Slide 1
Same element, different neutron count
Isotopes of an element have the same number of protons but different numbers of neutrons. Their chemical identity remains tied to the proton count.
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Carbon-12 and carbon-13 each have six protons, but seven neutrons occur in carbon-13 rather than six. The number after the name is the total proton-plus-neutron count, called mass number.
Slide 2
Mass number is not exact mass
Mass number counts nucleons, the protons and neutrons in a nucleus. It is an integer rather than the precise mass of the atom.
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Nuclear binding changes the mass relative to the sum of separated constituents. High-precision atomic masses therefore need measurement and cannot be obtained simply by assigning exactly one mass unit to each nucleon.
Slide 3
Stability can differ
Some isotopes are stable and others undergo radioactive decay. Changing neutron number can strongly affect nuclear stability without changing the element initially.
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An unstable nucleus can emit particles or radiation and become a different nuclear state or element. Ordinary chemical bonding mainly involves electrons, so it is distinct from these nuclear transformations.
Slide 4
Mass can leave a chemical trace
Isotopes usually have similar chemistry, but their different masses can alter reaction rates and physical processes slightly. These isotope effects are measurable.
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Lighter isotopes can be preferentially distributed during processes such as evaporation. Ratios measured in samples can therefore carry information about their history, provided the relevant fractionation processes are understood.
Slide 5
An average reflects a mixture
An element’s listed atomic weight often reflects a weighted mixture of naturally occurring isotopes. It need not equal any one isotope’s mass.
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Samples with different isotopic proportions can have slightly different average atomic weights. That is why precision work distinguishes an individual isotope, a sample’s composition and a conventional table value.
Sources
- Isotope Basics
National Isotope Development Center
- The Periodic Table and Energy-Level Models
American Chemical Society