6.1 Shapes of molecules and ions Flashcards
What is the electron pair repulsion theory
Model used for explaining and predicting the shapes of molecules and polyatomic ions
What determines the shape of a molecule or ion
The electron pairs surrounding a central atom
They repel one another so they are arranged as far apart as possible
The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape
Different numbers of electron pairs result in different shapes
Example - Methane molecule CH4
Symmetrical with four C-H covalent bonds
Four bonded pairs of electrons surround the central carbon atom
The four electron pairs repel one another as far apart as possible in three-dimensional space
What shape is a methane molecule
Tetrahedral shape with four equal H-C-H bond angles of 109.5 degrees
How do we show 3D shapes to visualise structures
Using lines and wedges
What does a solid line represent
A bond in the plane of the paper
What does a solid wedge show
Comes out of the plane of the paper
What does a dotted wedge show
Going into the plane of the paper
What are wedges useful for representing
Bonds in organic molecules
A lone pair of electrons is slightly _____ to the central atom, and occupies _____ space, than a bonded pair. Therefore, a lone pair repels _____ strongly than a bonding pair
Closer
More
More
Relative repulsions between lone pairs and bonding pairs
bonded-pair/bonded-pair < bonded-pair/lone-par < lone-pair/lone-pair
INCREASING REPULSION ->
Things to remember about molecular shapes from four electron pairs
The four electron pairs around the central atom repel one another as far apart as possible into a tetrahedral arrangement
Lone pairs repel more strongly than bonded pairs
Therefore lone pairs repel bonded pairs slightly closer together, decreasing the bond angle - the angle between the bonded pairs of electrons
The bond angle is reduced by about 2.5 degrees for each lone pair.
What can each multiple bond in a molecule containing multiple bonds be treated as
A bonding region
For example - CO2 has 2
Things to remember about molecular shapes from other numbers of electron pairs
Electron pairs around the central atom repel each other as far apart as possible
The greater the number of electron pairs, the smaller the bond angle
Lone pairs of electrons repel more strongly than bonded pairs of electrons
What shape does Boron trifluoride have
A trigonal planar shape with equal bond angles of 120 degrees