Atomic structure and the periodic table

• an atom is the smallest part of an element that can exist
• compounds contain two or more elements chemically combined in fixed proportions
• a mixture consists of two or more elements not chemically combined together
• mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography
• before the discovery of the electron atoms were thought to be tiny spheres that could not be divided
• JJ Thomson discovered the electron in 1897
• the discovery of the electron led to the plum pudding model of the atom
• the plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it
• the plum pudding model was incorrect because it didnt explain why atoms are neutral
• Ernest Rutherford carried out an experiment where he fired alpha particles at thin sheets of gold foil
• rutherfords experiment showed that the mass of the atom was concentrated in the centre, and that the nucleus was charged
• Niels bohr suggested that electrons orbit the nucleus in shells of fixed distance
• later experiments showed that the nucleus also had positively charged protons
• james chadwick discovered the neutron in 1932
• an atoms atomic number is the number of protons in the nucleus of an atom
• atoms are very small and have a radius of 0.1nm
• the radius of a nucleus is about 1 x 10^-14
• the mass number of an element is the sum of the protons and neutrons in the nucleus
• elements in the same group of the period table have the same amount of electrons in their outer shell which gives them similar chemical properties
• before the discovery of protons neutrons and electrons, elements were arranged by their atomic weights
• mendeleev overcame problems in the early periodic table by leaving gaps for elements and in some places changed the order based on atomic weights
• elements with properties predicted by mendeleev were later discovered, which were explained by knowledge of isotopes
• elements that react to form positive ions are metals
• elements that react to form negative ions are non-metals
• the elements in group 0 are called the noble gases
• the noble gases are unreactive and do not easily form molecules because their atoms have stable electron arrangements
• all noble gases have 8 electrons in their outer shells except helium which has 2
• the noble gases are all colourless monatomic gases at room temperature
• as you go down group 0 the boiling point increases
• elements in group 1 are known as the alkali metals
• alkali metals are very reactive, less dense, softer and have lower melting points
• metal + water = metal hydroxide + hydrogen
• metal + chlorine = metal chloride
• metal + oxygen = metal oxide
• the elements in group 7 are known as the halogens
• reactions of alkali metals in water:

  -metal floats and moves around

  -metal fizzes due to the hydrogen

  -for potassium, the hydrogen produced is ignited and burns with a lilac flame
• all group 7 elements exist as diatomic molecules
• a more reactive halogen can displace a less reactive halogen from a compound
• in group 7 reactivity decreases down the group because the outer electron is further from the nucleus
• as you go down group 7 the melting and boiling point gets higher