Periodicity of Chemical Properties of the Elements in Period 3 (Cambridge (CIE) A Level Chemistry): Flashcards

NaCl and MgCl₂ have giant ionic structures. Polar water molecules are attracted to the ions, hydrating them and breaking down the lattice — no chemical reaction occurs, only dissolving.

O^2- (aq) + H₂O (l) → 2OH^- (aq)

This occurs in solutions of ionic oxides such as Na₂O and MgO, which is why they produce alkaline solutions.

False.

Anhydrous AlCl₃ is covalent and exists as the dimer Al₂Cl₆. It only behaves ionically when dissolved in aqueous solution, where it dissociates into Al³⁺ and Cl^- ions.

False.

SiO₂ is an acidic oxide. Its giant covalent bonding means it has high melting points, but the acid-base character is determined by the electronegativity of Si relative to oxygen, which leads to covalent bonding and acidic behaviour.

When SiCl₄ is hydrolysed by water, it produces a white precipitate of SiO₂ and white fumes of HCl.

Al₂O₃ reacts with acids to form a salt and water, and with hot, concentrated NaOH (alkali) to form NaAl(OH)₄.

Hydrolysis is a chemical reaction in which a compound reacts with water and is broken down. In Period 3 chemistry, covalent chlorides such as SiCl₄ and PCl₅ undergo hydrolysis to produce HCl gas.

Na and Mg oxides are ionically bonded. Their oxide ions (O^2-) react with water to form OH^- ions, giving alkaline solutions. P and S oxides are covalently bonded and react with water to form acids (e.g. H₂SO₄, H₃PO₄) that donate H⁺ ions.

PCl₅ (s) + 4H₂O (l) → H₃PO₄ (aq) + 5HCl (g)

Both H₃PO₄ and dissolved HCl are acidic, giving the solution a pH around 2.

False.

Mg(OH)₂ is a basic compound only. It is Al(OH)₃ that is amphoteric, reacting with both HCl to form AlCl₃ and with NaOH to form NaAl(OH)₄.

False.

The hydrolysis of SiCl₄ produces HCl (g), some of which dissolves in water to give an acidic solution (pH ≈2).

SO₃ reacts with water to form H₂SO₄ (aq), which then donates H⁺ to water to produce an acidic solution.

The highly charged Al³⁺ ion becomes hydrated. Its strong charge polarises the O–H bonds in the coordinated water molecules, making it easier for one water molecule to release an H⁺ ion into solution. This gives the solution its acidic character (pH < 7).

Oxygen has an electronegativity of 3.5, much higher than Na (0.9) and Mg (1.2). The large difference means electrons are transferred from the metal to oxygen, forming ionic bonds in Na₂O and MgO.

Period 3 chlorides with covalent bonding hydrolyse in water to give acidic solutions, while those with ionic bonding simply dissolve to give neutral or near-neutral solutions.

Al(OH)₃ (s) + NaOH (aq) → NaAl(OH)₄ (aq)

This demonstrates the acidic behaviour of aluminium hydroxide, acting as an acid towards a base.

SiCl₄ (l) + 2H₂O (l) → SiO₂ (s) + 4HCl (g)

This is a rapid reaction producing a white precipitate of SiO₂ and fumes of HCl.

Electronegativity is the power of an atom to draw the bonding electrons towards itself in a covalent bond.

Across Period 3, nuclear charge increases while shielding stays roughly constant (electrons are added to the same shell). This means the nucleus attracts bonding electrons more strongly, increasing electronegativity from Na to Cl.

True.

Aluminium donates 3 electrons per atom to the delocalised sea, creating more charge carriers and stronger metallic bonding than sodium (1 electron) or magnesium (2 electrons), making Al the best conductor of the three.

Across Period 3, bonding changes from metallic in Na, Mg and Al to covalent in Si, P, S and Cl.

P, S and Cl exist as simple molecules with only weak instantaneous dipole-induced dipole forces between them. They have no delocalised electrons, so there are no mobile charge carriers to carry an electric current.

False.

Silicon has a giant molecular (covalent) structure in which each Si atom is covalently bonded to four neighbouring Si atoms. This is fundamentally different from phosphorus, which exists as simple P₄ molecules.

In a giant metallic lattice, positive metal ions are arranged in a regular lattice surrounded by a 'sea' of delocalised electrons from the outer (valence) shell of each atom.

Phosphorus exists as P₄, sulfur as S₈ and chlorine as Cl₂. These are all simple molecular structures with strong covalent bonds within the molecules and weak instantaneous dipole-induced dipole forces between them.

Al³⁺ has a higher ionic charge and contributes more delocalised electrons (3 per atom) than Na⁺ (1 per atom). The greater electrostatic forces between the Al³⁺ ions and the larger 'sea' of electrons result in stronger metallic bonding.

NaCl has ionic bonding and a giant ionic structure. SiCl₄ has covalent bonding and a simple molecular structure. This reflects the shift from ionic to covalent character going across Period 3.

False.

Al₂O₃ has ionic bonding with some covalent character, unlike Na₂O and MgO which are purely ionic. This mixed bonding explains why Al₂O₃ is amphoteric.

Going across Period 3, chlorides and oxides become more covalent in bonding and shift from giant ionic to simple molecular structures.

A giant ionic structure is a three-dimensional lattice of oppositely charged ions held together by strong electrostatic forces. Examples in Period 3 include NaCl, MgCl₂, Na₂O and MgO.

From Na₂O to SO₃, the bonding changes from ionic (Na₂O, MgO) through mixed ionic-covalent (Al₂O₃) to covalent (SiO₂, P₄O₁₀, SO₂, SO₃). The structure changes from giant ionic to giant covalent to simple molecular.

False.

PCl₅ has covalent bonding and a simple molecular structure. It undergoes hydrolysis when added to water, unlike ionic chlorides which simply dissolve.

Covalent chlorides such as SiCl₄ and PCl₅ are hydrolysed by water, breaking covalent bonds. Ionic chlorides such as NaCl and MgCl₂ simply dissolve as their ions are already separated. Hydrolysis requires bond breaking and is more vigorous.

SiO₂ has a giant covalent structure, while P₄O₁₀ and SO₂ have simple molecular structures.

Al₂Cl₆ (anhydrous aluminium chloride) has covalent bonding and a simple molecular structure, existing as a dimer. This is in contrast to NaCl and MgCl₂, which are ionic with giant lattice structures.