Chapter 6.1 - Shapes of molecules and ions Flashcards
What does the electron-pair repulsion theory explain?
The electron-pair repulsion theory is a model used in chemistry to explain and predict the shapes of molecules and polyatomic ions.
What determines the shape of a molecule or ion?
The shape of a molecule or ion is determined by the electron pairs surrounding the central atom.
How do electron pairs behave around a central atom? What does this do?
- Electron pairs repel one another so that they are arranged as far apart as possible.
- This arrangement minimizes repulsion and determines the shape of the molecule.
What causes different shapes in molecules and ions?
Different numbers of electron pairs result in different shapes.
How is the shape of a methane (CH₄) molecule described?
Methane (CH₄) is symmetrical with four C-H covalent bonds arranged in a tetrahedral shape. The bond angle is 109.5°.
What is the role of wedges in molecular representations?
Wedges are used to represent three-dimensional molecular shapes on flat paper.
What are the types of wedges, and what kind of bond do they show?
- Solid line: A bond in the plane of the paper.
- Solid wedge: A bond coming out of the plane of the paper.
- Dotted wedge: A bond going into the plane of the paper.
How do lone pairs differ from bonded pairs in terms of repulsion?
Lone pairs are closer to the central atom and occupy more space than bonded pairs.
Lone pairs repel more strongly than bonded pairs.
What is the order of repulsion strength?
Increasing repulsion:
Bonded-pair/Bonded-pair < Bonded-pair/Lone-pair < Lone-pair/Lone-pair
How do lone pairs affect bond angles?
Lone pairs push bonded pairs closer together, reducing the bond angle by about 2.5° per lone pair.
How does the electron repulsion theory apply to multiple bonds?
In molecules with multiple bonds, each multiple bond is treated as a single bonding region.
What happens to bond angles as the number of electron pairs increases?
The greater the number of electron pairs, the smaller the bond angle due to increased repulsion.
Can the electron-pair repulsion theory predict the shape of ions?
Yes, the theory can explain and predict the shape of ions.
What is the shape of the ammonium ion (NH₄⁺)?
The ammonium ion (NH₄⁺) has four bonded pairs around the central nitrogen atom, giving it a tetrahedral shape with bond angles of 109.5°, similar to methane.
What must you consider when predicting molecular shapes and bond angles?
- The number of electron pairs (bonded and lone) surrounding the central atom.
- Lone pairs cause stronger repulsion and reduce bond angles.
