The method and the results are on the atoms and molecules lab. Do the analysis yourself first, then check the answers below.
Quick answer: An atom is the smallest particle of an element, and a molecule is two or more atoms bonded together. These answers go with the grade 8 science lab on atoms and molecules: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
The answers use the sample results printed in the table. Your own values will differ a little, but the pattern of balls matching the subscripts should be the same.
Add the subscripts of the formula. CH₄: 1 + 4 = 5 balls. NH₃: 1 + 3 = 4 balls. These match the table, so balls = total of the subscripts.
Question 2 Mass per ball
H₂ 6.1 ÷ 2 = 3.05 g. O₂ 6.0 ÷ 2 = 3.00 g. H₂O 9.2 ÷ 3 = 3.07 g. CO₂ 8.9 ÷ 3 = 2.97 g. CH₄ 15.1 ÷ 5 = 3.02 g. NH₃ 12.0 ÷ 4 = 3.00 g. Mean = 18.11 ÷ 6 = 3.0 g per ball.
Question 3 Spread of the results
Highest 3.07 g (water), lowest 2.97 g (carbon dioxide). Range = 3.07 − 2.97 = 0.10 g. Differences come from the balls not being exactly the same size when rolled by hand.
Question 4 Sticks and valency
Carbon has 4 sticks touching it, hydrogen 1, oxygen 2 and nitrogen 3 (a double bond counts as two sticks). C 4, H 1, O 2, N 3.
Question 5 Predict ethanol
Balls = 2 + 6 + 1 = 9. Sticks: the 9 atoms form one connected group with no rings, so 9 − 1 = 8 sticks. Mass = 9 × 3.0 = 27 g (9 balls, 8 sticks).
Question 6 Limits of the model
Real atoms are not coloured and are not solid balls; the sticks do not exist as objects; the sizes of the balls do not show the real relative sizes of atoms. Two limits: colour and shape, and size.
Question 7 Improving the lab
Weigh every ball before use and discard any outside 3.0 ± 0.1 g. Repeat each model twice and average. Weigh balls first, repeat and average.
