal atom to the non-metal atom to form ions that come together in an ionic compound.
The metal atom becomes a cation, and the non-metal atom becomes an anion, and as oppositely-charged ions attract, this forms a three-dimensional giant ionic lattice.

Question 2

Dot-and-cross diagrams of ionic compounds

What are the rules for drawing dot-and-cross diagrams for ionic compounds?

Answer 2

The metal atom, who becomes a cation, should not have any dots or crosses. e.g 2 [Na]+.
The non-metal atom, who becomes an anion, should be clear on which electrons were transferred from the metal atom. This is shown through using a different way of expression (dot/cross).

Question 3

Physical Properties of Ionic Compounds

What are the physical properties of ionic compounds?

Answer 3

1. High Melting and Boiling Points
2. Soluble in Water
3. Conducts electricity in molten and aqueous states
4. Hard, Rigid and Brittle

Question 4

Physical Properties of Ionic Compounds

Why do ionic compounds have high melting and boiling points?

Answer 4

Ionic Compounds have a giant ionic lattice structure. In melting, large amounts of energy is supplied to overcome the strong electrostatics forces of attraction between the oppositely-charged ions to break the giant ionic lattice structure and form free ions.

Question 5

Physical Properties of Ionic Compounds

What are the factors influencing the strength of ionic bonds?

Answer 5

1. Effect of ionic charge: The greater the charge of ions, the stronger the ionic bonds. (e.g Al2O3 has stronger ionic bonds compared to Na2O)
2. Effect of ionic size: The bigger the ions, the greater the ionic radii, and the weaker the ionic bonds. Electrons are at a greater distance from the nucleus, thus electrostatic forces of attraction are weaker.

Question 6

Physical Properties of Ionic Compounds

When comparing the melting/boiling point of ionic compounds, what is necessary in the question.

Answer 6

Comparison: Similarity + Difference.
E.g (NaCl, and MgO)

Both ionic compounds have a giant ionic lattice structure. (similarity) However, Mg2+ and O2- have greater charge and smaller ionic radii as compared to Na+ and Cl-, hence the strength of ionic bonds in MgO is greater than that in NaCl. As such, MgO has a higher melting/boiling point, as more energy is required to overcome the stronger electrostatic forces of attraction between the oppositely charged Mg2+ and O2- ions.

Question 7

Physical Properties of Ionic Compounds

Why are ionic crystals soluble in water?

Answer 7

When an ionic salt dissolves, each ion on the crystal’s surface is attracted to the oppositely-charged poles of the polar water molecules, and the ions become hydrated. This hydration process (or solvation) releases energy, overcoming the strong electrostatic forces of attraction between oppositely-charged ions in the ionic crystal, and hence the solid breaks down,

Question 8

Physical Properties of Ionic Compounds

Why are ionic compounds able to conduct electricity only in the aqueous/molten state?

Answer 8

Ionic Compounds do not conduct electricity in the solid state as the ions are fixed in position and are unable to move due to the strong electrostatic forces of attraction. However, in the aqueous or molten states, the ions are mobile, and hence are free to move and are able to flow through the oppositely-charged electrodes acting as mobile charge carriers, carrying a current.

Question 9

Physical Properties of Ionic Compounds

Why are ionic compounds hard, rigid and brittle?

Answer 9

In an ionic lattice, oppositely charged ions are held in fixed positions by strong electrostatic forces of attraction throughout the lattice. Moving the ions out of position therefore requires a large amount of energy to break these bonds. Ionic Compounds are therefore extremely hard and rigid.

Ionic compounds are also brittle. If enough pressure is applied, such as by knocking or cutting, ions of like charges are brought next to each other. Repulsion between the ions of like charges causes the lattice to shatter apart. Ionic lattices are therefore quite brittle.

Question 10

Metallic Bonding and Physical Properties of Metals

What is metallic bonding?

Answer 10

Metals have a giant metallic lattice structure, which is held together by metallic bonding. Metallic bonding refers to the strong electrostatic forces of attraction between a lattice of metal cations and a sea of delocalised electrons.

Question 11

Metallic Bonding and Physical Properties of Metals

How to determine the stength of metallic bonding?

Answer 11

1. Number of delocalised electrons within the structure. The larger the number of valence electrons contributed per atom, the greater the number of delocalised electrons and hence the stronger the metallic bonding.
2. Charge and radius of the metal cation. The higher the charge and the smaller the radius of the metal cation, the greater the electrostatic forces of attraction between the cations and the sea of delocalised electrons.

Question 12

Metallic Bonding and Physical Properties of Metals

What are the physical properties of metals?

Answer 12

1. High Melting and Boiling Points
2. Conducts Electricity
3. Good Conductor of Heat
4. Malleable and Ductile
5. Shiny Surface
6. Hard

Question 13

Metallic Bonding and Physical Properties of Metals

Why do metals have a high melting and boiling point?

Answer 13

Steps:
1. Describe the structure and the type of bond
2. State the strength of the bond
3. Explain the relationship between bond and physical property

Metals have a giant metallic lattice structure which consists of a lattice of metal cations and a sea of delocalised electrons, and the electrostatic forces of attraction between the cations and the electrons constitue the metallic bonds.
Melting and boiling points of metals are relatively high as a larger amount of energy is required to overcome the strong electrostatic forces of attraction between the cations and the delocalised electrons.

Question 14

Metallic Bonding and Physical Properties of Metals

Why are metals good electrical conductors?

Answer 14

Metals are good electrical conductors due to the sea of delocalised electrons which act as mobile charge carriers are free to move about in the solid structure.

Question 15

Metallic Bonding and Physical Properties of Metals

Why are metals good heat conductors?

Answer 15

Metals are good heat conductors due to the presence of mobile delocalised electrons. The delocalised electrons disperse the heat energy more quickly from a region of higher temperature to a region of lower temperature.

The more energetic electrons collide with and speed up the slower, less energetic electrons, thereby allowing for good thermal conductivity.

Question 16

Metallic Bonding and Physical Properties of Metals

Why are metals malleable and ductile?

Answer 16

When an external force is applied, the layers of metal ions slide past each other and end up in new positions. The overall shape changes but the metal does not break as the sea of delocalised electrons prevent repulsions between the cations as they move past one another.
The metallic bond strength thus remains the same.

Question 17

Alloys and Their Uses

How does alloying make metals harder?

Answer 17

Alloys are mixtures of metals involving incorporation of small quantities of other elements into the pure metal.
Alloying makes metals harder, as the ions of the added metal have a different size from the main metal ions. This disrupts the arrangement of the main metal ions, so that the layers of main metal ions are no longer able to slide past each other easily when an external force is applied.