Ch 3 · Isotopes · O Level Chemistry 5070
Topic 3 of 7 in Atoms, Elements and Compounds — O Level Chemistry 5070 — 48 lessons.
Introduction
Not every atom of an element is identical. Chlorine, for example, is a mixture of atoms of mass 35 and mass 37, and carbon dating relies on a rare heavy form of carbon. These variants are isotopes. The definition is a guaranteed source of marks, symbol notation is needed throughout the course, and candidates must be able to turn isotope abundances into a relative atomic mass.
Key ideas — What isotopes are
Because they have the same number of protons, isotopes have the same proton number and are the same element, sitting in the same place in the Periodic Table. Because they have different numbers of neutrons, they have different mass numbers and slightly different masses.
🧑🏫 Teacher's explanation
Imagine you have a collection of identical-looking coins. Every coin is a 'Quarter' because they all have the same face value and design. However, some quarters might be slightly heavier than others because they were minted with a tiny bit more extra metal inside. Even though they weigh differently, they still function exactly the same way in a vending machine because their 'identity' (the face value) hasn't changed.
📖 Definitions
🔍 Worked example — Easy
An atom of Magnesium has a proton number of 12 and a mass number of 24. Calculate the number of neutrons in this atom.
🔍 Worked example — Standard
Chlorine has two main isotopes: Chlorine-35 and Chlorine-37. If an atom of Chlorine-35 has 17 protons, how many neutrons does it have?
🔍 Worked example — Exam-style
Calculate the relative atomic mass (A_r) of an element that consists of 75% of isotope X (mass 36) and 25% of isotope Y (mass 38).
⚠️ Common errors
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🎯 Exam question
Key ideas — Using symbols for atoms and ions
The full symbol of an atom shows the mass number (top left) and the proton number (bottom left) next to the element symbol:
🧑🏫 Teacher's explanation
Think of an element like a specific brand of soda. The 'identity' of the soda is determined by its flavor (the number of protons). Even if you change the weight of the bottle or the amount of sugar inside (the number of neutrons), it is still the same brand of soda. In chemistry, these different 'weights' of the same brand are called isotopes.
📖 Definitions
🔍 Worked example — Easy
Determine the number of protons, neutrons, and electrons in the ion ^37_17Cl^-.
🔍 Worked example — Standard
Calculate the relative atomic mass (A_r) of an element that consists of 75% isotope ^35_17X and 25% isotope ^37_17X.
🔍 Worked example — Exam-style
An ion has the symbol ^24_12mathrmMg^2+. State the electronic configuration of this ion.
⚠️ Common errors
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🎯 Exam question
Key ideas — Chemical properties of isotopes
Isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration. Chemical reactions involve only the electrons, particularly those in the outer shell, so chlorine-35 and chlorine-37 both react with sodium to form sodium chloride in exactly the same way. The extra neutrons affect only physical properties that depend on mass, such as density and the rate of diffusion, and whether the nucleus is stable.
🧑🏫 Teacher's explanation
Imagine you have two identical-looking LEGO cars. One is made of standard bricks, and the other is made of the same bricks but has a slightly heavier lead weight hidden inside the chassis. If you crash them into a wall, they behave exactly the same way because their shape and structure (the bricks) are identical. However, the one with the lead weight will be harder to push and will roll differently due to its extra mass.
📖 Definitions
🔍 Worked example — Easy
An atom of Magnesium has 12 protons and 12 neutrons. Another isotope of Magnesium has 12 protons and 14 neutrons. State the mass number for both isotopes.
🔍 Worked example — Standard
Calculate the relative atomic mass (A_r) of an element that consists of 75\% isotope A (mass 35) and 25\% isotope B (mass 37).
🔍 Worked example — Exam-style
Chlorine has two isotopes: ^35Cl (abundance 75\%) and ^37Cl (abundance 25\%). Calculate the relative atomic mass of Chlorine.
⚠️ Common errors
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🎯 Exam question
Key ideas — Relative atomic mass from isotopes
calculating relative atomic mass. The relative atomic mass, A_r, of an element is the weighted average mass of its naturally occurring atoms, which is why values in the Periodic Table are often not whole numbers. If the percentage abundances of the isotopes are known:
🧑🏫 Teacher's explanation
Imagine you are buying a bag of mixed fruit. If you have 90 apples that weigh 100g each and 10 oranges that weigh 200g each, the 'average' weight of a piece of fruit in that bag isn't just the middle point (150g). Because there are so many more apples, the average weight will be much closer to 100g than 200g. This is exactly how relative atomic mass works.
📖 Definitions
🔍 Worked example — Easy
An element X has two isotopes. Isotope A has a mass of 10 and an abundance of 80%. Isotope B has a mass of 12 and an abundance of 20%. Calculate the relative atomic mass, A_r.
🔍 Worked example — Standard
Copper has two naturally occurring isotopes: Cu-63 (abundance 69%) and Cu-65 (abundance 31%). Calculate the relative atomic mass of copper.
🔍 Worked example — Exam-style
An element has three isotopes. The masses are 20, 21, and 22. The abundances are given as a ratio of 5:3:2. Calculate the relative atomic mass.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. Naturally occurring copper consists of two isotopes: 69.2% is copper-63 and 30.8% is copper-65.
🔍 Worked example — Easy
An atom of Magnesium-25 has a proton number of 12. Calculate the number of neutrons in this isotope.
🔍 Worked example — Standard
Chlorine has two isotopes: Chlorine-35 (abundance 75%) and Chlorine-37 (abundance 25%). Calculate the relative atomic mass (A_r) of chlorine.
🔍 Worked example — Exam-style
Element X has three isotopes. Isotope A (mass 10, abundance 90%), Isotope B (mass 11, abundance 5%), and Isotope C (mass 12, abundance 5%). Calculate the relative atomic mass of X.
⚠️ Common errors
Common mistakes
Practice
Answers Atoms of the same element with the same number of protons [B1] but different numbers of neutrons [B1]. ^18_8O: 8 protons, 10 neutrons, 8 electrons [B1]; ^16_8mathrmO^2-: 8 protons, 8 neutrons [B1], 10 electrons [B1]. They differ in mass / number of neutrons / density [B1]; they have identical chemical properties [B1] because they have the same number of electrons and the same electronic configuration [B1]. (10 × 20) + (11 × 80)/100 [M1] = 10.8 [A1]. (24 × 79) + (25 × 10) + (26 × 11)/100 = 1896 + 250 + 286/100 [M1] = 24.3 [A1].