RI Y4 Chem 2022 FE revision Chemical Bonding ANS

1 Raffles Institution Raffles Programme Year Four Chemistry Name: ____________________________ ( ) Class: ______ Date: _______ 2022 FE Revision Chemical Bonding ANSWERS Elements may be classified as metals or non-metals. Ionic bonds are formed by the loss of electrons, usually from a metal atom to a non-metal atom. The metal atom forms a positive ion or cation. The non-metal atom gains electrons to form a negative ion or anion. In an ionic compound, strong electrostatic forces of attraction exist between the ions of opposite charges. A covalent bond forms when two atoms share a pair of electrons. Atoms of an element combine with other atoms to form ionic or covalent bonds. The atoms achieve a noble gas electronic configuration (e.g. in water: H, 2; O, 2,8) A metallic bond is the electrostatic force of attraction between positively charged metal ions and negatively charged delocalised outer shell electrons. Physical properties substance electrical conductivity ionic compound molten and aqueous states, the ions are mobile/freely moving and can therefore act as charge carriers. covalent element or compound (one exception – Graphite, which has delocalised electrons as charge carriers.) metallic element good conductors of electricity in the solid and liquid/molten states, delocalised electrons are the charge carriers. Covalent bonds and bond polarity A covalent bond may be polar or non -polar. A polar bond is formed between 2 atoms of different electronegativities. The more electronegative atom pulls the shared electrons toward itself, forming a δ− end of the bond. The opposite end is thus labelled δ+. The greater the difference in electronegativities between the 2 bonded atoms, the greater the polarity of the bond. Molecules with polar bonds may or may not be polar molecules. Polar molecules can interact with water molecules (which are polar). This explains why simple covalent substances made up of polar molecules (e.g. ammonia, NH 3) are soluble in water. A substance that is made up of molecules which are more polar will be more soluble in water because the molecules can form stronger attractions to the water molecules. Practice Questions 1 A 2 D 3 B 4 C 5 D 6 (a) D (b) C 7 C 8 (a) CaCO3(s) → CO2(g) + CaO(s) (b) (i)

2 (ii) For calcium oxide, strong electrostatic forces of attraction exist between calcium ions and oxide ions. The solid has a giant ionic structure. High heat is required to overcome the above strong attractions between the ions when melting. Calcium oxide has a high melting point. For carbon dioxide, weak intermolecular forces of attraction exist between carbon dioxide molecules. Solid carbon dioxide has a simple molecular/covalent structure. Hence, only low heat energy is needed to overcome the weak attractions between molecules. Carbon dioxide has a low melting point. Note: The strong covalent bonds between C and O atoms in each CO2 molecule are not broken. 9 (a) (b) Methane exists as simple or discrete molecules. Low heat is required to overcome the weak intermolecular forces of attraction. Hence, at room temperature, methane already exists as a gas. Sodium chloride is a giant ionic solid with strong electrostatic forces of attraction between ions of opposite charge. A lot of energy is needed to overcome these attractions, so at room temperature, it is a solid. (c) sodium hydride (i) ionic bonding (ii) 10 (a) Sodium chloride is made up ions that are held in fixed positions in a giant ionic lattice by strong electrostatic forces of attraction. The ions are not freely-moving or not mobile and cannot act a charge carriers. (b) S: 12 outer shell electrons (c) Dilute sulfuric acid is fully dissociated into ions in aqueous solution. So the ions are mobile and can acts as charge carriers to conduct electricity. H2SO4(aq) → 2H+(aq) + SO42−(aq) 11 (a) XCl2 liquid YCl2 solid (b) (i) XCl2 covalent bonding between X and Cl atoms, with weak intermolecular forces, between XCl2 molecules that are easily overcome by a small amount of energy, hence a low melting point. (ii) YCl2 ionic bonding, which are strong thus requiring a lot of energy to overcome, hence a high melting point. (c) (i) X is a non-metal, since it forms a covalent compound with chlorine (a non-metal). (ii) Y is a metal, since it forms an ionic compound with chlorine (a non-metal).