Ch 2 · Isotopes · AS & A Level Chemistry 9701
Topic 2 of 4 in Atomic structure — AS & A Level Chemistry 9701 — 47 lessons.
Introduction
Not every atom of a given element is identical: atoms of the same element can differ in the number of neutrons they contain. This lesson defines isotopes, explains how to read isotope notation, and shows why isotopes share chemical behaviour but differ in physical properties that depend on mass.
Key ideas — Defining isotopes
Isotopes are atoms of the same element (the same atomic number, Z, and therefore the same number of protons) that have different numbers of neutrons, and therefore different mass numbers, A. Because the number of protons is fixed for a given element, all isotopes of that element also have the same number of electrons in the neutral atom, and hence the same electronic configuration.
🧑🏫 Teacher's explanation
Imagine you have several identical-looking LEGO bricks. They are all the same shape and color, so they fit into the same slots in a build. However, some of these bricks are made of heavy lead, while others are made of light plastic. Even though they look and act the same in the build, one set is much heavier than the other.
📖 Definitions
🔍 Worked example — Easy
An atom of Carbon has 6 protons and 8 neutrons. Write its symbol in isotope notation.
🔍 Worked example — Standard
Compare the chemical properties of ^12_6C and ^14_6C.
🔍 Worked example — Exam-style
Calculate the relative atomic mass (A_r) of an element that consists of 75\% of isotope X (mass 35) and 25\% of isotope Y (mass 37).
⚠️ Common errors
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🎯 Exam question
Key ideas — Isotope notation
An isotope is written as ^A_ZX, where X is the element symbol, A is the mass number (protons + neutrons) written as a superscript to the left of the symbol, and Z is the atomic number (protons only) written as a subscript to the left of the symbol. For example, ^35_17Cl and ^37_17Cl are two isotopes of chlorine: both have 17 protons, but the first has 35 - 17 = 18 neutrons and the second has 37 - 17 = 20 neutrons. Isotopes are also written in words as, for example, chlorine-35 and chlorine-37, using the mass number only, since the atomic number is fixed by the element's identity.
🧑🏫 Teacher's explanation
Think of isotopes like different versions of the same smartphone model. Every 'iPhone 15' has the same internal operating system and buttons (the protons), which determines how it functions and interacts with other apps (chemical properties). However, one version might have 128GB of storage while another has 512GB (the neutrons). Even though they look and act the same in most ways, the version with more storage is physically heavier and denser.
📖 Definitions
🔍 Worked example — Easy
An atom of Magnesium has an atomic number of 12 and a mass number of 24. Determine the number of protons and neutrons in this isotope.
🔍 Worked example — Standard
Calculate the relative molecular mass (M_r) of a molecule of water (H_2O) if the isotopes present are ^1_1H and ^16_8O.
🔍 Worked example — Exam-style
An unknown isotope of an element has 26 protons and 30 neutrons. Write its symbol in the form ^A_ZX and provide its name.
⚠️ Common errors
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🎯 Exam question
(a) State the number of protons and neutrons in this atom. [2] (b) Explain why this isotope has different physical properties compared to ^34_16S. [2] [4 marks]
Key ideas — Why isotopes have identical chemical properties
Chemical properties are determined by the number and arrangement of electrons, particularly the outer-shell (valence) electrons, since these govern how an atom bonds with other atoms. Because isotopes of the same element have identical proton numbers and therefore identical electronic configurations, they have identical chemical properties: the extra neutrons in the nucleus have no effect on electron arrangement or on the strength of attraction between the nucleus and the bonding electrons, since neutrons are uncharged and do not alter the nuclear charge.
🧑🏫 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 small, heavy lead weight hidden inside the chassis. If you try to crash them into a wall, they will react exactly the same way (they both dent or break in the same pattern) because their shape and structure are identical. However, the car with the lead weight will be harder to push and will hit the wall with more force because it is heavier.
📖 Definitions
🔍 Worked example — Easy
An atom of Chlorine has an atomic number of 17 and a mass number of 37. Determine the number of protons and neutrons in this isotope.
🔍 Worked example — Standard
Calculate the relative molecular mass (M_r) of a molecule containing one atom of ^12C and two atoms of ^1H.
🔍 Worked example — Exam-style
Explain why ^16O and ^18O have different densities.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Why isotopes differ in physical properties
Physical properties that depend directly on the mass of individual atoms differ between isotopes, because isotopes have different numbers of neutrons and therefore different masses, even though their chemical behaviour is unchanged. Two properties are affected in this syllabus:
🧑🏫 Teacher's explanation
Think of isotopes like two different models of the same smartphone. Both phones have the same operating system and the same buttons (the electrons), so they perform the exact same apps and react to the same touch commands (chemical reactions). However, one model might have a heavier, metal-reinforced casing while the other has a lightweight plastic casing. Even though they function identically, one is physically heavier and denser than the other.
📖 Definitions
🔍 Worked example — Easy
An atom of Chlorine has an atomic number of 17 and a mass number of 35. Calculate the number of neutrons in this isotope.
🔍 Worked example — Standard
A sample contains two isotopes of Magnesium: ^24_12Mg and ^26_12Mg. If the mass of a single ^24Mg atom is 3.99 imes 10^-23 g, calculate the mass of a single ^26Mg atom, assuming the mass of the extra neutrons is 1.67 imes 10^-24 g each.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Explain why these isotopes show identical chemical properties. [2] (b) Explain why a sample of ^37X is denser than a sample of ^35X of the same volume. [2] [4 marks]
Worked example
Question. Boron exists as two isotopes, boron-10 and boron-11. (a) State the number of protons and neutrons in one atom of each isotope. [2] (b) Explain why boron-10 and boron-11 have identical chemical properties. [2] (c) A sample of pure boron-11 and an equal-sized sample of pure boron-10 are compared. State and explain which sample has the greater density. [2]
🔍 Worked example — Easy
An atom of Carbon-14 is represented as ^14_6C. Determine the number of protons and neutrons in this atom.
🔍 Worked example — Standard
Explain why ^35_17Cl^- and ^37_17Cl^- have identical chemical properties.
🔍 Worked example — Exam-style
A sample of Magnesium-24 and a sample of Magnesium-25 occupy the same volume. Which sample has a higher density and why?
⚠️ Common errors
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Practice
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