EduNinja

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same electron arrangement and therefore the same chemical properties, while their different masses can give different physical properties.

$A_r = ∑(isotope mass × fractional abundance)$

Convert percentage abundances to fractions, multiply each isotope mass by its fractional abundance, then add the contributions. The result is a weighted mean, not an unweighted average.

For a two-isotope sample containing 75% mass 35 and 25% mass 37, the weighted mean is (35 × 0.75) + (37 × 0.25) = 35.5. An answer between the isotope masses is a useful check; simply averaging 35 and 37 would ignore abundance.

Read isotope identity from the pair of nuclear symbols: ³⁵₁₇Cl and ³⁷₁₇Cl are both chlorine because Z = 17 in each, but they contain 18 and 20 neutrons respectively because A differs. Mass number belongs to one nuclide; relative atomic mass is the abundance-weighted mean for a sample.

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Calculate the relative atomic mass of bromine from the sample, giving your answer to two decimal places.

Retrieve the sequence: use nuclear notation to count particles, distinguish isotopes by neutron number, then read mass-spectrum positions and heights as isotope mass and abundance evidence.

When checking an answer, ask: Did I separate A, Z, and charge? Did I explain isotope properties through electron arrangement? Did I weight each isotope by its abundance?

1.2.2—Isotopes

Isotopes and Relative Atomic Mass

The Nuclear Atom Summary

Applying Isotope Abundance