Topic 5 – Bonding and Structure
• Atoms form bonds to get a full outer shell of electrons • There are three types of bonding: ionic, covalent and metallic • The structures produced by forming bonds are either giant or simple • The possible combinations of structure and bonding are giant ionic, simple covalent, giant covalent and giant metallic • Simple covalent is sometimes called simple molecular • Giant covalent is sometimes called giant molecular or macromolecular • To melt a substance the forces holding the particles together need to be broken • To conduct electricity there must something charged that can move (ions or electrons). Technically this is called a mobile charged species • To dissolve in a particular solvent the substance must interact with the solvent
Types of Bonding
▪ Ionic bonding – metals transfer electrons to non-metals producing positive metal ions and negative non-metal ions. An ionic bond is an electrostatic attraction between oppositely charged ions. Dot-cross diagrams show outer electrons only e.g. NaCl
▪ Covalent bonding – A covalent bond is a shared pair of electrons. Only non-metals can get a full shell by sharing electrons. The bond is the attraction of the shared electrons for the two nuclei. Dot-cross diagrams show outer electrons only e.g. Cl2 [pic]
▪ In dative covalent bonds, one atom provides both of the shared pair of electrons e.g. formation of an ammonium ion, NH4+, from ammonia, NH3, and H+
▪ Metallic bonding – metals lose their outer shell electrons to produce a lattice of positive metal ions surrounded by delocalised (free) electrons.
Types of Structure
Giant ionic lattices e.g. sodium chloride
o Lattice of oppositely charged ions.
o High melting and boiling points (strong forces of attraction between ions need to be broken). o Do not conduct when solid (ions not free to move). o Conduct when molten or dissolved in water (ions then free to move). o Most are soluble in polar solvents like water. The ions interact with the dipoles in the solvent molecules o Tend not to dissolve in non-polar organic solvents like cyclohexane. The ions do not interact with non-polar solvents
Simple covalent lattices e.g. iodine and ice
o Consists of molecules held together by weak intermolecular forces (see section 5 below) o Low melting and boiling points (weak forces of attraction between molecules are easily broken) o Do not conduct (no mobile charge carriers)
o Most are insoluble in polar solvents, like water, because they do not interact with the dipoles in the solvent. Alcohols, however, can hydrogen bond to water molecules o Tend to dissolve in non-polar organic solvents, like cyclohexane, because the solvent can interact with the simple covalent substance
Giant metallic lattices e.g. magnesium, copper
o Lattice of metal ions surrounded by delocalised electrons. o High melting and boiling points usually (strong forces of attraction between metal ions and free electrons need to be broken). o Conduct when solid (free electrons).
o Insoluble in all solvents (some react with water)
Giant covalent lattices e.g. diamond, graphite
o Lattice of non-metal atoms joined by strong covalent bonds o Very high melting and boiling points usually (many strong covalent bonds to be broken) o Diamond doesn’t conduct (no mobile charge carriers). Graphite is the only non-metal that conducts as a solid (structure contains delocalised electrons) o Insoluble in polar solvents, like water, because they do not interact with the dipoles in the solvent in water
Electronegativity and bond polarity
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