Grade 11 · Chemistry
Atomic number, mass and isotopes
Why it matters
Protons, neutrons, electrons, isotopes and average atomic mass.
Core20m readingPrerequisite: The structure of the atom
What are isotopes?
Short answer
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, so they share an atomic number but differ in mass number. Because chemical behaviour is set by the electrons, isotopes of an element react almost identically. The atomic mass on the periodic table is the weighted average of an element’s naturally occurring isotopes.
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Frequently asked questions
- Do isotopes of an element react differently?
- No, they react almost identically, because chemistry depends on electrons and isotopes have the same electron arrangement.
- Why is atomic mass usually not a whole number?
- It is the weighted average of all naturally occurring isotopes, based on their masses and relative abundances.