1. REVISION 1 - Chemical Bonding & Structure Properties of Materials ANSWERS

Pei Hwa Secondary School 4N Science (Chemistry) Revision 1 Chp 4 & 5: Chemical Bonding & Structure and Properties of Materials Name: Class / TG: Date: Ionic Bonding: • Transfer of electron(s) from metal atom(s) to non-metal atom(s). • Structure: giant ionic lattice structure “Dot and cross” diagram of ionic compounds 1. lithium chloride 2. sodium oxide Properties of ionic compound: Properties Explanation 1. High melting and boiling point Large amount of energy is needed to overcome Strong electrostatic forces of attraction between oppositely charged ions Held in a giant lattice structure 2. Poor conductor of electricity in solid state Good conductor of electricity in aqueous or liquid (molten) state Ions are held in fixed positions and are not mobile. Giant lattice structure has broken down and ions are mobile to act as charge carriers

3. Hard but brittle 4. Usually soluble in water. Usually insoluble in organic solvent Covalent Bonding: • sharing of electrons between non-metal atoms. • Structure: simple covalent molecule Drawing of covalent molecules using “dot and cross”: 1. A molecule of fluorine, F2 2. A molecule of nitrogen, N2 3. A molecule of ammonia, NH3 4. A molecule of carbon dioxide,CO2 Properties of covalent molecules Properties Explanation low melting and boiling point A small amount of energy is needed to overcome Weak intermolecular forces of attraction between molecules held In simple molecular structure Poor conductor of electricity in all states No mobile ions or electrons to act as charge carriers Usually insoluble in water Usually soluble in organic solvent

Metals and Alloys: • High melting point and boiling point. Alloys, being a mixture, melt over a range of temperatures. • Good conductor of heat • Good conductor of electricity due to sea of delocalised electrons that are mobile throughout the giant metallic lattice structure. • Alloys are harder and stronger than the pure metals. In a pure metal, atoms are of the same size and packed regularly in neat rows. Layers of atoms slide over one another easily when a force is applied. Hence, pure metals are too malleable / ductile In an alloy, atoms are of different sizes, hence the regular arrangement of atoms is disrupted. Layers of atoms cannot slide easily over each other when a force is applied, and so an alloy is harder and stronger. force applied metal atoms of the same size force applied

Practice Questions 1 Which two elements combine to form an ionic compound? A calcium and fluorine B hydrogen and carbon C hydrogen and chlorine D sulfur and oxygen 2 (a) When atoms combine with other atoms, they either gain, lose or share a number of electrons. Showing only valence electrons, draw ‘dot and cross’ diagrams to show the electronic structure of (ii) oxygen gas; [2] (ii) magnesium chloride. [2] (b) Magnesium ribbon burned in oxygen to form a white solid, magnesium oxide. Write a balanced chemical equation, including state symbol s for the reaction. [2] 2 Mg (s) + O2 (g) → 2MgO (s)

(c) The physical properties of different compounds are shown in the table below. name of substance boiling point (°C) magnesium oxide 3600 oxygen gas –183 Explain, in terms of bonding, the difference in boiling points between magnesium oxide and oxygen gas. MgO has a higher melting point than oxygen gas. Larger amount of energy is needed to overcome strong electrostatic forces of attraction between oppositely charged ions in giant lattice structure of MgO. [1] Less energy is needed to overcome weak intermolecular forces of attraction between simple molecules in O2 gas. [1] [2]