Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
Water
Oxygen has six outer electrons and shares one pair with each hydrogen. Water contains two O-H bonds and two lone pairs on oxygen. Each hydrogen reaches two electrons and oxygen reaches eight.
Ammonia
Nitrogen has five outer electrons and shares one pair with each of three hydrogen atoms. NH3 therefore has three N-H bonds and one lone pair on nitrogen.
Methane
Carbon has four outer electrons and shares one pair with each of four hydrogen atoms. CH4 has four C-H bonds and no lone pairs on carbon.
Methanol
In CH3OH, carbon forms three C-H bonds and one C-O bond. Oxygen forms the C-O and O-H bonds and retains two lone pairs.
Checking diagrams
Count the total outer electrons, show a pair between every bonded atom and include all lone pairs. Use one symbol consistently for each atom origin.
Practical or data skill
Draw H2O, NH3, CH4 and CH3OH using dots and crosses. Verify the outer shell of every atom.
Examination tip
Missing lone pairs are a common source of lost marks.
Review questions and suggested answers
Question 1
Bonds in methane?
Suggested answer
Four.
Question 2
Lone pairs on oxygen in water?
Suggested answer
Two.
Question 3
Pairs around nitrogen in ammonia?
Suggested answer
Three bonding pairs and one lone pair.
