Bonding Chapter 3 IGCSE Chemistry

Covalent bonds O = C = O

Covalent bonds  Electrons shared between atoms  One pair of shared electrons = one bond  Between non-metals

Ionic bonds

 Electrons are donated or accepted  Ions (charged atoms) are formed  Between metals and non-metals  Metals form cations (positive)  Non-metals form anions (negative)  More easy to lose/gain one or two electrons

Metallic bonding

Metallic Bonding  General properties of metals:  Hard  High melting point  High boiling point  Malleable  Ductile  Conducts heat and electricity

Metallic Bonding  The properties are due to very strong forces that hold the metal together.  Metal structure: ‘sea of electrons’  Electron orbitals/shells overlap  Electrons become detached from the parent electron and are said to be delocalised.  Electrons are free to move: conductivity

Metallic Bonding  The attraction between positive ions and the electrons holds the structure together.  High MP and BP  Group 2 metals have a higher BP and MP than group 1 metals. Why?  More electrons in the ‘sea’ = stronger forces between particles.

Intermolecular Forces  Forces of attraction between molecules.  Gas if intermolecular forces are very weak  Solid if intermolecular forces are strong  Ionic compounds usually have strong intermolecular forces  Covalent compounds can have weak or strong intermolecular forces

Giant and simple structures  Simple structures have a fixed number of atoms in the molecule eg. H 2 O, C 6 H 12 O 6  They can be quite big molecules eg. Proteins, DNA  The protein haemoglobin always has the same number of C, H, O, N, S, Fe, etc  Giant structures do not have a fixed number of atoms.

Giant ionic structures A lattice of ions held together by + and - Strong attractions High mp and bp Crystalline Brittle Soluble in water Conduct electricity when molten or dissolved in water

Giant Covalent Structures Eg. Diamond Each Carbon bonded to 4 other carbon atoms Tetrahedral Very hard High mp and bp Does not conduct electricity Does not dissolve in water

Giant covalent structure Eg graphite Layer structure Each layer hard, but layers rub off easily – feels slimy High mp and bp Insoluble Less dense than diamond Can conduct electricity – free e -

Simple molecular structure Very weak intermolecular forces Low mp and bp Insoluble in water (unless they react with it) Don’t conduct electricity

Polar Molecules  Uneven sharing of electrons  This uneven sharing causes attractions between neighboring molecules.  Heating a substance affects these forces.  Liquid has a weaker force than a solid.  Forces are broken when the substance becomes a gas.  Covalent bonds are not affected during changes of state.