Summary Periodicity Notes - AQA Chemistry A Level

Periodicity





 Elements are classified as s,p,d or f block, according to which sub-level the highest energy
electrons are in
 Periodicity – Repeating trends in properties across a period

Trends Across Period 3
Atomic Radius
 DECREASES
 Increasing nuclear charge
 Outer electrons entering same principle energy level
 Similar shielding
 Greater attraction between outer electron and nucleus
 Outer electrons are pulled closer towards nucleus

First Ionisation Energy
 INCREASES
 Increasing nuclear charge
 Outer electrons entering same principal energy level
 Similar shielding
 Smaller atomic radius
 Greater attraction for outer electron from nucleus
 Outer electron is harder to remove
 Drop at Al
o Outer electron is removed from 3p sublevel in Al which is higher in energy than outer
electron in 3s in Mg
 Drop at S
o Pair of eletrons in S’s 3p sub-level – spin-pair repulsion outweighs increase in nuclear
charge

Electronegativity
 INCREASES

,  Increasing nuclear charge
 Outer electrons entering same principal energy level
 Similar shielding
 Smaller atomic radius
 Greater attraction for outer electron from nucleus

Melting & Boiling Points
Metallic – Na,Mg,Al
 High melting points
o Strong electrostatic forces of attraction between metal cations and sea of delocalised
electrons
 Increasing charge on ion – Na+ to Al3+
 More delocalised electrons
 Stronger metallic bonding

Giant Covalent – Si
 Many strong covalent bonds which require lots of energy to break – high melting point
o Each Si atom is covalently bonded to 4 other Si atoms in a 3D lattice
 Boiling point not as high relative to other elements as melting point
o Majority of covalent bonds have already been broken, only forces in the liquid need to
be overcome

Simple Molecular – P4,S8,Cl2
 All have weak Van der Waals forces between the molecules which require little energy to
overcome
 Van der Waals forces depend on molecular mass

Atomic – Ar
 Only very weak Van der Waals forces between atoms
 Ar has low Ar, weaked attractive forces, lowest melting point

Reactions with Water
Na
 Fizzes, floats, moves around surface and melts into ball
 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)

Mg
 Very slow reaction in water
 In steam burns with bright white light
 Mg(s) + 2H2O(l) -> Mg(OH)2(aq) + H2(g)
 Mg(s) + 2H2O(g) -> Mg(OH)2(s) + H2(g)

Cl
 Dissolves into pale green solution
 Cl2 + H2O ⇌ HCl + HClO