6.1 - Shapes of molecules and ions

Question 1

What is electron pair repulsion theory?

Answer 1

The electron pair repulsion theory is a model used in chemistry used in chemistry to explain and predict the shapes of molecules and polyatomic ions, because electron pairs repel one another

Question 2

What determine the shape of a molecule or ion?

Answer 2

The electron pairs surrounding a central atom determine the shape

Question 3

How do electron pairs repel?

Answer 3

The electron pairs repel one another so they are arranged as far apart as possible

Question 4

What does the arrangement of electron pairs minimise?

Answer 4

The arrangement of electron pairs minimised repulsion and thus holds bonded atoms in definite shape

Question 5

What result in different shapes?

Answer 5

Different numbers of electron pairs

Question 6

Explain the shape of a molecule of methane

CH4

Answer 6

Methane is symmetrical with four C-H covalent bonds

• four bonded pairs of electrons surround central carbon atom
• four electron pairs repel one another as far apart as possible in 3D space

The result is a tetrahedral shape with your equal H-C-H bond angles of 109.5 degrees

Question 7

What wedges show what structure in 3 dimensions?

Answer 7

A solid line represents a pond in line with the plate of paper

A solid wedge comes out of the plane of the paper

A dotted wedge goes into plane of paper

Question 8

What is a lone pair?

Answer 8

pair of electrons not bonded and part of central atom

• occupies more space than bonded pair
• repels more strongly than bonded pair

Question 9

What are the relative repulsions between lone pairs and bonded pairs?

Answer 9

Bonded-bonded < bonded-lone < lone-lone

Question 10

For each lone pair, what happens to the bond angle?

Answer 10

It reduces by 2.5 degrees

Question 11

What is the shape, bonded and lone pairs and bond angle of NH3?

Answer 11

shape - pyramidal
bonded pairs - 3
lone pairs - 1
bond angles - 107 degrees

Question 12

What is the shape, bonded and lone pairs and bond angle of H2O?

Answer 12

shape - non-linear
bonded pairs - 2
lone pairs - 2
bond angles - 104.5 degrees

Question 13

What is the bonding and shape of a carbon dioxide molecule?

Answer 13

Each multiple bond (2 oxygen and 2 carbon electrons) is treated as a bonding region - the two bonding regions repel one another as far as possible

shape - linear
bond angle - 180 degrees

Question 14

What are the principles of electron-pair repulsion theory/

Answer 14

• Electron pairs around central atom repel each other as far 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 is the bond angles and shape of BF3 and why?

Answer 15

Boron trifluoride only has 3 bonded pairs around central boron atom. Electron pair repulsion gives trigonal planar shape with equal bond angles of 120 degrees

Question 16

What is the bond angles and shape of SF6 and why?

Answer 16

Sulfur hexafluoride has 6 bonded pairs of electrons around central sulfur atom, giving octahedral shape with equal bond angles of 90 degrees

Question 17

What is the bond angles and shape of NH4 and why?

Answer 17

Tetrahedral shape and bond angles of 109.5 degrees

Question 18

What is the bond angles and shape of CO3 2- and NO3 2- ions?

Answer 18

They have 3 regions of electron density surrounding central atom so they have same shape as BF3 molecule

Question 19

What is the bond angles and shape of SO4 2- and why?

Answer 19

They have 4 centres of electron density around central sulfur atoms so have same shape as methane molecule

Question 20

What enables you to predict the arrangement of electron pairs around a central atom of unfamiliar molecules and ions?

Answer 20

The principles of electron-pair repulsion