6.1 Shapes of molecules and ions
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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
What shape does Sulfur hexafluoride have
An octahedral shape with equal bond angles of 90 degrees
What can we do with electron-pair repulsion theory
Explain and predict the shapes of ions
Shapes of ammonium
Four bonded pairs surrounding the central nitrogen atom Has the same number of bonded pairs of electrons around the central atom as a methane molecule Has the same tetrahedral shape and bond angles as a methane molecule.
What shape do CO3^2- and NO3- ions have
Same as Boron trifluoride, trigonal planar shape They have three regions of electron density
How many centres of electron density does So4^2- have, and what shape does it have
Four centres of electron density around the central sulfur atoms and have the same shape as a methane molecule