In the periodic table, the ATOMIC NUMBER is the SMALLER number (it cannot exceed the mass number as you cannot have negative neutrons).
Isotopes
ISOTOPES are atoms of the SAME ELEMENT with the SAME ATOMIC NUMBER but DIFFERENT MASS NUMBERS.
Same protons โ same element โ same chemical properties.
Different neutrons โ different mass โ different physical properties (e.g. density, rate of diffusion).
Isotopes have IDENTICAL CHEMICAL behaviour โ because chemical reactions depend on electron configuration (determined by proton number, which is the same).
Examples:
CARBON ISOTOPES:
Carbon-12 (ยนยฒC): 6p + 6n โ the standard (98.9% of carbon)
Carbon-13 (ยนยณC): 6p + 7n โ stable, ~1.1%
Carbon-14 (ยนโดC): 6p + 8n โ radioactive, used in carbon dating
CHLORINE ISOTOPES:
Chlorine-35 (ยณโตCl): 17p + 18n โ ~75% of chlorine
Chlorine-37 (ยณโทCl): 17p + 20n โ ~25% of chlorine
Because of this mixture, the Ar of chlorine โ 35.5 โ between the two isotope masses.
Relative Atomic Mass
The RELATIVE ATOMIC MASS (Ar) is the WEIGHTED AVERAGE mass of all atoms of an element compared to 1/12 of the mass of a carbon-12 atom.
Because most elements have multiple isotopes in different abundances, Ar is NOT a whole number.
Formula:
Ar = ฮฃ (% abundance ร mass number) รท 100
Example โ chlorine:
Ar = (75 ร 35 + 25 ร 37) รท 100
Ar = (2625 + 925) รท 100
Ar = 3550 รท 100 = 35.5
This explains why periodic table Ar values are often not whole numbers โ they are weighted averages across all naturally occurring isotopes.
โ ๏ธ Common Mistake
Ar is a WEIGHTED average โ not a simple average. Chlorine has Ar 35.5 because ~75% is Cl-35 and only ~25% is Cl-37. If both were 50/50, Ar would be 36. The heavier isotope has LESS influence because it's less abundant. Also: isotopes have SAME chemical properties (same electrons) but DIFFERENT physical properties (different mass).
๐ Key Equations
Mass number = protons + neutrons
Neutrons = mass number โ atomic number
Ar = ฮฃ(% abundance ร mass number) รท 100
๐ Key Note
Atomic number = protons. Mass number = protons + neutrons. Neutrons = mass number โ atomic number. Isotopes: same element, same protons, different neutrons โ same chemical, different physical properties. Ar = weighted average of isotope masses.
๐ฏ Matching Activity โ Atomic Number, Mass Number and Isotopes
Match each term to its correct definition. โ drag the symbols on the right to match the component names on the left.
Atomic number
Drop here
Mass number
Drop here
Isotopes
Drop here
Relative atomic mass
Drop here
Number of neutrons
Drop here
Number of protons โ unique to each element
Mass number minus atomic number
Weighted average of all isotope masses โ often not a whole number
Same element, same protons, different number of neutrons
Total protons + neutrons in the nucleus
โฝ FIFA Worked Examples
Relative Atomic Mass Calculation
Boron has two isotopes: 20% boron-10 and 80% boron-11. Calculate the relative atomic mass of boron.
F
Ar = ฮฃ(% abundance ร mass number) รท 100
I
Ar = (20 ร 10) + (80 ร 11) รท 100
F
Ar = (200 + 880) รท 100 = 1080 รท 100
A
Ar = 10.8
โญ Higher Tier Only
Use mass spectrometry data to calculate Ar from isotope masses and percentage abundances. Interpret mass spectra showing relative abundance on y-axis and mass/charge on x-axis. Understand that non-integer Ar values result from isotope mixtures.
๐ฏ Test Yourself
Question 1 of 2
1. Chlorine-35 and chlorine-37 are isotopes. What is the same and what differs between them?
2. An element has two isotopes: 60% at mass 63 and 40% at mass 65. What is its Ar?
โญ How Well Do You Understand This Topic?
Be honest with yourself โ this helps you know what to revise!
Don't get itGetting thereNailed it!
๐ค Ask Mr Badmus AI
Stuck? Just ask! ๐ฌ
I'll use FIFA for calculations and flag Higher/Triple content clearly.