3.1 The periodic Table

• Arrangement of periodic table:

  -increase in atomic number

  -groups have the same number of outer shell electrons
• Period trends:

  increase in 1st ionisation energy across a period -similair shielding, increased attraction between outer electrons and nucleus

  atomic radius decreases across a the period - increased attraction between electrons and nucleus brings it closer together
• Exceptions in ionisation energy:

  -Grp.2 to grp.3, Be and B, 2p electron in B is at higher energy so easy to remove

  • Grp.5 to grp.6, N and O, in 2p orbital paired electrons are repelled so its easier to remove
• First ionisation energy:

  the energy change to remove one electron from one mole of its gaseous atom
• Blocks in the periodic table

  
• Period 3 melting point Trends:

  -Na to Al, giant metallic lattice, Al>Mg>Na, Al3+ more positive ions attracting delocalised electrons

  -Si, giant covalent lattice, strong covalent bonds between all atoms

  -P4, S8, Cl2, simple molecular lattice, more electrons in S8 so more london force, S8>P4>Cl2

  -Ar, atomic, london forces

  
• Trend in reactivity group 2:

  reactivity increases down - more shielding, less attraction between nucleus and outer electron, outer electron is more easily lost as energy is required
• Group 2 Reactions:

  with oxygen - Mg + 1/2O2--> MgO

  with water - Mg + 2H2O --> Mg(OH)2

  with water - UI changes to blue, bubbling and fizzing, metal disappears
• Group 2 Oxide reactions:

  MgO + H2O --> Mg(OH)2 , pH 8

  CaO + H2O --> Ca(OH)2 , pH 10

  SrO + H2O --> Sr(OH)2 , pH 12
• Uses of group 2 elements and compounds:

  magnesium hydroxide and calcium carbonate - anatacids, indigestion, neutralises stomach acid

  calcium hydroxide - agriculture to treat soil, neutralise acidic soil
• Halogens:

  group 7 due to the 7 outer electrons, they are in p block as their outer electrons are in the p subshell

  diatomic elements
• Trends in group 7:

  -atomic radius increases down group, more shells, more shielding, less attraction between outer electron and nucleus

  -boiling point increases down group, more electrons in molecule, stronger london forces in molecule so more energy needed to overcome this

  -reactivity decreases down the group, more shells, more shielding, less attraction between outer electrons gained and nucleus, so electron gains less readily
• Halogens reactivity:

  Cl2>Br2>I2
• Displacement Reactions:

  
• Disproportionation:

  a redox reaction in which the same element is both oxidised and reduced
• Chlorine and Water:

  Cl2 + H2O --> HCl + HClO

  reduction - Cl2 0 to -1

  oxidation - Cl2 0 to +1

  kills bacteria, water treatment
• Chlorine and cold dilute aqueous sodium hydroxide:

  Cl2 + 2NaOH --> NaCl + NaClO + H2O

  forms sodium chlorate one, aka bleach
• Benefit and risk of chlorine in water treatment:

  -benefit, kills bacteria, cleaner water, saves lives

  -risk, the gas is toxic, reaction with organics forms chlorinated hydrocarbons, these can cause cancer (carcinogenic)
• Positive test for chlorine:

  -silver nitrate, white precipitate

  -silver chloride and ammonia, dissolves in dilute
• Positive test for Bromide:

  -silver nitrate, cream precipitate

  -silver bromide and ammonia, dissolves in concentrated
• Positive test for Iodide:

  -silver nitrate, yellow precipitate

  -silver iodide and ammonia, does not dissolve
• Anion tests:

  1 CO32- reaction with acid forming CO2, bubble gas in limewater, white precipitate

  2 SO42- reaction with Ba2+ forming BaSO4

  3 halide tests

  (has to be done in order as carbonate and sulfate both insoluble in water and produce a white precipitate)
• cation test:

  4 NH4+, warm with aqueous sodium hydroxide, moisture with litmus paper, turns blue