The Periodic Table.
1864 - only 63 elements discovered
> arranged them in order of atomic weight.
> found similarities with every 8th element
> some were placed in wrong group because of atomic weight.
He knew more elements would be discovered
> left blanks / predicted other elements.
The Modern Periodic Table.
Subatomic particles (protons, neutrons, electrons) and electronic structure provided more evidence to arrange the elements.
Now arranged by their electronic structure (electrons on the outer shells)
Group 1 : ALKALI METALS
- Low melting / boiling points. (decrease as you go down the group.)
- Have a low density.
- Become more reactive down the group (because the electron is easily lost as it isnt as attracted to the nucleus.)
All have the same amount of electrons on the outer shell (1)
Reactions & Alkali Metals.
Have to be stored in oil - reacts with Oxygen & Water.
(When it reacts with Water, a metal hydroxide is given off)
Alkali Metals react with non-metals to create ionic compounds. The metal always looses the electron, so it is positively charged.
Group 7. HALOGENS
5 elements - non metals.
- Melting / Boiling points increase as you go down the group
- Have coloured vapoured (especially bromine & chlorine)
- Become less reactive down the group.
All have the same amount of electrons on the outer shell (7)
Reactions & Halogens.
Halogens react with metals to produce ionic salts.
Halogens gain atoms and form halide ions which is -ve.
Halogen react with non-metals to form molecular compounds
- Form coloured compounts
- Have ions with different charges. (eg Fe2+ & Fe3+)
- Can be used as a catalyst to speed up chemical reactions
- Good conductors of heat and electricity.
- Can be easily bent or hammered into place
In comparison to Group 1.
- Higher density
- Higher melting points (apart from Mercury)
- Harder, Stronger
- Less reactive
Acids and Alkalis
Some compounds react with water to create acidic and alkaline solutions. These compounds need water to act as a base.
All Acids in aqueous solutions dissociate to produce hydrogen ions (H+) The H+ ion is a proton hydrated with water.
All Alkalis in aqueous solutions dissociate to produce hydroxide ions (OH-).
Acid = Proton Donor.
Base = Proton Acceptor. (usually water)
Strength of Acids and Alkalis
Acids and Alkalis are classified by the extent of ionisation in water.
A strong acid or alkali will dissociate all its ions in water
A weak acid or alkali will only dissociate partically in water.
Strong Acids = Hydrochloric Acid, Sulfuric Acid, Nitric Acid
Weak Acids = Ethanolic Acid, Citric Acid, Carbonic Acid
Strong Alkalis = Sodium Hydroxide, Potassium Hydroxide
Weak Alkalis = Ammonia Solution.