Chemical Bonding and Structure: notes

Ionic and Covalent Bonding

• Ionic bond: attraction between oppositely charged ions. A metal atom loses electrons and a non-metal atom gains them. Sodium gives one electron to chlorine, forming Na+ and Cl-.
• Ionic compounds: giant lattices with high melting points. They conduct when molten or dissolved, but not when solid, because the ions cannot move.
• Covalent bond: a shared pair of electrons between non-metal atoms. In water, H2O, oxygen shares one electron with each hydrogen.
• Simple molecules: such as Cl2 and CH4, have weak forces between molecules, so low melting points and no conduction.

Giant Covalent and Metallic Structures

• Diamond: each carbon bonds to four others. It is very hard, has a very high melting point and does not conduct.
• Graphite: each carbon bonds to three others in layers that slide, so it is soft. One free electron per carbon lets it conduct.
• Silicon dioxide: a giant covalent structure like diamond, found in sand.
• Metallic bond: attraction between positive metal ions and a sea of delocalised electrons. Metals conduct because electrons move freely, and are malleable because layers of ions slide.

Structure and Properties

• Melting point: giant structures have high melting points because many strong bonds must break. Simple molecular substances have low ones because only weak forces break.
• Conductivity: only substances with free electrons or mobile ions conduct, such as graphite, metals and molten sodium chloride.