These eight questions cover the four lessons in matter and atomic structure. Try each one on paper first.
Questions
Question 1. Classify each as atom, molecule or ion: (a) He, (b) NH₃, (c) K⁺. [3 marks]
Answer
(a) Atom: one neutral particle. (b) Molecule: neutral, four atoms joined. (c) Ion: carries a positive charge.
Question 2. Give the numbers of protons, neutrons and electrons in ²⁷₁₃Al. [3 marks]
Answer
Protons = 13. Neutrons = 27 − 13 = 14. Electrons = 13, because the atom is neutral.
Question 3. Give the numbers of protons, neutrons and electrons in ¹⁹₉F⁻. [3 marks]
Answer
Protons = 9. Neutrons = 19 − 9 = 10. Electrons = 9 + 1 = 10, because the fluoride ion has gained one electron.
Question 4. State the definition of isotopes. [2 marks]
Answer
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Question 5. Atom X has 17 protons and 18 neutrons, and atom Y has 17 protons and 20 neutrons. Are they isotopes, and what is each nucleon number? [3 marks]
Answer
Yes, because they have the same proton number and different neutron numbers. The nucleon number of X is 17 + 18 = 35, and of Y is 17 + 20 = 37.
Question 6. Neon has two isotopes: 90% neon-20 and 10% neon-22. Calculate the relative atomic mass. [3 marks]
Answer
(20 × 90) + (22 × 10) = 1 800 + 220 = 2 020. Then 2 020 ÷ 100 = 20.2.
The answer lies between 20 and 22 and is closer to 20, the more abundant isotope.
Question 7. A solid melts at 55 °C. Explain why the temperature stays at 55 °C while it melts. [3 marks]
Answer
The heat energy absorbed is used to overcome the forces of attraction between the particles. Kinetic energy does not increase during this time. The average kinetic energy stays the same, so the temperature is constant until all the solid has melted.
Question 8. Magnesium has isotopes Mg-24 (79%), Mg-25 (10%) and Mg-26 (11%). Calculate the relative atomic mass to one decimal place, and state how many neutrons Mg-26 has (proton number 12). [4 marks]
Answer
(24 × 79) + (25 × 10) + (26 × 11) = 1 896 + 250 + 286 = 2 432. Then 2 432 ÷ 100 = 24.32, which is 24.3.
Neutrons in Mg-26 = 26 − 12 = 14.
If you got these wrong
Question 1 uses distinguishing atoms, ions and molecules. Questions 2 and 3 use calculating protons, neutrons and electrons.
Questions 4, 5, 6 and 8 use interpreting isotopes and relative atomic mass. Question 7 uses explaining heating and cooling curves using particles.
Log each wrong answer in the mistake log and paper-error review. For a teacher to mark your working, see online one-to-one Chemistry tuition.