Development of the Periodic Table

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  • C3.1: Development of the Periodic Table
    • C3.1.1: The early periodic table
      • John Dalton
        • Arranged the elements in order of their mass, which he measured in various chemical reactions
        • 1808: published a table of elements in his book, A New System of Chemical Philosophy
      • John Newlands
        • Built on Dalton's ideas with his Law of Octaves: he arranged the known elements in mass order and noticed that the properties of every eighth element were similar
        • 1864: he produced his table of octaves, but he had assumed that all the elements had been discovered so filled the octaves in though some weren't similar at all; the rule only worked up to calcium
      • Dmitri Mendeleev
        • 1869: 50 elements had been identified, so Mendeleev arranged them in a table in order of atomic weights, then in a periodic pattern so properties could be seen
        • He left gaps for elements not yet discovered and used the table to predict what their properties were. This meant that years later, new elements discovered fitted this.
    • C3.1.2: The modern periodic table
      • Soon after finding out about protons and electrons, scientists developed models of the arrangement of electrons in atoms. The elements were arranged in order of their atomic no.s and lined up in vertical groups.
      • Groups have similar properties bc their atoms have the same no. of electrons in their outer shell. For the main groups, the no. of electrons in the outer shell is the same as the group no.
      • Reactivity in groups
        • Going down a group, there are more occupied energy levels and the atoms get larger. As they get larger, the electrons in the highest occupied shell are not attracted as strongly to the nucleus, so are easier to lose.
        • When metals react, they lose electrons, so the reactivity of metals in a group increases going down the group
        • When non-metals react, they gain electrons, so the reactivity of non-metals decreases going down a group.
    • C3.1.3: Group 1 (Alkali Metals)
      • Metals that react readily with air and water
      • Soft solids at room temp. with low melting and boiling points that decrease going down the group
      • React with water to produce hydrogen gas and a metal hydroxide that is an alkali e.g. sodium + water --> sodium hydroxide + hydrogen
      • All have one electron in outer shell, which they lose in reactions to form ionic compounds in which their ions have a single +ve charge
      • React with halogens (group 7) to form salts that are white  or colourless crystals e.g. sodium + chlorine --> sodium chloride
      • Their compounds dissolve in water, forming solutions that are usually colourless
      • Reactivity increases going down the group bc outer electron is less strongly attracted to the nucleus as the no. of occupied energy levels increases and the atoms get bigger
    • C3.1.4: The transition metals/ elements (between groups 2&3)
      • Higher melting and boiling points than alkali metals (exc. mercury)
      • Malleable and ductile
      • Good conductors of heat and electricity
      • React slowly, or not at all, with xygen and water at ordinary temp.
      • Most are strong and dense and are useful as building materials, often as alloys
      • Form +ve ions with various charges
      • Their compounds are often brightly coloured
      • Many transition metals and their compounds are catalysts for chemical reactions
    • C3.1.5: Group 7 - The Halogens
      • Non-metallic
      • Small molecules made up of pairs of atoms
      • Low melting and boiling points that increase going down the group (at room temp., fluorine is a pale yellow gas, chlorine is a green gas, bromine is a red-brown liquid and iodine is a grey solid which easily vaporises to a violet gas
      • Form ionic compounds with metals in which the halide ions have a charge of  -1
      • Bond covalently with non-metals, forming molecules
      • Reactivity decreases going down the group bc attraction of outer electrons to the nucleus decreases as the no. of full shells increases (a more reactive halogen can displace a less reactive one from an aqueous solution of a halide compound)


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