HISTORY OF THE PERIODIC TABLE
1) John Dalton arranged the known elements in order of ATOMIC MASS (people did not yet know about atomic structure and did not believe that atoms were divisible).
2) John Newlands discovered the LAW OF OCTAVES (every eighth element has similar properties) and arranged a table in rows of seven. Because many elements were not yet discovered, the pattern was often broken.
3) Dimitri Mendeleev rearranged Newlands' table to leave room for undiscovered elements. This put elements with similar properties in the same group and enabled properties of undiscovered elements to be predicted.
4) The elements were arranged as we now know them in the twentieth century- in increasing ATOMIC NUMBER.
- Arranged in increasing atomic number (proton number).
- Columns are groups of elements with similar properties (the way a substance reacts) and the same number of electrons in their outer shells (same as group number). There are eight groups.
- Rows are periods. There are seven periods.
GROUP ONE- ALKALI METALS
- Lithium, Sodium, Potassium, Rubidium, Caesium, Francium.
- One electron in outer shell. React to form 1+ ions.
- All very reactive and stored in oil to prevent reactions with the air. Reactivity increases going down the group as the outer electron becomes further from and less strongly attracted to the positive nucleus.
- Fresh cut (metals quite soft, harder down group) surface is shiny and silvery but quickly tarnishes with air, forming a dull oxide layer.
- Low melting and boiling points (for metals). Decrease down group.
- Low density (for metals). Density increases down group.
- React with non-metals to form ionic compounds. These salts are white solids which easily dissolve to form solutions.
- React violently with water to form hydrogen gas and a metal hydroxide which dissolves into the water to form an alkaline solution.
GROUP SEVEN- HALOGENS
- Fluorine, Chlorine, Bromine, Iodine, Astatine.
- Seven electrons in outer shell. React to form 1- ions..
- Reactivity decreases down group because outer electrons are further from positive nucleus and so attracted less strongly.
- Poisonous, diatomic (paired), non-metals that form coloured vapours (darker going down the group) and do not conduct heat or electricity.
- Low melting and boiling points due to weak intermolecular forces.
- React covalently with non-metals to form simple molecules with weak intermolecular forces.
- React with metals to form ionic compounds.
- A more reactive halogen will displace a less reactive halogen from an aqueous solution of its salt, e.g.
Cl2 (g) + 2KBr (aq) > 2KCl (aq) + Br2 (l)
When the halogen Chlorine is added to a solution of Potassium Bromide, it displaces the Bromine because Chlorine is more reactive than Bromine; Potassium Chloride and Bromine are produced.
- Large block of 'normal' metals (conduct heat and electricity, strong, hard, malleable, ductile, high density, high melting point, high boiling point... compared to alkali metals) between groups two and three.
- Form coloured compounds.