Properties of Period 3 elements and their oxides

3.2.4

Trend in atomic radius across period 3

Atomic radius decreases

Same shielding

Increase in nuclear charge

So outer shell is more strongly attracted to nucleus, compacting.

Trend in 1^st ionisation energy across period 3

Overall Increase in 1^st ionisation energy

Decrease in atomic radius

Increased nuclear charge

So stronger electrostatic attraction between nucleus and outer electron, requiring more energy to remove.

---

Al 1^st ionisation energy lower than Mg

Mg outer electron in 3s orbital

Al outer electron in 3p orbital

3p is of higher energy than 3s

So more energy required to remove electron from 3s orbital.

---

S 1^st ionisation energy lower than P

S has one spin pair in the 3p orbital which repel each other

So requires less energy to remove outer electron from S.

Trend in electronegativity across period 3

Increase in electronegativity

Atomic radius decreases

Nuclear charge increases

So stronger electrostatic attraction between the nucleus and electron in a bond.

Trend in melting point across period 3

Na, Mg, Al are metallic lattices.

Strong electrostatic attraction between oppositely charge ions.

---

Si is a giant covalent.

Many strong covalent bonds, require a lot of energy to overcome.

---

P, S, Cl are simple covalent molecules.

Van der Waal’s forces are stronger the larger the molecule.

S₈ > P₄ > Cl₂

So S₈ requires most energy to overcome VDW.

---

Ar exists as single atoms.

Explain the pH of solution of Al₂O₃

Has ionic lattice structure with covalent character.

Covalnt character means it is not water soluable.

So pH of solution remains at 7

Explain the pH of solution of SiO₂

Macromolecular covalent

Many strong covalent bonds need to be broken for it to dissolve in water.

So solution remains at pH 7.

Explain the pH of solution of Na₂O & MgO

Ionic lattices, Ions free to move, so solution is basic.

Na₂O_(s) + H₂O_(l) → 2NaOH_(aq) pH 14

MgO_(s) + H₂O_(l) → Mg(OH)_2(aq) pH 9

Mg²⁺ creates a stronger ionic bond than Na⁺, so MgO is less soluble, so less O^2-, so lower OH^-. Therefore MgO is a weaker base.

Explain the pH of solution of P₄O₁₀ , SO₃, SO₂

Simple covalent molecular

P₄O_{10 (s)} + 6H₂O → 4H₃PO_{4 (aq)} pH 1

SO_{3 (g)} +H₂O → H₂SO_{4 (aq)} pH 1

SO_{2 (g)} +H₂O → H₂SO_{3 (aq)} pH 3

Forms strong acid so has lower pH.

Sulfuric acid and sulfate (VI) anion, SO₄^2-

Phosphoric acid and Phosphate anion, PO₄^3-

Sulfuric acid (IV) and Sulfate (IV) anion, SO₃^2-

Melting points of period 3 oxides

Sodium and water

Rapid with cold water.

Na_(s) + H₂O_(l) → NaOH_(aq) + H_{2 (g)}

Magnesium and water

Reacts slowly with cold water (l)

Faster with steam (g)

Mg_(s) + 2H₂O_(l) → Mg(OH)_{2 (aq)} + H_{2 (g)}

Mg_(s) + H₂O_(g) → MgO _(aq) + H_{2 (g)}

Which solution is a stronger alkali, Mg(OH)₂ or NaOH?

NaOH is stronger as it is more soluble in water so release more OH^-.

phophorus oxide

P₄O₁₀

Silicon oxide

SiO₂

Aluminium oxide

Al₂O₃

Magnesium oxide

MgO

Sodium oxide

Na₂O

Oxides of sulfur

SO₃

SO₂

Which period 3 oxides react with acids?

The basic oxides, magnesium oxide and sodium oxide

Which period 3 oxides react with bases?

The acidic oxides, phosphorus oxide, sulfur dioxide and sulfur trioxide, react with bases.

Does Aluminium oxide react with acids or bases?

Aluminium oxide is amphoteric, it reacts with solutions of strong bases and with solutions of strong acids.

Does SiO₂ oxide react with acids or bases?

Silicon dioxide reacts with hot concentrated alkaline solutions

Aluminium oxide reacts with acid and base

Al₂O₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂O

---

Al₂O₃ + 2NaOH + 3H₂O → 2Na⁺ + 2[Al(OH)₄]^-

Silicon dioxide reaction with base

SiO₂ + 2NaOH → Na₂SiO₃ +H₂O

Acid + Base

→ Salt + Water

P₄O₁₀ add base

P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O

Al2O3 + H2O

Al2O3 is insoluble in water so no reaction occurs