Double or triple bonds are formed in Lewis structures when doing so helps satisfy the octet rule for all involved atoms and results in a more stable arrangement of electrons.
- Core idea: A single bond uses one pair (two electrons) shared between two atoms. A double bond uses two shared pairs (four electrons), and a triple bond uses three shared pairs (six electrons).
- When to form them:
- If a central atom does not yet have an octet (eight electrons around it), forming one or more additional bonds can help complete its octet.
- If expanding the octet is possible and appropriate for the atoms involved (common for elements in or beyond the third period, e.g., sulfur, phosphorus, chlorine in certain molecules).
- When a terminal atom has lone pairs that can be shared with the central atom to satisfy its octet more efficiently, creating a double or triple bond in the process.
- In molecules where multiple bonds are necessary to place enough electrons around the atoms to meet their typical valence needs (for example, O=C=O in carbon dioxide or N≡N in nitrogen gas).
Notes and common guidance:
- The octet rule is a guiding principle for main-group elements; some atoms can exceed eight electrons (expanded octet) without issue.
- Resonance can mean a structure with one or more double bonds is just one contributing form; the actual molecule is a resonance hybrid of those forms.
- When drawing Lewis structures, start with the framework (skeletal bonds), place valence electrons, ensure all atoms satisfy their typical valence (octet or expanded octet as appropriate), and adjust by forming additional bonds as needed.
If you’d like, I can walk through a specific example (e.g., CO2, SO3, or CN−) step by step to show exactly when and why double or triple bonds are formed.
