Element — a substance which cannot be broken down into two or more substances by any means. Only contains one type of atom.
Compound – a substance in which atoms of two or more elements have been chemically combined.
- Metals bonding with non-metals
- Involves transfer of electrons from metal to non-metal, so that both species obtain full outer shell electron configurations (Noble gas configuration).
- Metals lose electrons to form positive ions (Cations).
- Non-metals gain electrons to form negative ions (Anions).
- The ions are held together by strong forces of electrostatic attraction. The ions are held in a giant three-dimensional regular lattice.
- High melting and boiling points — due to strong electrostatic forces between ions.
- Soluble in water (polar solvent), but not in hexane — due to the attraction of the atoms on the water molecules for the positive and negative ions.
- Conduct when molten, or in solution but not when solid — ions are free to move when molten or in solution.
Example: Sodium Chloride
Na [2,8,1] Cl [2,8,7] Na+ [2,8] Cl– [2,8,8]
(Possible diagram needed for this example)
- Between non-metals and non-metals
- Involves the sharing of electrons. Atoms share electrons, so that both can achieve a full outer shell electron configuration.
Two forms of covalent compounds: simple and giant.
Simple Covalent Compounds
Simple covalent compounds are usually gases, liquids or solids with relatively low melting and boiling points. This is because the forces between the molecules are relatively weak.
Simple covalent compounds do not conduct electricity, as there is no overall electric charge on the molecules.
Examples of simple covalent compounds
Methane (CH4), ammonia (NH3), hydrogen chloride (HCl)
Methane (CH4)— electrons from carbon represented by x, electrons form hydrogen represented by o.
(Diagrams of these examples needed)
This line represents a covalent bond – shared pair of electrons.
Giant Covalent Compounds
Giant covalent structures usually have high melting and boiling points, because the covalent bonds themselves need to be broken.
Examples of giant covalent compounds
E.g. silicon dioxide (SiO2), graphite and diamond (allotropes of carbon). Silicon dioxide has the same structure as diamond.
Structure and properties of diamond and graphite
(Diagrams of structure needed)
In Diamond each carbon is bonded to four other carbon atoms.
In Graphite each carbon is bonded to three other carbon atoms. It conducts electricity because it has mobile electrons between the layers
- Found in metals, which have giant structures.
- An array of cations surrounded by a sea of “delocalised” (mobile) electrons.
- High melting point and boiling points — the atoms are packed close together and the bonds holding them are very strong. The energy needed to melt the metals is high.
- Conduct heat and electricity — electrons in the “sea” are mobile. A current (flow of electrons) can pass.
- Insoluble in water and organic solvents, but may react with water.