3.6 The shapes of molecules and ions

Question 1

Determining the no. of electron pairs

Answer 1

• Identify the central atom bonded to all other atoms.
• Find the number of outer shell electrons of the central atom using its group number.
• Add one electron for each bonded atom.
• If the species is an ion, add one electron for each negative charge or subtract one electron for each positive charge.
• Divide the total number of electrons by two to get the number of electron pairs.
• Subtract the number of bonds from the number of electron pairs to determine the number of lone pairs.

Question 2

NH4+ ion bond angles

Answer 2

Question 3

Repulsion order

Answer 3

Lone pair-lone pair > Lone pair-bonding pair > Bonding pair-bonding pair.

Question 4

bond angles

Answer 4

Question 5

Electron pair repulsion theory

Answer 5

A molecule adopts a shape that minimises the repulsion between its electron pairs due to their negative charge:

• Electron pairs, whether bonding or lone, repel each other.
• Lone pairs cause more repulsion than bonding pairs because they are closer to the nucleus.
• Electron pairs arrange themselves as far apart as possible to minimise repulsion.

Question 6

Bonding pairs vs Lone pairs

Answer 6

Bonding pairs- Involved in forming covalent bonds with other atoms.
Lone pairs - Not involved in bonding and remain on the central atom.