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IsotopesCambridge IGCSE Chemistry: Revision notes

Section 1

What Are Isotopes?

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.

  • Because they have the same number of protons, isotopes of an element have the same proton (atomic) number
  • Because they have different numbers of neutrons, isotopes have different mass (nucleon) numbers
  • Isotopes of the same element are still the same element — they just have slightly different masses
Key termsisotopes
Example

Carbon-12 and carbon-14 are both isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons.

Section 2

How Do We Write Isotope Symbols?

Atoms and ions are represented using symbols that show both the mass number and proton number:

  • The mass number is written at the top left of the element symbol
  • The proton number is written at the bottom left of the element symbol
  • For an ion, the charge is written at the top right

For example, an atom of carbon-12 is written with mass number 12 above and proton number 6 below the symbol C. A chloride ion formed from chlorine-35 is written with mass number 35 above and proton number 17 below the symbol Cl, plus a minus charge, showing it has gained one electron to form a 1− ion.

Exam tip

To find the number of neutrons from an isotope symbol, subtract the bottom number (proton number) from the top number (mass number).

Section 3

Why Do Isotopes of the Same Element Have the Same Chemical Properties?

Chemical properties are determined by the number and arrangement of electrons, not by the number of neutrons.

  • Isotopes of the same element have the same number of protons, so (as neutral atoms) they also have the same number of electrons
  • Having the same number of electrons means they have the same electronic configuration
  • This means isotopes of the same element react in exactly the same way chemically — only their mass, and some physical properties linked to mass, differ slightly
Common mistake

Don't confuse isotopes having different masses with them having different chemical behaviour — chemical reactivity depends on electron arrangement, which is identical across isotopes of an element.

Section 4

How Is Relative Atomic Mass Calculated From Isotopes?

Most elements exist as a mixture of isotopes in fixed natural proportions. The relative atomic mass (Ar) takes this into account by being a weighted average.

To calculate Ar from isotope data:

  1. Multiply the mass number of each isotope by its percentage abundance
  2. Add these values together
  3. Divide the total by 100 (if abundances are given as percentages)

Ar = [sum of (isotope mass x % abundance)] / 100

Example

Chlorine is 75% chlorine-35 and 25% chlorine-37. Ar = [(35 x 75) + (37 x 25)] / 100 = (2625 + 925) / 100 = 35.5.

Must Know

  • Isotopes are atoms of the same element with the same proton number but different mass numbers (different numbers of neutrons)
  • Isotope symbols show mass number (top) and proton number (bottom) next to the element symbol
  • Isotopes of the same element have identical chemical properties, because chemical behaviour depends on electron number/arrangement, not neutron number
  • Relative atomic mass is a weighted average of the masses of all isotopes of an element, based on their natural abundance
  • Neutrons in an isotope = mass number − proton number

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