Topic 1a - Atoms, Elements, Compounds and Mixtures

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Structure of the atom: Tiny particles that make up everything. Radius: 0.1 nm (1 X 10^-10 m). Nuclear model: Protons, electrons, neutrons (subatomic particles). Electron shells/energy levels (2, 8, 8, 2, 8, 8, etc). Nucleus and electrons: Nucleus radius: 1 x 10^-14 (1/10000 of the size of an atom). Electrons: Negatively charged, tiny, occupy shells around the nucleus, relative mass of 1/2000 (1/1840), relative charge -1. Protons: Positively charged, relative mass of 1, relative charge +1. Neutrons: No charged, relative mass 1, relative charge 0. Ions: An atom or group of atoms that has lost or gained electrons.

Elements: A substance only contains one type of atom. The smallest part of an element is an atom. Nuclear symbols: e.g. 24 Mg 12. 24 = the mass number (protons+ neutrons). 12 = the atomic number/the proton number (protons).

Isotopes: Different forms of the same element. Just different amounts of neutron. Relative atomic mass: Ar = sum of (isotope abundance x isotope mass number) / sum of abundances of all the isotopes (should always equal 100 if given as percentages).

Compounds: Atoms combine within the elements to form compounds (small molecules or great big lattices). Elements can't be separated by physical processes.Chemically bonded (share electrons) and the atoms of the elements are in fixed proportions throughout the compound. Metals and non-metals react. The metal atom lose electrons to form positive ions, and the non-metal atoms gain electrons to form negative ions (ionic bonding). Non-metals can form a compound, usually consists of molecules. Share electrons (covalent bonding).

Mixture: Not chemically bonded, not necessarily in fixed positions. Separate substances can be physically separated e.g. chromatography (chromatograms), filtration, crystallisation, simple distillation (boiling), fractional distillation (distillation with a fractionating column).

History of the atom: Plum pudding model: John Dalton at the start of the 19th century described atoms as solid spheres and different spheres made up different elements. In 1897, J J Thomson disproved this and proved electrons. Thus came the plum pudding model. Originally thought to be a ball of positive mass, with electrons scattered within. Ernest Rutherford disproved this. Fired positively charged alpha particles at an extremely thin sheet of gold. Expected particles to pass straight through the sheet or be slightly deflected at most. Results different. Some particles were deflected by much more than expected, even deflected backwards. Rutherford had to explain this, and thus the nuclear model of the atom was created. Tiny, positively charged nucleus, with a cloud of electrons. Most of the atom is empty space. Niels Bohr's nuclear model turned the cloud of electrons into electron shells which were evenly distanced. James Chadwick proved neutrons. The model had to be updated.

Electronic structure (an electronic configuration): Electron shells: 2, 8, 8. Atoms are much happier when they have a full outer electron shell. Most atoms outer shell isn't full, which is why atoms react (to obtain a full outer shell). Potassium (2, 8, 8, 1). Calcium (2, 8, 8, 2).