A level Chemistry: Periodicity, Group 2 and Halogens FULL A* SUMMARY NOTES

Periodicity, Group 2 and Halogens Summary


 An element is classified as s, p, d or f block according to its position in
the Periodic Table, which is determined by its proton (atomic)
number:
 all elements in a period have the same number of shells (not taking
into account s and p subshells)

 all elements in a group have the same number of outer shell electrons.
(exception: group 0, in which helium has 2 outer shell electrons and
the rest have 8 outer shell electrons). Hence, all elements in a
particular group have similar chemical properties.

TRENDS ACROSS PERIOD 3

 Explaining the trends in atomic radius of Na–Ar:

 Across period 3, atomic radius decreases.
 This is due to increased nuclear charge.




 Explaining the trends in first ionisation energy of Na–Ar:

 Across period 3, first ionisation energy increases.
 This is also due to increased nuclear charge.
 Drop from Mg – Al: Al has new 3p orbital, which has an e -
that is little further from nucleus and is shielded by the 3s 2
electrons. It is therefore easier to remove. Drop from P-S:
electron pair repulsion in sulfur means it is easier to
remove the electron.




 The trends in melting points of Na–Ar, and reasons for these
trends in terms of structure and bonding:


 Na – Al are metals and hence display metallic bonding.
MPs increase from Na to Al as the number of delocalised
electrons increases (ions become increasingly positive),
hence increasing the metal-metal bonds.


 Si is macromolecular and a tetrahedral structure. It has
strong covalent bonds that need to be broken during melting. These require a huge amount of energy
to break.
 P-S are simple molecular and display VDW forces. Sulfur is higher than phosphorus as it is larger and
therefore has stronger VDW forces. Sulfur exists as S 8, Phosphorus as P4, and Chlorine as Cl2.
 Ar is monatomic and also displays VDW forces. Since it exists as a single atom, VDW forces between
atoms are weakest compared to the rest of period 3.

TRENDS DOWN GROUP 2 (alkaline earth metals)

, Periodicity, Group 2 and Halogens Summary


 Explaining the trends in atomic radius of Mg–Ba:

 Down group 2, atomic radius increases.
 This is due to increased electron shielding as an
extra shell is added each time.




 Explaining the trends in first ionisation energy of Mg–
Ba:



 Down group 2, ionisation energy decreases.
 This is due to increased electron shielding as an extra
shell is added each time.
 It is also due to increased atomic radius.



 Explaining the trends in melting point of Mg-Ba in terms of their structure and bonding:

 Down group 2, melting points generally decrease.
 Since they are all metals, they display metallic
bonding. Down gp 2, the metal ions get bigger but
number of delocalised (outer) electrons remains as
2.
 Therefore, the ionic radius increases and makes it
easier to lose the 2 delocalised electrons.




 The reactions of Mg–Ba with water:

M(s) + H20(l)  M(OH)2 + H2, where ‘M’ is any of the group 2 metals.

When group 2 metals react, they become oxidised: M  M2+ + 2e-

Going down the group from Mg to Ba, reactivity with water increases due to decreasing first ionisation energy.

 The use of magnesium in the extraction of titanium from TiCl 4:

(The main titanium ore from which titanium is extracted is TiO 2. Firstly, TiO2 is heated with carbon in a stream
of chloride gas to form TiCl4. The TiCl4 is then purified by fractional distillation.)

Next, TiCl4 is reduced by liquid magnesium. This is done in a furnace at 1000°C.

TiCl4(g) + 2Mg (l)  2MgCl2(l) + Ti(s)