Edexcel A-Level Chemistry AS Paper 1, June 2025: Question 1

Structure, Polarity & Intermolecular Forces in Cl₂ and SiCl₄

📌 What this question tests

This question assesses core physical and inorganic chemistry concepts from Topic 2 (Bonding and Structure):

Recalling the exact IUPAC definition of electronegativity.
Drawing an accurate dot-and-cross diagram for a simple covalent molecule with lone pairs.
Explaining molecular polarity by linking electronegativity differences, bond dipoles, and 3D molecular symmetry.
Comparing London forces between molecules based on total electron count and relating this to boiling temperatures.

Part (a) Definition of Electronegativity

Multiple Choice (1 Mark)

✅ Correct Answer

B: a pair of electrons from a covalent bond

[1 Mark] for selecting option B.

💡 Key Knowledge

Standard Definition: Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond.

❌ Why Other Options Fail

A is incorrect: It attracts an electron pair, not just a single electron.
C is incorrect: It refers strictly to covalent bonds, not vague electron density from a neighbouring atom.
D is incorrect: Electronegativity applies to neutral atoms sharing electrons in a covalent bond, not positive ions.

Part (b) Dot-and-Cross Diagram of SiCl₄

Drawing Molecular Structure (2 Marks)

✅ Required Diagram Features

Central Silicon: 4 single covalent bonds to 4 chlorine atoms (each bond consisting of one cross × from Si and one dot • from Cl). Zero lone pairs on Si.
Surrounding Chlorines: Each of the 4 chlorine atoms must have 3 lone pairs (6 non-bonding dots •), giving each Cl a full octet of 8 outer electrons.

          ••
       • Cl •
          •×
••                ••
• Cl •×  Si  ×• • Cl •
••        •×      ••
       • Cl •
          ••

Mark 1: Four bonding pairs and no lone pairs on the central Si atom.
Mark 2: 3 lone pairs (6 non-bonding electrons) on each of the four Cl atoms.

❌ Common Errors & Examiner Traps

Missing the lone pairs on Cl: By far the most common error. Students draw the 4 shared pairs but leave the chlorine outer shells empty, losing Mark 2.
Incorrect symbols: The question specifically states: crosses (×) for silicon and dots (•) for chlorine. While reversed symbols are allowed, mixing them inconsistently risks losing credit.
Inner shell electrons: The prompt states outer shells only. Drawing inner shells wastes valuable exam time.

Part (c) Explaining Molecular Non-Polarity

Applying Electronegativity & Molecular Geometry (3 Marks)

✅ Model 3-Mark Answer

Chlorine (Cl₂): Both chlorine atoms have identical electronegativities (no electronegativity difference), so there is no dipole / the bond is non-polar.
Si–Cl Bonds: Chlorine is more electronegative than silicon, meaning each Si–Cl bond is polar (has a permanent dipole).
SiCl₄ Molecule: The SiCl₄ molecule is symmetrical (tetrahedral shape), so the individual bond dipoles cancel out, leaving no overall molecular dipole.

Mark 1: Cl₂ has no electronegativity difference (so no dipole).
Mark 2: Si–Cl bond is polar / has a dipole (Cl is more electronegative than Si).
Mark 3: Symmetrical / tetrahedral arrangement causes bond dipoles to cancel out.

🧠 Exam Technique: "Polar Bond" vs "Polar Molecule"

To score full marks on molecular polarity questions, always structure your answer in two clear stages:

Stage 1 (Bonds): State whether individual bonds are polar by comparing the electronegativity of the bonded atoms.
Stage 2 (Whole Molecule): State whether the dipoles cancel or reinforce by referring to symmetry or shape (e.g. tetrahedral).

❌ Common Errors

Stating that "SiCl₄ has non-polar bonds" — incorrect! The Si–Cl bond is distinctly polar; it is the geometry that makes the molecule non-polar.
Forgetting to explain both substances when the question asks why both chlorine and silicon tetrachloride are non-polar.

Part (d) Boiling Temperature Comparison

Intermolecular Forces (2 Marks)

✅ Model Answer

Both chlorine and silicon tetrachloride are simple molecular substances held together by London forces (instantaneous induced dipole–dipole forces) between molecules.

Chlorine molecules have fewer electrons than silicon tetrachloride molecules, so the London forces in Cl₂ are weaker and require less energy to overcome.

Mark 1: Identifying that both molecules are held together by London forces (or dispersion / van der Waals forces).
Mark 2: Cl₂ has fewer electrons, so its London forces are weaker (or converse for SiCl₄).

📐 Supporting Electron Count Check

Cl₂: 2 × 17 = 34 electrons
SiCl₄: 14 + (4 × 17) = 82 electrons
More electrons → larger electron cloud → greater temporary fluctuations → stronger induced dipoles → stronger London forces.

❌ Fatal Error: Breaking Covalent Bonds

Never say "covalent bonds are broken during boiling"! Boiling simple covalent molecules only breaks weak intermolecular forces (London forces). Breaking covalent bonds scores 0 marks for the explanation.

🧠 Acceptable Terminology

The mark scheme accepts: London forces, instantaneous induced dipole–dipole forces, dispersion forces, or van der Waals' forces. Always mention the number of electrons, not atomic mass or molecular mass.