chemistry (yr9)

• Plum Pudding Model: JJ Thomson discovered the electron. He proposed that atoms were balls of positive charge with tiny negatively charged electrons dotted randomly through them like plums in a pudding.
• Rutherford discovered nucleus the positively charged and the large areas of empty space when he fired alpha particles at a thin piece of gold foil. Much later, James Chadwick discovered another particle in the nucleus – the neutron.
• Development of the Periodic Table: Scientists initially ordered elements in order of atomic weight. Then they tried to organise them using their properties.

  Dmitri Mendeleev succeeded by

  • ordering by atomic weight and properties

  • swapping elements to make them fit

  • leaving gaps for undiscovered elements.
• Non metal atoms need to gain electrons to get a full outer shell. They can do this by overlapping their outer shell and sharing a pair of electrons. One electron in each pair must come from each atom. This is called a covalent bonds.
• Some atoms need more than one electron to give them a full outer shell example: oxygen needs 2 electrons to complete its outer shell. Oxygen shares two electrons per atom to make a covalent bond.
• Properties of simple covalent compounds: simple covalent compounds have low melting points and are often gases at room temperature because they have strong covalent bonds within molecules but weak intermolecular forces (between molecules.)
• Giant covalent diamond: 3D, high MP hard, insulator.
• Structure graphite: 2D, high MP, soft, conductor.
• Group 1 Alkali Metals: All have 1 electron in their outer shell. Reactivity increases down the group as atoms increase in size. Valence electrons is easier to remove: - further from + nucleus and - shielded by more electrons in shells.
• Group 7 Halogens: All have 7 electrons in their outer shell. Reactivity decreases down the group as atoms increase in size. It is harder to attract a valence electron - further from + nucleus and - shielded by more electrons in shells.
• Group 0 Noble gases: All have a full outer shell and all unreactive (INERT.) Boiling point increases as you go down the group as relative atomic mass increases.