Atomic Structure & Periodic Table | AQA GCSE Chemistry Notes | Topic 1

• An atom is the smallest part of an element that can exist.
• Compounds are two or more elements chemically combined.
• Mixtures are two or more elements not chemically combined.
• Mixtures can be separated by filtration, crystallisation, simple distillation, fractional distillation and chromatography.
• Filtration: use filter paper and a funnel.
• Distillation: heating, allowing some of the mixture to evaporate faster.
• Crystallisation: allowing the remaining parts to form crystals.
• Fractional distillation: organic chemistry (topic 7)
• Chromatography: topic 8
• Atoms have a small radius of about 1 × 10^-10 metres.
• The radius of a nucleus is less than 1/10 000 of the radius of an atom.
• Atoms have a positively charged nucleus with protons and neutrons, surrounded by negative electrons.
• Most of the mass of an atom is in the nucleus.
• The electrons are arranged in shells. They may change arrangement if electromagnetic radiation is absorbed (move further from the nucleus; a higher energy level). 2 in 1st shell, 8 in 2nd and 3rd shell.
• Atoms have the same number of protons and electrons unless they get charged.
• Atomic number: number of protons (should be the number of electrons too).
• Mass number: number of protons and neutrons added up (top number)
• Ions: charged particles (when an atom has lost or gained an electron).
• Isotopes: atoms with the same number of protons but different number of neutrons.
• Before the discovery of the electron: atoms as tiny spheres that could not be divided.
• Plum Pudding Model: the atom as a ball of positive charge with negative electrons embedded in it.
• Alpha particle scattering experiment: they shot alpha particles at gold foil. Most particles went straight through, some deflected, very few reflected. Conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged.
• Bohr: improved the nuclear model, saying that electrons orbit the nucleus at specific distances (shells). Experiments also found each proton as one unit of positve charge.
• Chadwick: showed the existence of neutrons in the nucleus 20 years after the nucleus was found.
• Relative Mass: Proton (1), Neutron (1), Electron (0)
• Relative Charge: Proton (+1), Neutron (0), Electron (-1)
• Relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
• Relative Atomic Mass = [(% x Ar) + (% x Ar) + (% x Ar) + ...]/100%
• Mendeleev arranged the periodic table in order of proton number and left gaps.
• Metals are group 1, 2, 3 and form positive ions.
• Transition metals are in the middle.
• Non-metals are in group 4-8 and form negative ions.
• Ionic charge: group 1: 1+, group 2: 2+, group 7: 1-, group 6: 2-...
• Diatomic: anything ending with -gen always exists in pairs e.g. H2, O2, N2, including all Halogens (Group 7)
• Group no. tells us the no. of electrons on the outer shell
• Group 8 (also called Group 0) are the noble gases. They are very stable and do not want to gain or lose electrons.
• The BP in group 8 increases going down the group (greater Mr).
• Group 1 (alkali metals) form 1+ ions. They get more reactive as you go down because they have more shells, so the outer electrons are further away from the nucleus, so there is more electron shielding so less nuclear attraction. Electrons can escape more easily.
• Group 1 metals react with oxygen, chlorine and water to form metal oxides, metal chlorides and metal hydroxides.
• Group 7 (halogens) want to gain one electron. Reactivity increases as you go up group 7 because there are less shells so there is less electron shielding and less nuclear attraction so it is easier to attract an extra electron.
• A more reactive halogen can displace a less reactive halogen
• e.g. fluorine + sodium chloride --> chlorine + sodium fluoride

Topic 1 AQA GCSE Chemistry Notes for Triple Science Students Only

• Transition Metals compared to Group 1: higher melting point, more dense, stronger, harder, less reactive with oxygen, water and halogens.
• They have ions with different charges, form coloured compounds and are useful as catalysts.