Atomic Structure, Isotopes and Relative Atomic Mass
Atomic structure at IGCSE builds on the basic proton-neutron-electron model with electronic configurations for the first twenty elements, the electron arrangement of common ions, and how a mass spectrometer measures the mass and relative abundance of isotopes to calculate relative atomic mass. Edexcel 4CH1 and Cambridge 0620/0971 both expect students to interpret mass spectrometer data (a set of isotope peaks with masses and percentage abundances) rather than just being given percentage abundances in a sentence.
Before you start
Make sure you're comfortable with these topics first:
Method
- Use the atomic number (Z) and mass number (A) to find the number of protons (= Z), neutrons (= A - Z) and electrons (= Z, in a neutral atom); remember isotopes of an element have the same number of protons but a different number of neutrons and mass number.
- Write electron configurations for elements 1 to 20 using the shell-filling rule (first shell holds up to 2 electrons, the second and third shells hold up to 8 each before a fourth shell starts) - for example, sodium (Z=11) is 2,8,1 and calcium (Z=20) is 2,8,8,2.
- Write the electron configuration of a simple ion by adding electrons for a negative charge or removing electrons from the outermost shell for a positive charge - for example, a chloride ion Cl- (Z=17) is 2,8,8, not the atom's 2,8,7.
- To read mass spectrometer data, treat each peak as one isotope: its position (mass to charge ratio, m/z) gives the isotope's mass number, and its height or the percentage given gives its relative abundance.
- Calculate relative atomic mass from mass spectrometer data using: relative atomic mass = (sum of each isotope's mass number x its % abundance) / 100, using every peak shown in the data.
- Remember relative atomic mass is a weighted average, so it usually is not a whole number, and it lies closer to the mass of whichever isotope has the greater abundance.
Worked example
A sample of chlorine is analysed in a mass spectrometer, producing two peaks: one at m/z = 35 with 75% abundance, and one at m/z = 37 with 25% abundance. Calculate the relative atomic mass of chlorine from this data.
- Identify the two isotope peaks: mass number 35 with 75% abundance, and mass number 37 with 25% abundance.
- Multiply the first isotope's mass number by its abundance: 35 x 75 = 2625.
- Multiply the second isotope's mass number by its abundance: 37 x 25 = 925.
- Add the two results together: 2625 + 925 = 3550.
- Divide by 100, the total percentage: 3550 / 100 = 35.5.
- Final answer: the relative atomic mass of chlorine is 35.5.
Practice questions
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Q1Give the meaning of the term isotopes.Show answer
Answer: Atoms of the same element (the same number of protons/atomic number) that have a different number of neutrons, and therefore a different mass number.
Q2Write the electron configuration of a potassium atom (atomic number 19).Show answer
Answer: 2,8,8,1
Q3Write the electron configuration of a magnesium ion, Mg2+ (atomic number 12).Show answer
Answer: 2,8 (the atom, 2,8,2, loses the two electrons in its outer shell to form the ion, leaving a full outer shell of 2,8)
Q4A mass spectrometer shows a single peak at m/z = 27 for element X. State what this tells you about element X's isotopes, and its relative atomic mass.Show answer
Answer: Element X has only one naturally occurring isotope (mass number 27), so its relative atomic mass is 27.
Q5Bromine has two isotopes: bromine-79 (50.7% abundance) and bromine-81 (49.3% abundance). Calculate the relative atomic mass of bromine to 3 significant figures.Show answer
Answer: 80.0. (79 x 50.7) + (81 x 49.3) = 4005.3 + 3993.3 = 7998.6; 7998.6 / 100 = 79.986, which rounds to 80.0.
Q6State the number of protons, neutrons and electrons in an atom of carbon-14 (atomic number 6).Show answer
Answer: 6 protons, 8 neutrons (14 - 6), and 6 electrons.
Q7Explain why the relative atomic mass of chlorine (35.5) is not a whole number, even though every individual chlorine atom has a whole number of protons and neutrons.Show answer
Answer: Relative atomic mass is the weighted average mass of all the isotopes of chlorine, taking into account their relative abundances (about 75% chlorine-35 and 25% chlorine-37); an average of two different whole-number masses does not itself have to be a whole number.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
A sample of lithium gives a mass spectrum with a peak at m/z = 6 with 7.5% abundance, and a larger peak at m/z = 7 with 92.5% abundance. Calculate the relative atomic mass of lithium in this sample, giving your answer to 3 significant figures.
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Write the electron configurations of a sulfur atom (atomic number 16) and a sulfide ion, S2- (atomic number 16), and explain the difference between them.
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Silicon has three isotopes: silicon-28, silicon-29 and silicon-30. A sample of silicon is analysed in a mass spectrometer, producing three peaks with relative abundances of 92.2% (mass number 28), 4.7% (mass number 29) and 3.1% (mass number 30). (a) Describe, in terms of protons and neutrons, what these three isotopes have in common and how they differ. (b) Calculate the relative atomic mass of silicon from this data, giving your answer to 3 significant figures.
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Free printable worksheet
Want more practice on paper? Download the atomic structure, isotopes and relative atomic mass worksheet pack - 6 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 12 of IGCSE Science Workbook, the whole course as one free printable PDF.
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