6.1 Shapes of molecules and ions

Question 1

What is the electron pair repulsion theory

Answer 1

Model used for explaining and predicting the shapes of molecules and polyatomic ions

Question 2

What determines the shape of a molecule or ion

Answer 2

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

Question 3

Example - Methane molecule CH4

Answer 3

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

Question 4

What shape is a methane molecule

Answer 4

Tetrahedral shape with four equal H-C-H bond angles of 109.5 degrees

Question 5

How do we show 3D shapes to visualise structures

Answer 5

Using lines and wedges

Question 6

What does a solid line represent

Answer 6

A bond in the plane of the paper

Question 7

What does a solid wedge show

Answer 7

Comes out of the plane of the paper

Question 8

What does a dotted wedge show

Answer 8

Going into the plane of the paper

Question 9

What are wedges useful for representing

Answer 9

Bonds in organic molecules

Question 10

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

Answer 10

Closer
More
More

Question 11

Relative repulsions between lone pairs and bonding pairs

Answer 11

bonded-pair/bonded-pair < bonded-pair/lone-par < lone-pair/lone-pair

INCREASING REPULSION ->

Question 12

Things to remember about molecular shapes from four electron pairs

Answer 12

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.

Question 13

What can each multiple bond in a molecule containing multiple bonds be treated as

Answer 13

A bonding region

For example - CO2 has 2

Question 14

Things to remember about molecular shapes from other numbers of electron pairs

Answer 14

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

Question 15

What shape does Boron trifluoride have

Answer 15

A trigonal planar shape with equal bond angles of 120 degrees

Question 16

What shape does Sulfur hexafluoride have

Answer 16

An octahedral shape with equal bond angles of 90 degrees

Question 17

What can we do with electron-pair repulsion theory

Answer 17

Explain and predict the shapes of ions

Question 18

Shapes of ammonium

Answer 18

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.

Question 19

What shape do CO3^2- and NO3- ions have

Answer 19

Same as Boron trifluoride, trigonal planar shape

They have three regions of electron density

Question 20

How many centres of electron density does So4^2- have, and what shape does it have

Answer 20

Four centres of electron density around the central sulfur atoms and have the same shape as a methane molecule