Relative atomic mass
5 marks · HardDescribe what isotopes are.
The element magnesium has isotopes with mass numbers 24, 25, and 26. Give the number of protons and neutrons in each of these isotopes.
| Isotope | Number of protons | Number of neutrons |
|---|---|---|
| ^24Mg | ||
| ^25Mg | ||
| ^26Mg |
Explain why magnesium-24, magnesium-25, and magnesium-26 all have the same chemical properties.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Atoms of the same element (same number of protons) with different numbers of neutrons. · must show Same number of protons, Different number of neutrons
(b) — · must show ^24Mg: 12 protons, 12 neutrons, ^25Mg: 12 protons, 13 neutrons, ^26Mg: 12 protons, 14 neutrons
(c) All isotopes have the same number and arrangement of electrons, so they react in the same way. · must show Same electron arrangement, Chemical properties depend on electrons
Common mistakes: Mixing up neutron numbers; Confusing atomic and mass numbers; Explaining properties using neutrons instead of electrons
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Marking points
B1 (a) · 1 mark · Correct definition of isotopes as atoms with same protons but different neutrons.
B2 (b) · 1 mark · ^24Mg: 12 protons, 12 neutrons.
B3 (b) · 1 mark · ^25Mg: 12 protons, 13 neutrons.
B4 (b) · 1 mark · ^26Mg: 12 protons, 14 neutrons.
B5 (c) · 1 mark · Recognises same electron arrangement in all isotopes, so they react the same way.
Worked solution
a) Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
b) Magnesium has atomic number 12 (12 protons in each isotope).
| Isotope | Number of protons | Number of neutrons |
|---|---|---|
| ^24Mg | 12 | 12 |
| ^25Mg | 12 | 13 |
| ^26Mg | 12 | 14 |
c) All isotopes of magnesium have the same number and arrangement of electrons, so their chemical properties are identical.