1.4 Periodicity

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First Ionisation Energy

 Generally INCREASES across Period 3

  • Nuclear charge increases
  • Shielding stays the same
  • Electrostatic attraction is stronger so electrons are harder to remove

DECREASES SLIGHTLY between Mg and Al

  • Outer electron is in new p-orbital in Al
  • Better shielded so easier to remove

Also DECREASES SLIGHTLY between P and S

  • Sulphur's outer electron is paired
  • Repulsion between paired electrons makes them easier to remove
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Atomic Radius

DECREASES across Period 3

  • Nuclear charge increases
  • Shielding stays the same
  • Electrostatic attraction increases
  • Outer electrons are pulled closer to the nucleus
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Electronegativity

INCREASES across Period 3

  • Nuclear charge increases
  • Shielding stays the same
  • Electrons are more strongly attracted to atom
  • Atom therefore has a larger share of electrons in a covalent bond

 

Electronegativity: the tendency of an atom to attract a bonding pair of electrons towards itself.

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Melting Point

INCREASE between Na And Si

  • Na, Mg and Al are metals with metallic bonding
  • Across Period 3, charge on metal ions in the metallic structure increases
  • Number of delocalised electrons also increases
  • Strength of metallic bonding increases

STRONG INCREASE from Al to Si

  • Silicon is a metalloid with giant covalent bonding
  • Strong covalent bonds in silicon's giant strucuture require a lot of energy to break

Generally DECREASE from Si to Ar

  • P, S and Cl form simple molecular structures with weak Van der Waal's forces
  • S8 > P4 > Cl2 (Number indicates atoms in a molecule of  each element)
  • Larger molecules = higher melting point due to more VDW forces
  • So slight increase in melting point from P to S
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Boiling Point

INCREASES from Na to Al

  • Same reasons as for melting point

DECREASES from Al to Ar

  • P, S and Cl are non-metals which form simple molecular structures
  • Weak VDW forces act between molecules
  • Take little energy to overcome
  • Slight increase at S
  • Sulphur molecules are formed from a ring of 8 atoms
  • Large number of atoms = stronger Van der Waal's forces
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