AQA AS Chemistry Unit 1 Notes: Periodicity

Covering trends across Period 3.


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Trends across Period 3
Atomic radius decreases across Period 3
This is because across the period, the atoms have more protons and the
electrons in the outer shell are pulled closer to the positive nucleus.
Melting and boiling point fluctuates across Period 3
This is because of the bonding between molecules and structures the atoms form.
Elemen Na Mg Al Si P S Cl Ar
Bondin Metalli Metalli Metalli Macromolecula Simple Simple Simple Non
g c c c r Covalen Covalen Covalen e
t t t
Melting 371 922 933 1683 317 392 172 84
Sodium, magnesium and aluminium all form giant metallic crystals. The
melting point increases from Na to Al because each atom loses more
electrons to become metal ions in the metallic structure (Na+, Mg2+, Al3+), as
well as because the atomic radius decreases. These two factors combined

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Silicon is a giant covalent (macromolecular) tetrahedral structure with
strong covalent bonds between atoms that require lots of energy to break.
Phosphorus, sulphur and chlorine are all simple covalent (molecular)
structures. Because of the nature of their electron shells, these atoms form
P4, S8 and Cl2 molecules respectively. Bigger molecules have stronger Van
der Waals forces between them, explaining the increase in melting point
between P and S and decrease between S and Cl.…read more


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