Chemistry (9701) | Periodicity of chemical properties of the elements in Period 3

🧠 Mastering Periodicity: Period 3 Elements (Na to Ar)

Hello future Chemists! This chapter is incredibly important because it takes all those fundamental atomic structure concepts (like nuclear charge and shielding) and applies them to real-world trends. Period 3 elements (Sodium to Argon) are the perfect group to study because they show dramatic and predictable changes in properties as we move across the table.

Understanding these trends—known as periodicity—will help you predict the behaviour of any unknown element! Let's dive into how structure, bonding, and electrons dictate everything.

Quick Reminder: The Elements of Period 3

The elements are: Na, Mg, Al, Si, P, S, Cl, Ar. They are filling the third electron shell, meaning they all have the same amount of electron shielding from inner shells (the 1s²2s²2p⁶ electrons).

9.1 Periodicity of Physical Properties

9.1.1 Atomic and Ionic Radii

Trend in Atomic Radius

Across Period 3, the atomic radius decreases.

• The Explanation: As you move from Na (Z=11) to Ar (Z=18), the number of protons (the nuclear charge) increases.
• The number of occupied electron shells remains the same (three shells), so the shielding effect remains approximately constant.
• The increasing nuclear charge pulls the outer electrons closer to the nucleus, resulting in a smaller atomic radius.

Trend in Ionic Radius

The trend here is trickier because we compare two different types of ions: positive ions (cations) on the left and negative ions (anions) on the right.

1. Cations (Na⁺, Mg²⁺, Al³⁺):
These ions are much smaller than their parent atoms because they have lost their entire outer (third) shell. They also decrease in size from Na⁺ to Al³⁺ because the nuclear charge increases (11+ to 13+) pulling the remaining electrons (10 electrons) tighter.

2. Anions (P^3-, S^2-, Cl^-):
These ions are much larger than their parent atoms because they have gained electrons, increasing inter-electron repulsion within the third shell, causing the cloud to swell. They decrease in size from P^3- to Cl^- because the nuclear charge increases.

9.1.2 Melting Point and Structure/Bonding

The variation in melting points is complex because Period 3 elements exhibit four different fundamental types of structure and bonding. Melting points generally increase sharply to Silicon and then decrease dramatically.

Analogy: Think of melting point as breaking the construction. Si is a massive, complex skyscraper (covalent network), while Ar is a single, detached Lego block (weak forces).

9.1.3 Electrical Conductivity

Electrical conductivity depends entirely on the presence of mobile charged particles (usually delocalised electrons or ions).

Across Period 3, conductivity increases sharply then drops to zero.

9.2 Periodicity of Chemical Properties (Reactions)

The chemical properties depend on how the elements react, which is governed by the electronegativity and the tendency to achieve a stable electronic configuration.

9.2.1 Trend in Oxidation Number in Oxides and Chlorides

Across Period 3, the maximum oxidation state generally increases, reflecting the number of valence electrons available for bonding.

Oxides: The maximum oxidation state shown by the elements in their oxides increases from +1 (Na) to +7 (Cl).

• Na₂O (+1)
• MgO (+2)
• Al₂O₃ (+3)
• SiO₂ (+4)
• P₄O₁₀ (+5)
• SO₃ (+6)
• Cl₂O₇ (+7)

Note: Sulfur dioxide (SO₂, S is +4) and Phosphorus pentachloride (PCl₅, P is +5) show that elements in Period 3 can expand their octet due to vacant d orbitals, unlike Period 2 elements.

Chlorides: The maximum oxidation state in chlorides also follows this trend.

• NaCl (+1)
• MgCl₂ (+2)
• AlCl₃ (+3)
• SiCl₄ (+4)
• PCl₅ (+5)

Explanation (9.2.2): The increase in oxidation number corresponds to the number of outer shell electrons available for bonding. For the metals, this results in the formation of ions; for the non-metals, this involves sharing electrons, often utilizing energetically accessible vacant 3d orbitals (for Si, P, S, Cl) to expand the octet.

9.2.2 Reactions of Elements with Oxygen, Chlorine, and Water (9.2.1)

The reactions become progressively less vigorous across the period, especially towards water.

9.2.3 & 9.2.4 Acid/Base Behaviour of Oxides and Reactions with Water

This is the most crucial chemical trend! When dissolved in water (or reacted with acid/base), Period 3 oxides show a transition from Basic to Amphoteric to Acidic behaviour.

Memory Aid: Think BAA - Basic, Amphoteric, Acidic.

1. Basic Oxides (Metals: Na₂O, MgO)

These oxides are ionic. The O^2- ion reacts strongly with water to form OH^- ions, increasing the pH (alkaline solution).

• Sodium Oxide (Na₂O): Very soluble, strongly basic.

\(\text{Na}_2\text{O}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow 2\text{NaOH}(\text{aq})\) (pH ~ 14)

• Magnesium Oxide (MgO): Sparingly soluble, weakly basic.

\(\text{MgO}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{Mg(OH)}_2(\text{aq})\) (pH ~ 9)

2. Amphoteric/Insoluble Oxides (Metalloids: Al₂O₃, SiO₂)

These oxides have stronger covalent character and are insoluble in water, but they react with both strong acids and strong bases.

• Aluminium Oxide (Al₂O₃): Insoluble, Amphoteric.

Reacts with strong acid (e.g., HCl):
\(\text{Al}_2\text{O}_3(\text{s}) + 6\text{HCl}(\text{aq}) \rightarrow 2\text{AlCl}_3(\text{aq}) + 3\text{H}_2\text{O}(\text{l})\)
Reacts with strong base (e.g., hot NaOH):
\(\text{Al}_2\text{O}_3(\text{s}) + 2\text{NaOH}(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{Na}[\text{Al(OH)}_4](\text{aq})\) (Sodium tetrahydroxoaluminate)

• Silicon Dioxide (SiO₂): Insoluble, slightly acidic. Reacts only with very strong bases (like NaOH).

3. Acidic Oxides (Non-Metals: P₄O₁₀, SO₂, SO₃)

These oxides are covalent molecules. When reacted with water, they hydrolyse completely to form strong acids, resulting in low pH solutions.

• Phosphorus(V) Oxide (P₄O₁₀): Highly reactive with water, forms phosphoric acid.

\(\text{P}_4\text{O}_{10}(\text{s}) + 6\text{H}_2\text{O}(\text{l}) \rightarrow 4\text{H}_3\text{PO}_4(\text{aq})\) (pH ~ 2)

• Sulfur Dioxide (SO₂): Dissolves, forms sulfurous acid (weak acid).

\(\text{SO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{H}_2\text{SO}_3(\text{aq})\) (pH ~ 3)

• Sulfur Trioxide (SO₃): Highly reactive with water, forms sulfuric acid.

\(\text{SO}_3(\text{l}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{H}_2\text{SO}_4(\text{aq})\) (pH ~ 1)

9.2.5 Reactions of Chlorides with Water

The behaviour of Period 3 chlorides in water also shifts from simple dissolution to violent hydrolysis, which is directly related to the change in bonding (9.2.6 & 9.2.7).

Why do the covalent chlorides hydrolyse violently? (9.2.6)

Covalent chlorides like SiCl₄ and PCl₅ hydrolyse easily because the central atom (Si or P) has vacant d orbitals (3d) that can accept a lone pair of electrons from a water molecule (acting as a nucleophile). This forms an unstable intermediate which then breaks down, releasing H⁺ ions (HCl gas) and leading to a highly acidic solution.

9.2.6 & 9.2.7 Explaining Trends using Bonding and Electronegativity

We can tie all these trends together by looking at the change in bonding:

1. Sodium to Aluminium: The bonding is predominantly ionic (metal + non-metal), resulting in basic properties. However, as the metal ion charge increases (Na⁺ to Al³⁺) and ion size decreases, the power of the ion to distort the electron cloud of the anion (its polarising power) increases. This pulls the bonding towards being more covalent. AlCl₃ shows clear covalent properties (dimer Al₂Cl₆, lower melting point, violent hydrolysis).

2. Silicon to Chlorine: These compounds are formed between two non-metals, leading to covalent bonding (simple molecular or giant molecular).

Deducing Bonding from Properties (9.2.7)

If you are given physical properties, you can deduce the bonding type:

• High MP, conducts electricity: Giant Metallic (Na, Mg, Al)
• Extremely High MP, does not conduct: Giant Molecular Covalent (Si)
• Low MP, does not conduct, hydrolyses violently in water: Simple Molecular Covalent (SiCl₄, PCl₅, SO₃)
• Dissolves in water, gives neutral/basic solution: Giant Ionic (NaCl, Na₂O, MgO)