Molecular shape depends on how electron domains are arranged around atoms. Bonding and lone pairs influence angles and therefore the observed geometry.
The essential idea
Molecular shape depends on how electron domains are arranged around atoms. Bonding and lone pairs influence angles and therefore the observed geometry.
Example
CH₄ is tetrahedral around carbon, whereas H₂O is bent because oxygen's lone pairs alter the shape.
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Mini problem
Why do CO₂ and H₂O have different shapes?
Show the solution
CO₂ has two main bonding domains around carbon with no central lone pair; H₂O has two bonds and two lone pairs around oxygen.
Common mistake
Do not replace reasoning with a memorized rule. Always check geometry, units, coefficients or model assumptions.
Guided exercise
Predict the shape of NH₃ around nitrogen using VSEPR.
Show the detailed solution
Nitrogen has three N–H bonds and one lone pair: four electron domains in total. Electron geometry is tetrahedral, while molecular shape is trigonal pyramidal.
Avoid this
Do not confuse electron geometry with molecular shape: lone pairs count in the first but are not atoms in the second.
Electron-rich regions repel. VSEPR treats bonds and lone pairs as electron domains around the central atom.
Key idea
Molecular shape
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Key idea
Molecular shape
Nitrogen has three N–H bonds and one lone pair: four electron domains in total. Electron geometry is tetrahedral, while molecular shape is trigonal pyramidal.
Guided example
Molecular shape
Nitrogen has three N–H bonds and one lone pair: four electron domains in total. Electron geometry is tetrahedral, while molecular shape is trigonal pyramidal.
Put it into practice
Molecular shape
Choose a concrete case linked to Molecular shape. Write down the known information, what you are trying to find, and the units. Then state the rule, model or trend you will use. If a calculation is involved, keep the units visible at every line; if it is conceptual, state what changes and what remains fixed.
Check your answer
A strong answer contains four things: the starting situation, the variable that changes, the direction or numerical result, and one check. The check can be a unit, an order of magnitude, a limiting case or a comparison with a familiar example.