Isotopes
Introduced| English | Meaning |
|---|---|
| isotopes/ˈaɪsətəʊps/ | Atoms with the same proton number but different neutron numbers. |
| relative atomic mass/ˈrelətɪv əˈtɒmɪk mæs/ | The weighted mean mass of an element's atoms relative to one twelfth of the mass of a carbon-12 atom. |
Same element, different mass
- All atoms of an element have the same number of protons — but they don't all weigh the same.
- Atoms of the same element with different numbers of neutrons are isotopes 同位素.
Isotope lab
Classify isotope facts by proton, neutron and mass number.
Isotopes are atoms of the same element with different numbers of:
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Isotopes differ only in neutron number.
Chlorine-37 has 17 protons. How many neutrons does it have?
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Neutrons = mass number − protons = 37 − 17 = 20.
Isotopes of an element always have the same number of ______.
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Same protons means it is still the same element.
What isotopes are
- Isotopes are atoms of the same element with the same protons but different neutrons.
- So they have the same atomic number but different mass numbers.
An atom's protons, neutrons and electrons — isotopes of an element differ only in their number of neutrons
Protons, neutrons & isotopes
Change the neutrons to make isotopes (same element, different mass number); change the electrons to make ions.
Isotopes of an element react chemically:
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Same electrons → same chemistry.
Same chemistry
- Isotopes have the same number of electrons, so they react chemically the same.
- Only their mass differs (and sometimes radioactivity).
Worked example. Chlorine-35 and chlorine-37 are isotopes: both have 17 protons (and 17 electrons), but one has 18 neutrons and the other 20. They behave identically in reactions.
Relative atomic mass is the weighted average mass of an element's isotopes.
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It accounts for how common each isotope is.
Relative atomic mass 相对原子质量
- Because an element is a mix of isotopes, we use an average mass: the relative atomic mass (Ar).
- It is the average mass of the isotopes, weighted by how common each is.
Isotopes differ in neutrons, not protons. Changing the proton number would change the element entirely. Isotopes are the same element — only the neutron count (and mass) changes.
You've got it
- isotopes = same element, same protons, different neutrons (different mass)
- they have the same electrons, so the same chemistry
- relative atomic mass (Ar) is the weighted average mass of an element's isotopes