1.4 Periodicity

 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

DECREASES across Period 3

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

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.

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

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