Bonding and Structure Problem Solving

This lesson brings together everything you have learned about bonding and structure into a problem-solving framework. A-Level Chemistry exams frequently ask you to draw structures, predict shapes, explain properties, and compare substances. The key to success is a systematic approach that connects structure to properties through clear reasoning.

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Drawing Dot-and-Cross Diagrams for Complex Molecules

Dot-and-cross diagrams (Lewis structures) are the starting point for understanding bonding. Here is a reliable method for drawing them correctly:

Step-by-Step Method

1. Count the total number of valence electrons. Sum the outer shell electrons of all atoms. For ions, add electrons for negative charges and subtract for positive charges.

2. Identify the central atom. This is usually the least electronegative atom (excluding hydrogen, which is always terminal).

3. Place single bonds between the central atom and each surrounding atom. Each bond uses 2 electrons.

4. Distribute remaining electrons as lone pairs. Start with the outer atoms (give each atom an octet, or 2 for hydrogen). Then place any leftover electrons on the central atom.

5. Check octets. If the central atom has fewer than 8 electrons, convert lone pairs on outer atoms into double or triple bonds to the central atom.