AQA UNIT 3 GCSE REVISION NOTES (Higher only in italics)
The Periodic Table
At the start of the 1800s quite a lot of elements had been discovered, but there had been no real attempt to organize them.
Newlands, an English chemist and Mendeleev, a Russian chemist both worked on the problem by arranging the elements or order of their atomic weight (mass) and looking for patterns in their properties. a
Both were successful to some extent, but Mendeleev was much more successful because.
a) He recognized that there were some elements still to be discovered, so he left gaps in his table
b) He was not afraid to change the order of some elements in the table where it seemed to make sense.
c) He was able use the table to successfully predict the properties of some elements which had not yet been discovered.
Later when more elements had been discovered and chemists realized that the elements should be in order of atomic number rather than atomic mass, Mendeleev’s Table was shown to be extremely successful.
The Modern Table
The periodic tables patterns are now known to be based on the structure of the atom.
Elements in the periodic table are arranged in order of atomic number (number of protons in the nucleus)
The group number of an element shows the number of electrons in the outer shell.
The period number shows the number of shells of electrons
e.g. Sodium is element number 11 and has the structure 2.8.1 It therefore has 11 protons, it is in Group1, period 3 of the periodic table.
GROUP 1 , The Alkali Metals.(Li, Na, K, Rb, Cs, Fr)
1) All elements have 1 electron in the outer shell:
Li: 2.1 Na: 2.8.1 K: 22.214.171.124 etc.
2) For this reason they all form 1+ ions when they react:
Ions are Li+, Na+, K+ etc.
3) They have similar physical properties. All are very light, soft metals which are shiny when cut.
4) They have similar chemical properties. In particular they react vigorously with water giving hydrogen and an alkaline solution of the hydroxide.
(e.g. 2Na + 2H2O = 2NaOH + H2)
For this reason, they are called alkali metals
Because of the violence of this reaction with water, alkali metals are stored under oil, and only very small amounts should ever be reacted with water using safety screen, goggles etc.
1) The metals are all soft and light, but going down the group from Li to Cs, they
a) Have decreasing melting points (Li = 180oC, Na = 98oC, K = 64oC etc.)
b) Become softer
2) There is a definite increase in reactivity going down the group from Li to Cs. Li reacts quite mildly, Na more vigorously, K quite violently etc.
This is because the atoms lower down have more inner shells of electrons. These 'shield'…