1. 1
   A mass spectrometry laboratory is analysing a sample of naturally occurring chlorine gas taken from an industrial process. Their instrument detects two types of chlorine atom present in the sample, chlorine-35 and chlorine-37, in different proportions, and the technicians must calculate an accurate relative atomic mass to report to their client.

   (a)

   A mass spectrometry laboratory analyses a sample of naturally occurring chlorine gas and finds it contains two isotopes: chlorine-35 and chlorine-37. Both isotopes have the same atomic (proton) number. What causes two atoms of the same element to have different mass numbers, as seen here?

   [1 mark]

   • AA different number of protons
   • BA different number of electrons
   • CA different number of neutrons
   • DA different electrical charge on the nucleus

   (b)

   The laboratory's report states that the relative atomic mass of chlorine is 35.5, which is not a whole number. Which explanation for this is correct?

   [1 mark]

   • ARelative atomic mass is always a whole number and 35.5 must be an error
   • BIt is a weighted average that accounts for the relative abundance of each isotope present
   • CIt is simply the mass number of the least abundant isotope
   • DIt is the mass of a single chlorine-35 atom measured in grams

   (c)

   The sample contains 75% chlorine-35 atoms and 25% chlorine-37 atoms. Calculate the relative atomic mass of this chlorine sample, showing your working.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A history of science museum is designing a new exhibit explaining how the modern periodic table developed. The exhibit compares early attempts to order the elements by atomic weight with Dmitri Mendeleev's table, which left gaps for undiscovered elements and sometimes placed elements out of strict atomic weight order.

   (a)

   A history of science museum is designing an exhibit about Dmitri Mendeleev's early periodic table, which left gaps for elements not yet discovered. What did Mendeleev use to decide where to leave these gaps?

   [1 mark]

   • AHe arranged elements strictly by atomic number, leaving gaps at unused numbers
   • BHe used the number of electron shells only, ignoring chemical properties
   • CHe arranged elements alphabetically by name
   • DHe grouped elements with similar properties together, leaving gaps where the pattern of properties was broken

   (b)

   The exhibit explains that some early tables, ordered strictly by atomic weight, placed certain elements in the wrong group. What later discovery explained why ordering strictly by atomic weight sometimes gave the wrong result?

   [1 mark]

   • AThe discovery of isotopes
   • BThe discovery of the noble gases
   • CThe invention of the mass spectrometer
   • DThe discovery of radioactivity

   (c)

   Explain why the discovery of isotopes showed that ordering elements strictly by atomic weight (relative atomic mass) could give the wrong sequence, whereas ordering by atomic (proton) number does not have this problem.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A fertiliser company sources boron compounds for one of its products and sends a sample to be analysed. The analysis identifies the sample as a mixture of two isotopes of boron, present in different proportions, and the company's chemist needs to calculate the relative atomic mass and describe the atomic structure of the isotopes involved.

   (a)

   A fertiliser company tests a sample of boron used in one of its products and finds it is made of two isotopes: boron-10, present in 20% abundance, and boron-11, present in 80% abundance. Calculate the relative atomic mass of this boron sample, showing your working, and state the number of neutrons in one atom of boron-11 given that boron has an atomic number of 5.

   [3 marks]

   (b)

   The chemist also needs to explain to a non-specialist colleague, using the boron example, what an isotope is and why isotopes of the same element have identical chemical properties despite their different masses. Give a full explanation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A university outreach team is preparing a school talk on the historical development of the periodic table, from early classification systems through to the modern table arranged by atomic number, to help students understand why scientific models change as new evidence emerges.

   (a)

   A university outreach team is preparing a talk for school students on how the periodic table developed from early classification attempts to the modern table used today. Describe the historical development of the periodic table, including early attempts to order elements by atomic weight, Mendeleev's contribution, and how the discovery of protons, neutrons and isotopes led to the modern arrangement by atomic number.

   [6 marks]

   (b)

   The outreach team also wants students to appreciate how relative atomic mass calculations connect to this historical story. Using a named example of an element with isotopes, explain how relative atomic mass is calculated from isotopic abundance, and evaluate why this modern understanding of relative atomic mass is more reliable evidence for classifying elements than the atomic weight values available to 19th-century chemists.

   [6 marks]

   Total for question 4: 12 marks

End of questions