VSEPR Theory and Molecular Shapes

20 Question-and-Answer flashcards covering VSEPR theory, the effect of lone pairs, common molecular shapes, determination steps, and the importance of molecular geometry.

What theory explains the shapes of molecules?

The Valence Shell Electron Pair Repulsion (VSEPR) theory, also called the electron-pair-repulsion theory.

According to VSEPR theory, how do electron pairs arrange themselves around a central atom?

Electron pairs repel one another and position themselves as far apart as possible to minimise repulsion.

What are bonding pairs?

Shared pairs of electrons that form covalent bonds between atoms.

What are lone pairs (non-bonding pairs)?

Pairs of valence electrons located on the central atom that are not involved in bonding.

How do lone pairs influence bond angles compared with bonding pairs?

Lone pairs repel more strongly than bonding pairs, compressing bond angles between bonding pairs.

By roughly how many degrees does each lone pair reduce a bond angle?

About 2.5° per lone pair.

What is the shape, bond angle, and an example for a molecule with 2 bonding pairs and 0 lone pairs?

Linear; 180°; example: CO₂.

What is the shape, bond angle, and an example for a molecule with 3 bonding pairs and 0 lone pairs?

Trigonal planar; 120°; example: BF₃.

What is the shape, bond angle, and an example for a molecule with 4 bonding pairs and 0 lone pairs?

Tetrahedral; 109.5°; example: CH₄.

What is the shape, bond angles, and an example for a molecule with 5 bonding pairs and 0 lone pairs?

Trigonal bipyramidal; 90° and 120°; example: PCl₅.

What is the shape, bond angle, and an example for a molecule with 6 bonding pairs and 0 lone pairs?

Octahedral; 90°; example: SF₆.

What is the shape, approximate bond angle, and an example for a molecule with 3 bonding pairs and 1 lone pair?

Trigonal pyramidal; ~107°; example: NH₃.

What is the shape, approximate bond angle, and an example for a molecule with 2 bonding pairs and 2 lone pairs?

Bent (non-linear); ~104.5°; example: H₂O.

What is the shape, approximate bond angle, and an example for a molecule with 2 bonding pairs and 1 lone pair?

Bent (non-linear); ~118°; example: SO₂.

What is the shape, approximate bond angle, and an example for a molecule with 3 bonding pairs and 2 lone pairs?

T-shaped; ~90°; example: ClF₃.

What is the shape, approximate bond angle, and an example for a molecule with 5 bonding pairs and 1 lone pair?

Square pyramidal; ~90°; example: BrF₅.

What is the shape, bond angle, and an example for a molecule with 4 bonding pairs and 2 lone pairs?

Square planar; 90°; example: XeF₄.

List the steps for determining the shape of a molecule using VSEPR theory.

1) Count outer (valence) electrons on the central atom. 2) Add one electron for each bonded atom (single bonds). 3) Adjust for ionic charge (+ subtract 1 e⁻ per positive charge, − add 1 e⁻ per negative charge). 4) Divide the total by 2 to obtain the number of electron pairs (charge clouds). 5) Use VSEPR to assign the molecular shape.

Do double and triple bonds change the counted number of regions of electron density in VSEPR?

No. Multiple bonds count as one region of electron density (one charge cloud), the same as a single bond.

Why is the three-dimensional shape of a molecule important?

Shape influences physical properties, reactivity, polarity, boiling/melting points, and biological functions such as enzyme-substrate interactions.