2.4 Properties of Period 3 elements and their oxides

Key baseline facts

Period 3 incorporates eight elements, Na (Z=11) to Ar (Z=18).

All Period 3 elements have three filled inner energy levels (electron shells) corresponding to Ne ([Ne]=1s2 2s2 2p6), which shields the nuclear charge from the outermost valence electrons.

Across the Period, there is successive filling of the outermost 3s (3sand 3s2), then, filling of the even higher energy 3p orbitals (3p1 to 3p6). There is also additional proton nuclear charge. However, given there is no significant increase of electron shielding, the resulting increasing proton nuclear charge results in a general trend of:
•increasing effective nuclear charge and decreasing atomic radii so an increasing effective nuclear charge density
•increasing electronegativity
•increasing first ionisation energy (with small dips at Group III and VI)
•increasing maximum oxidation state (in accordance with the number of its valence electrons).

Across the Period, there is a trend from:
metallic (Na, Mg, Al) to metalloid (Si) to non-metal (P, S, Cl, Ar)
• ionic to covalent compound formation that applies to Period 3 oxides, halides and hydrides.

Across the Period, the increasing proton nuclear charge outweighs the effect of an extra electron in the same energy level (shell), hence no significant increase in shielding, so effective nuclear charge increases.

Reactions with water 

Sodium reacts vigorously with water resulting in sodium hydroxide solution (strong base)
2Na + 2H2O → 2NaOH + H2

Magnesium  reacts vigorously with steam to given a intense white light and white solid magnesium oxide:
Mg + H2O → MgO + H2

The rate of reaction of magnesium with with cold water is extremely slow and yields magnesium hydroxide (partially soluble weak base):
Mg + 2H2O → Mg(OH)2 + H2

Reactions with oxygen and trends

Formation of oxides:

4Na + O2   → 2Na2O
2Mg + O2   →2MgO
4Al +  3O2  →2Al2O3
Si   +  O2    → SiO2
P4 +   5O2   → P4O10
S8 +  😯2   →8SO2

Note: phosphorous, sulfur and chlorine form more than one oxide, so more than one oxidation state is possible

Reactions of the oxides of the elements with water: acid-base character of the oxides

Formula of oxide Na2O MgO Al2O3 SiO2 P4O10 SO2 SO3
state of oxide at 298K solid solid solid solid solid gas liquid
Bonding and structure
in oxide
ionic lattice ionic lattice ionic lattice giant covalent
Reaction of oxide with water reacts to form
NaOH (aq)
strong base
reacts to form
Mg(OH)2 (aq)
weak base
does not
react with
does not
react with
reacts to form
H3PO4 (aq)
weak acid
reacts to form
H2SO3 (aq)
weak acid
reacts to form
H2SO4 (aq)
strong acid
Aqueous solution pH 14 10 7 7 1 1 0
Nature of oxide basic basic amphoteric acidic acidic acidic acidic

Note the trend of:
1. Increasing acidity of the oxide and the corresponding decrease in the pH of their aqueous solutions.
2. Across the Period, the trend of ionic lattice to giant covalent to covalent molecular bonding of the oxide.
3. Aluminium oxide is amphoteric.

Selected reactions between oxides and given acids and bases

Basic Oxides

Na2+ H2O→2NaOH               NaOH (aq) is a strong base (pH 14)
Na2O + H2SO4 → Na2SO4 + H2O

MgO + H2O  → Mg(OH)2           Mg(OH)(aq) is only partially ionised and is a weak base (pH 10)
MgO + 2HCl  →MgCl2  + H2O

Amphoteric oxide: hydrated aluminium oxide behaves as if it an approximate formula of Al(OH)3

Al(OH)3  + 3H+ →   Al3+ + 3H2O
Al(OH)3 + OH  →  [Al(OH)4]

Acidic oxides:

SiO2+ 2NaOH→ Na2SiO3 + H2O
P4O10 + 6H2O →  4H3PO4
SO2 + H2O  → H2SO3
SO3 + H2O  → H2SO4


H3PO4 ionises in three stages to form H2PO4, HPO42− and PO43− ions.
The pKa values for the successive loss of the three protons are pKa1=2.2, pKa2=7.2 and pKa3=12.3
pKa2 value is close to neutral so it represents an important biological phosphate buffer (H2PO4/HPO42−)

H2SO4 ionises in two stages to form HSO4 and SO42−
H2SOalso ionises in two stages to form HSO3 and SO32−

[2015 syllabus onwards]
The structures of the acids and the anions formed when P4O10, SO2 and SO3 react with water

Phosphoric acid is tetrahedral


H3PO4 images

Sulfuric acid is tetrahedral

H2SO4 download

Sulfurous acid is pyramidal (steric 4; one lone pair; bond angle 106º)



Assessment and practical opportunities

Students could carry out reactions of elements with oxygen and test the pH of the resulting oxides.