Isotopes and relative atomic mass

Science · Matter & Materials · ages 14-15

Name ______________________ Date ____________

Magnesium-24 and magnesium-26 are written with different numbers after the name. What are those numbers?
- the protons packed in each nucleus
- the neutrons packed in each nucleus
- the electrons out in the shells
- the mass numbers of the two atoms
Two isotopes of one element have the same mass number.
Circle one: True False
A sample of boron is 20 per cent boron-10 and 80 per cent boron-11. What is the relative atomic mass of boron?
Answer: ______________
Oxygen-16 and oxygen-18 are isotopes with atomic number 8. How many neutrons does an oxygen-18 atom hold?
Answer: ______________
Bromine is 50 per cent bromine-79 and 50 per cent bromine-81. What is its relative atomic mass?
Answer: ______________
Which pair of atoms are isotopes of each other?
- atomic number 11 with mass number 23, and atomic number 11 with mass number 24
- atomic number 11 with mass number 23, and atomic number 12 with mass number 23
- atomic number 11 with mass number 23, and atomic number 12 with mass number 24
- atomic number 10 with mass number 20, and atomic number 12 with mass number 24
Neon is 90 per cent neon-20 and 10 per cent neon-22. What is its relative atomic mass?
Answer: ______________
An atom gains one extra neutron. What changes?
- its mass number, but not the way it reacts
- the way it reacts, but not its mass number
- both its mass number and the way it reacts
- neither its mass number nor the way it reacts
Asked for the relative atomic mass of an element that is 10 per cent mass-106 and 90 per cent mass-108, Sam writes (10 x 108 + 90 x 106) divided by 100, and gets 106.2. Where is the slip?
- he should have divided by 2 instead of 100
- he paired each share with the wrong mass number
- the two percentages do not add up to 100
- he should have added the masses before multiplying
Magnesium is 79 per cent magnesium-24, 10 per cent magnesium-25 and 11 per cent magnesium-26. What is its relative atomic mass?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

Isotopes and relative atomic mass W1-mt_NFoRZd6UN_-s1

the mass numbers of the two atoms · The number after the name is the mass number. Magnesium has 12 protons, so magnesium-24 holds 12 neutrons and magnesium-26 holds 14.
False · They share the atomic number, not the mass number. Different neutron counts push their mass numbers apart, which is how you tell them apart on a chart.
10.8 · Out of 100 atoms: 20 lots of 10 is 200, and 80 lots of 11 is 880. That is 1080 for 100 atoms, so 1080 divided by 100 is 10.8.
10 · The 18 is the mass number, so take the 8 protons off: 18 - 8 = 10 neutrons. Oxygen-16 holds only 8.
80 · Out of 100 atoms: 50 x 79 = 3950 and 50 x 81 = 4050. That is 8000 for 100 atoms, so the answer is 80.
atomic number 11 with mass number 23, and atomic number 11 with mass number 24 · Isotopes share the atomic number and differ in the mass number. Only one pair does that: the same 11 protons, one atom with 12 neutrons and one with 13.
20.2 · Out of 100 atoms: 90 x 20 = 1800 and 10 x 22 = 220. That is 2020, so the answer is 20.2.
its mass number, but not the way it reacts · A neutron adds 1 to the mass number. It adds no charge, so the electron count is untouched, and reactions are settled by electrons.
he paired each share with the wrong mass number · He has multiplied 10 by 108 and 90 by 106, which is the wrong way round. The correct working is (10 x 106 + 90 x 108) divided by 100, which is 10780 over 100, or 107.8.
24.32 · Out of 100 atoms: 79 x 24 = 1896, 10 x 25 = 250 and 11 x 26 = 286. Those add to 2432, so the answer is 24.32.