National 4 Chemistry Summary Notes Bonding, Structure, and Properties PDF

NATIONAL 4 CHEMISTRY SUMMARY NOTES Unit One: Chemical Changes and Structure 4. Bonding, Structure and Properties Covalent Bonding This occurs between 2 non-metal atoms. A covalent bond is a shared pair of electrons. In Molecular Elements There are 8 diatomic e...

NATIONAL 4 CHEMISTRY SUMMARY NOTES Unit One: Chemical Changes and Structure 4. Bonding, Structure and Properties Covalent Bonding This occurs between 2 non-metal atoms. A covalent bond is a shared pair of electrons. In Molecular Elements There are 8 diatomic elements (2 atoms held by covalent bonding): Hydrogen (H2), Oxygen (O2), Nitrogen (N2) and all of the Group 7 elements (Halogens). In molecular compounds When non-metal atoms join! Eg. Water (H2O), ammonia (NH3), Methane (CH4) or the diatomic elements. These have specific shapes because of the covalent bonds. Covalent molecular substances tend to have low melting and boiling points because they have weak bonds between the molecules. However, the covalent bonds between the atoms are strong! Covalent network substances (carbon in the form of diamond and graphite, silicon dioxide, silicon carbide) have high melting and boiling points because all the atoms are interlinked by strong covalent bonds which take a lot of energy to break. Ionic Bonding This is formed between a metal and a non-metal atom forming charged particles called ions. The metal loses its outer electrons to form a positive ion. The non-metal takes these electrons to form a negative ion. In both cases the ion ends up with a full outer electron shell, like the Noble gases. The electrostatic force of attraction holds these oppositely charged ions together very tightly. This is why ionic compounds tend to have high melting and boiling points. The structure is a large lattice. Metallic Bonding Found in metal elements. The atoms lose their outer electrons which are then free to move from one metal ion to the next. This free movement of electrons is why metals conduct electricity. Solubility LIKE DISSOLVES LIKE Ionic compounds dissolve in polar solvents, like water. Covalent compounds (like candle wax) dissolve in covalent solvents (like hexane). A solution is produced when a solute (solid) dissolves in a solvent (liquid). Electrical Conductivity Metals conduct when solid or liquid. Covalent compounds don’t conduct at all. Ionic compounds only conduct when molten (liquid) or in solution because the ions are then free to move. Type of Electrical conductivity Melting/boiling points bonding Solid Liquid Aqueous Metallic yes yes insoluble High Ionic no yes yes High Covalent no no no Low molecules An electrolyte is used to complete the circuit and is usually a solution of an ionic compound. This allows charge to flow.  5. Chemical Symbols & Formulae All elements have a chemical symbol with one capital letter and a small letter if a second is needed. Naming Compounds The rules for naming compounds are: 1. The names of the elements are written from left to right as they appear on the Periodic Table. 2. If there are only 2 elements in the compound the name of the 2nd element ends in ‘_ide’. 3. If the compound contains oxygen plus at least 2 other elements the second name ends in ‘_ate’ and oxygen is not used in the name, eg. copper sulphate. Working out formulae To work out the chemical formula of a compound you use the valency (number of bonds it makes) of the atoms present. Group No. 1 2 3 4 5 6 7 8 Valency 1 2 3 4 3 2 1 0 Eg. Magnesium chloride 1. Write the symbols of the elements Mg Cl 2. Write the valencies 2 1 3. Swap the numbers over to give the Mg Cl2 number of each element in the compound (if equal to 1, the number does not need to be put in). If prefixes are used, these tell us the number of each type of atom in a compound, eg. Dinitrogen tetraoxide = N2O4. Prefix Meaning Mono One Di Two Tri Three Tetra Four Penta Five Hexa Six Chemical Equations A word equation shows the reactants on the LHS and the products on the RHS of an arrow. hydrogen + oxygen water A formula equation replaces the words with the symbols and formulae. H2 + O2 H2O 6. Formula Mass The Formula Mass is the total of the relative atomic masses (RAMs found in the databook) in a formula of a substance. This is in atomic mass units (amu). Example Water has the formula H2O. Its formula mass is worked out as follows: 2 x RAM of Hydrogen = 2 x 1 = 2 1 x RAM of Oxygen = 1 x 16= 16 Total = 18 amu = Formula mass of water