Compare — 2 marks
A student is investigating different isotopes of carbon used in archaeological dating. Carbon-12 is the most abundant stable isotope, while carbon-14 is radioactive and used in radiocarbon dating of ancient artifacts. Both isotopes have 6 protons but differ in their number of neutrons.
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Compare the atomic structure of carbon-12 and carbon-14.
[2 marks]
Show mark scheme
- Both have the same number of protons (6) / same atomic number / same number of electrons (in neutral atoms)
- Carbon-14 has more neutrons than carbon-12 / carbon-12 has 6 neutrons while carbon-14 has 8 neutrons / different mass numbers
Explain — 5 marks
A student uses a mass spectrometer to analyse a sample of chlorine gas. The instrument separates chlorine atoms based on their mass-to-charge ratio. The results show two distinct peaks at mass numbers 35 and 37, with the peak at mass 35 being approximately three times taller than the peak at mass 37.
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(a) Explain why chlorine atoms with different mass numbers are detected in the mass spectrometer.
[2 marks]
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(b) The relative atomic mass of chlorine is 35.5. Explain how the presence of two isotopes with mass numbers 35 and 37, in a 3:1 ratio, results in this relative atomic mass value.
[2 marks]
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(c) Explain why both isotopes of chlorine have identical chemical properties despite their different mass numbers.
[1 mark]
Show mark scheme
- (a) Chlorine atoms have different numbers of neutrons (1 mark)
- (a) This gives them different mass numbers / different mass / different nucleon numbers (1 mark)
- (b) Three-quarters (or 75%) of chlorine atoms have mass number 35, one-quarter (or 25%) have mass number 37 (1 mark)
- (b) (3 × 35 + 1 × 37) ÷ 4 = 35.5 or equivalent calculation showing weighted average (1 mark)
- (c) Both isotopes have the same number of electrons (or protons/atomic number), which determines chemical properties (1 mark)
Explain — 5 marks
A student is learning about atomic structure. They observe a diagram showing a carbon atom with 6 protons, 6 neutrons, and 6 electrons. They wonder why the nucleus is so small compared to the overall size of the atom, and how the electrons stay in the atom without falling into the nucleus.
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(a) Explain why electrons do not fall into the nucleus despite the attractive force between them.
[2 marks]
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(b) Explain why the nucleus of a carbon atom is much smaller than the atom itself.
[2 marks]
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(c) Explain what would happen to a carbon atom if one of its electrons were removed.
[1 mark]
Show mark scheme
- (a) Electrons orbit/move around the nucleus in shells/energy levels (1 mark)
- (a) The centripetal force from their motion balances the electrostatic/attractive force from the nucleus (1 mark)
- (b) The nucleus contains protons and neutrons which are much more massive than electrons (1 mark)
- (b) Most of the atom is empty space/electrons are far from the nucleus compared to nuclear size (1 mark)
- (c) The atom would become a positively charged ion/have a net positive charge (1 mark)
Calculate — 2 marks
A chemist at a water treatment plant is analysing chlorine samples used to disinfect drinking water. Chlorine has atomic number 17. A chlorine atom has mass number 35 and forms a chloride ion (Cl⁻) by gaining one electron.
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(a) Calculate the number of neutrons in this chlorine atom.
[1 mark]
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(b) Calculate the number of electrons in the chloride ion (Cl⁻).
[1 mark]
Show mark scheme
- (a) 18 (neutrons)
- (b) 18 (electrons)
Describe — 2 marks
A chemist is preparing atomic models to demonstrate the structure of lithium atoms to students visiting an industrial chemistry laboratory. Lithium atoms contain protons, neutrons and electrons.
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(a) Describe the location of the protons and neutrons in a lithium atom.
[1 mark]
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(b) Describe where the electrons are found in a lithium atom.
[1 mark]
Show mark scheme
- (a) in the nucleus
- (b) in shells / energy levels (around the nucleus)
GCSE Perfect