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Updated Mar 14, 2026

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2 pages

Molecular Shapes and Their Arrangements

Rosie

@rosie_321

Understanding molecular shapes is crucial for predicting how compounds behave... Show more

# Shapes of molecules and polyatomic ions
- electron repulsions are responsible for the shape of a molecule
V S E P R
Valence Shell Electro

VSEPR Theory and Basic Molecular Shapes

VSEPR theory is your go-to tool for predicting molecular shapes, and it's actually quite straightforward once you get the hang of it. The basic idea is that electron pairs around a central atom repel each other and arrange themselves as far apart as possible.

Here's the step-by-step method: First, count the outer electrons on the central atom. Then add one electron for each atom attached to it. For charged ions, subtract one electron for each positive charge or add one for each negative charge. Finally, divide by 2 to get the total number of electron pairs.

The number of bonding pairs determines the basic shape. Two pairs give you a linear molecule (180°), three pairs create trigonal planar (120°), four pairs form tetrahedral, five pairs make trigonal bipyramidal, and six pairs result in octahedral geometry.

Quick Tip: Remember that CH₄ (methane) is your classic tetrahedral example - 4 carbon electrons + 4 hydrogen atoms = 8 electrons total, which means 4 bonding pairs.

How Lone Pairs Affect Molecular Shape

Here's where things get interesting - lone pairs $non-bonding electron pairs$ dramatically change the actual shape you observe. Even though the electron arrangement stays the same, the molecular geometry looks completely different.

Starting with tetrahedral electron arrangement: CH₄ has 4 bonding pairs and keeps its tetrahedral shape. But NH₃ has 3 bonding pairs and 1 lone pair, creating a trigonal pyramidal shape instead. Water (H₂O) has 2 bonding pairs and 2 lone pairs, resulting in an angular or bent shape.

The key insight is that lone pairs take up space but aren't visible in the final molecular shape. They're like invisible passengers that push the bonding pairs closer together, changing bond angles and overall geometry.

Remember: The electron pair geometry tells you where all electrons go, but the molecular geometry only describes where the atoms actually sit.

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129

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Updated Mar 14, 2026

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2 pages

Molecular Shapes and Their Arrangements

Rosie

@rosie_321

Understanding molecular shapes is crucial for predicting how compounds behave and react. The VSEPR theory (Valence Shell Electron Pair Repulsion) gives you a simple method to work out molecular geometry by considering how electron pairs repel each other around a... Show more

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VSEPR Theory and Basic Molecular Shapes

Quick Tip: Remember that CH₄ (methane) is your classic tetrahedral example - 4 carbon electrons + 4 hydrogen atoms = 8 electrons total, which means 4 bonding pairs.

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How Lone Pairs Affect Molecular Shape

Remember: The electron pair geometry tells you where all electrons go, but the molecular geometry only describes where the atoms actually sit.