Relative Atomic Mass & Isotopes

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CognitoRelative atomic mass and isotopes

Key facts & methods

Relative atomic mass (Aᵣ)

Atoms are so small that their actual masses are not practical — we use relative atomic mass (Aᵣ).
Aᵣ is measured relative to ¹²C (carbon-12), which is defined as exactly 12.
Aᵣ = weighted average mass of all naturally occurring isotopes of an element.
Formula: Aᵣ = Σ(% abundance × mass number) ÷ 100
Example: chlorine has ³⁵Cl (75%) and ³⁷Cl (25%). Aᵣ = (75×35 + 25×37) ÷ 100 = 35.5

Isotopes

Isotopes: atoms of the same element with the same number of protons but different numbers of neutrons.
Same atomic number (Z), different mass number (A).
Same chemical properties (same electron configuration) but different physical properties (different mass).
Example: ¹H, ²H (deuterium), ³H (tritium) are isotopes of hydrogen.
Radioactive isotopes (radioisotopes) have unstable nuclei that decay emitting radiation.

Mass spectrometry (Higher)

Mass spectrometer separates atoms by mass-to-charge ratio.
Sample is vaporised and ionised → accelerated → deflected by magnetic field → detected.
Heavier ions are deflected less; lighter ions more.
Output: mass spectrum — peaks at each isotope with height proportional to abundance.
Can be used to calculate Aᵣ from the relative heights of the peaks.

Exam questions — 4 questions · 10 marks · AQA 8462 style

3 marksCalculate AᵣAQA 8462 P1 style 🔢 Calculator

A sample of boron contains ¹⁰B (20% abundance) and ¹¹B (80% abundance). Calculate the relative atomic mass of this sample. (2 marks)

Hint: Ar = Σ(abundance × mass) ÷ 100. Multiply each isotope's mass by its % abundance, add together, divide by 100.

+30 XP

3 marksIsotope definition questionAQA 8462 style

(a) Define the term "isotope." (b) Chlorine has two isotopes: ³⁵Cl and ³⁷Cl. State the difference in their atomic structure. (c) Explain why they have identical chemical properties. (3 marks)

Hint: (a) Same protons, different neutrons. (b) Calculate neutrons = mass number − atomic number. (c) Link chemical properties to electron configuration.

+30 XP

1 markIsotope identificationAQA 8462 style

Which pair of atoms are isotopes of the same element?

A ¹²C and ¹⁴N

B ¹²C and ¹⁴C

C ¹H and ⁴He

D ²³Na and ²⁴Mg

3 marksMass spectrum questionAQA 8462 Higher style 🔢 Calculator

A mass spectrum of an element shows two peaks: mass 63 (abundance 69%) and mass 65 (abundance 31%). (a) Identify the element. (b) Calculate its relative atomic mass. (3 marks)

Hint: (a) Look up element with these mass numbers. (b) Apply the weighted average formula.

+30 XP

Relative Atomic Mass & Isotopes

Relative atomic mass (Aᵣ)

Isotopes

Mass spectrometry (Higher)

Module complete! 🎉