Summary notes for AQA A-Level Chemistry Unit 3.2.1 - Periodicity by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: Classification, Physical Properties of Period 3 Elements
Periodicity
Classi cation
An element is classi ed as s,p,d,f block when the highest energy electrons are in an s,p,d,f sub-shell.
Physical Properties of Period 3 Elements
Atomic Radius
Atomic radius decreases across a period because…
• As the no. of protons ↑ across the period, nuclear charge ↑ ∴ stronger nuclear attraction.
• ∴ outer electrons (which are in the same level across the period) are drawn closer to the nucleus.
Melting Point
1) The m.p.s of metallic elements ↑ across the period.
• Na ➜ Al, no. of outer electrons ↑
• ∴ more electrons can be delocalised, leading to a greater
attraction between +ve ions + delocalised electrons.
• Size of ions ↓ across the period, leading to smaller ions, + ∴
greater attraction between +ve ions + delocalised electrons.
• ∴ more energy required to overcome attraction.
2) The m.p. of silicon is very high.
• Si has a giant covalent structure (compared to giant metallic for Na, Mg + Al).
• ∴ a lot of energy is required to break the strong covalent bonds.
3) The m.p.s of phosphorus (P4), sulfur (S8), chlorine (Cl2) + argon (Ar) are low.
• P4, S8 + Cl2 are simple covalent molecules + little energy is required to overcome the weak van der
Waals’ forces between the molecules.
• Ar exists as atoms + very little energy is required to overcome the weak van der Waals forces
between the atoms.
4) The m.p.s of the elements after Si ↑ from phosphorus to sulfur then ↓ again.
• Attractions between molecules = van der Waals’ forces (as molecules = non-polar).
• Greater the no. of electrons, greater the induced dipole attractions…
• ∴ greater van der Waals’ forces of attraction between molecules.
• This ↑ energy required to overcome attractions.
- P4 no. of electrons < S8 no. of electrons ∴ m.p. ↑ P4 to S8.
- S8 no. of electrons > Cl2 no. of electrons and Ar has very low m.p. as monoatomic (resulting in
very weak van der Waals forces) ∴ m.p. ↓ again.
First Ionisation Energy
First ionisation energy increases across a period
because…
• ↑ nuclear charge w/ similar shielding across the
period, leading to stronger nuclear attraction.
• ∴ atomic radius ↓ across the period.
• ∴ outer electron closer to the nucleus.
• More energy required to remove outer electron.
First ionisation energies drop in group 3
First ionisation energy of atoms of group 3 elements is lower than expected because…
• Group 3’s outer electron is in the p sub-shell whilst group 2’s outer electron is in the s sub-shell.
• ∴ group 3’s outer electron is further from the nucleus than group 2’s + has more shielding than
group 2 (due to more inner electrons).
• ∴ weaker nuclear attraction, so electron more easily removed (less energy required to do so).
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