Bonding & Structure
Specification Point 1.32:
Explain why elements and compounds can be classified as:
b) simple molecular (covalent)
c) giant covalent
and how the structure and bonding of these types of substances results in different physical properties, including relative melting point and boiling point, relative solubility in water and ability to conduct electricity (as solids and in solution).
- All matter is composed of atoms which are bonded together, either by ionic, covalent or metallic bonds.
- The identify of the elements and chemical bonding present influences the structure and physical properties of a substance.
- The vast majority of known substances can be classified by their type of bonding and hence their structures into four main categories.
- These are:
- Simple molecular (covalent)
- Giant covalent
- Ionic compounds consist of a metal bonded to a nonmetal via electron transfer.
- They form regular shaped giant ionic lattices in which strong electrostatic forces act in all directions.
- A lot of energy is required to break so many strong bonds hence they have high melting and boiling points, so ionic compounds are usually solid at room temperature and are non-volatile.
- They are usually water soluble as both ionic compounds and water are polar molecules.
- Ionic compounds can conduct electricity in the molten state or in solution as they have ions that can move and carry charge.
- They cannot conduct electricity in the solid state.
Ionic lattice structure of a typical ionic compound
Simple covalent molecules
- Small covalent compounds consist of non-metal elements which have bonded together via electron sharing.
- They have low melting and boiling points so covalent compounds are usually liquids or gases at room temperature.
- This is because the weak intermolecular forces are easily overcome, meaning that these compounds are usually volatile, which is why many covalent organic compounds have distinct aromas.
- The intermolecular forces increase with molecular size, hence large molecules have higher melting and boiling points.
- Most covalent compounds are insoluble as they tend to be non-polar but can dissolve in organic solvents.
- Some do dissolve in water by forming intermolecular attractions between the water molecules. Examples include sucrose (common sugar C12H22O11) and CO2.
- They cannot conduct electricity as all electrons are involved in bonding so there are no free electrons or ions to carry the charge.
Diagram showing simple molecules of water
Giant covalent structures
- Giant covalent structures consist of many non-metals atoms bonded to other non-metal atoms via strong covalent bonds.
- They have high melting and boiling points as they have many strong covalent bonds.
- Large amounts of heat energy are needed to overcome these forces and break down bonds.
- Most cannot conduct electricity as they do not have free electrons but there are some exceptions such as graphite.
Silicon dioxide (silica) is a giant covalent lattice where each silicon atom is bonded to four oxygen atoms
- Metals consist of metal atoms are held together strongly by metallic bonding.
- Within the metal lattice, the atoms lose their valence electrons and become positively charged.
- The valence electrons no longer belong to any metal atom and are said to be delocalised, creating what is known as a sea of free electrons.
- The free electrons move freely in between the positive metal atoms.
- Metallic bonds are very strong and are a result of the attraction between the positive metal ions and the negatively charged delocalised electrons.
- Metals thus have very high melting and boiling points.
- They are usually insoluble in water although some do react with water.
- They can conduct heat and electricity due to the delocalised electrons.
- The layers of atoms in metals can slide over each other meaning metals are malleable and can be hammered and bent into shapes.
Diagram showing metallic lattice structure with delocalised electrons
Edexcel GCSE Chemistry Notes
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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