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QLDChemistryQuick questions
Unit 1: Chemical fundamentals (structure, properties and reactions)
Quick questions on Covalent bonding, Lewis structures, VSEPR and polarity (QCE Chemistry Unit 1)
11short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is drawing Lewis structures?Show answer
Procedure for a simple covalent species:
What is vSEPR theory?Show answer
VSEPR (Valence Shell Electron Pair Repulsion) predicts 3D shape from the count of electron regions around the central atom. An "electron region" is a single bond, a double bond, a triple bond, or a lone pair (each counts as one region). Electron regions arrange themselves to be as far apart as possible.
What is bond polarity and electronegativity?Show answer
A bond is polar if the two atoms have different electronegativities. The more electronegative atom carries a partial negative charge (delta-); the less electronegative atom carries a partial positive charge (delta+).
What is overall molecular polarity?Show answer
A molecule is overall polar if its bond dipoles do not cancel as vectors, and overall non-polar if they do cancel. Two questions to ask:
What is a workflow QCAA rewards?Show answer
For each molecule in a question, work in this order:
What is forgetting lone pairs on the central atom?Show answer
Lone pairs on the central atom change the shape (H_2O is bent, not linear).
What is predicting bond angles without adjusting for lone pairs?Show answer
A tetrahedral electron-region geometry with two lone pairs (H_2O) is not 109.5 degrees; it is approximately 104.5 degrees.
What is calling CCl_4 polar because the bonds are polar?Show answer
Symmetric geometry cancels the bond dipoles. Always check both bond polarity and geometry.
What is mis-counting electron regions in resonance structures?Show answer
A double bond counts as one region, not two. C in CO_2 has two regions, not four.
What is confusing electronegativity with ionisation energy or electron affinity?Show answer
They correlate but are distinct concepts. Electronegativity refers specifically to attraction in a bond.
What is treating "ionic versus covalent" as a sharp threshold?Show answer
The two are extremes of a continuum. Polar covalent bonds (like H-Cl) share the spectrum.