PARTICLES THAT SUBSTANCES ARE MADE OF - TOPIC 11 -

ATOMS AND COMPOUNDS - Atom = smallest particle that makes up substances - Very few substances found as SINGLE atoms (only the noble gases) - Atoms need to bond with other atoms to gain a full outer electron configuration - Atoms combine (via bonding) to form COMPOUNDS - Compound = a group of 2/more atoms attracted to each other by forces or bonds COMPOUNDS with COVALENT bonds - Covalent bonding = sharing of electrons Covalent bonding results in the formation of a MOLECULE Covalent molecular structures = LATTICES made of smaller, less complex, covalent compounds Consider water... 1) A single water molecule 2) Liquid phase water 3) Solid phase water (ice)

- When in the solid phase, water molecules are no longer moving randomly - The molecules arrange themselves in a UNIFORM HEXAGONAL LATTICE of individual molecules NB: the individual molecules DON’T REACT with one another, but there are intermolecular forces of attraction between the molecules (Hydrogen bonding) The hexagonal lattice is called a CRYSTAL LATTICE The intermolecular forces holding molecular substances together are usually weak This causes molecular solids to have LOW MELTING POINTS (easy to break IMF’s) Because molecular solids don’t contain ions or free electrons, they are POOR conductors! Some covalent molecular structures are made of a single type of atom (eg. SULFUR) It forms a ring shaped molecule (S8) These rings can combine to create lattices

- The lattice can assume different shapes = resulting in different ALLOTROPES - Allotrope = “Structurally different forms of the same element” - Covalent network structures = Giant repeating lattices of covalently bonded atoms - Example: Diamond - Diamond is made up of carbon atoms held in a lattice - The carbon atoms DON’T bond to form carbon molecules - BUT... In the carbon lattice, each carbon atom is bonded to 4 adjacent carbon atoms Each of those carbon atoms are bonded to 4 other carbon atoms (and so on...) Therefore, because no carbon molecules are formed in diamond, it has the formula: C

- Since it is a COVALENT BOND, and not IMF’s, holding the lattice together (high MP and BP) - This is also what makes diamond so hard - Since there are no free electrons or ions, diamond is a poor conductor GRAPHITE This is an allotrope of carbon Structure consists of linked flat 6-sided rings The plates can move over one another It is therefore easy to cut between layers - It contains some free electrons - Therefore it is a fairly good conductor

COMPOUNDS with IONIC bonds - Ionic bonding = transfer of electrons to form CATIONS and ANIONS Ionic bonding results in the formation of an IONIC COMPOUND IONIC COMPOUNDS = crystal LATTICES made up of positive and negative ions Consider table salt (sodium chloride)... - It is made up of positive sodium ions and negative chloride ions - Strong electrostatic forces, between oppositely charged particles, hold ions in the lattice . - When in the lattice, the ions are tightly held and cannot conduct electricity When dissolved in water, they are free to move around and can then conduct electricity The strong IMF’s cause ionic substances to be brittle and have high MP’s and BP’s

COMPOUNDS with METALLIC bonds - Metallic bonding = Lattice of positive nuclei surrounded by delocalised electrons The delocalised electrons make metals good conductors Summary of lattice types and their associated properties...