Isotopes
Introduced| English | Meaning |
|---|---|
| isotope/ˈaɪsətəʊp/ | An atom of an element with the same proton number but a different neutron number from another atom of that element. |
| nucleon number/ˈnjuːklɪən ˈnʌmbə/ | The total number of protons and neutrons in an atomic nucleus. |
| density/ˈdensɪti/ | Mass per unit volume. |
| 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 weight
- Isotopes 同位素 are atoms of the same element with a different number of neutrons.
- Same protons ($Z$), different nucleon number 核子数 ($A$).
- They behave the same chemically — but differ slightly physically.
Isotope lab
Classify isotope facts by what changes and what stays the same.
Isotopes of an element have:
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Isotopes share the proton number Z but differ in neutron number, so they have different nucleon numbers A.
Isotopes of an element have the same number of protons but different numbers of ______.
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Same protons (so same element), different mass.
Isotopes of an element react chemically in the same way.
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They have the same electron configuration, and chemistry depends on the electrons.
What makes an isotope
- Isotopes have the same proton number $Z$ but a different nucleon number $A$.
- We write one as $^{A}_{Z}\text{X}$ — e.g. chlorine has $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$.
Two chlorine isotopes: same protons, different neutrons
Same chemistry, different physics
- Chemical properties depend on the electrons — isotopes have the same electron arrangement, so they react identically.
- Some physical properties depend on mass — a heavier isotope (more neutrons) has a higher density 密度.
Isotopes of one element have the same chemical properties because:
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Chemistry depends on the electrons; isotopes have identical electron arrangements, so they react the same.
Which property can differ between two isotopes?
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A heavier isotope (more neutrons) has more mass and so a higher density; chemistry is unchanged.
Chlorine is 75% ³⁵Cl and 25% ³⁷Cl. Calculate its relative atomic mass: (35×75 + 37×25) ÷ 100.
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(35×75 + 37×25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5 — a weighted mean, closer to ³⁵Cl because it is more abundant.
Working out relative atomic mass 相对原子质量
Worked example. Chlorine is 75% ³⁵Cl and 25% ³⁷Cl. Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5.
Relative atomic mass is a weighted average — each isotope's mass weighted by how common it is
You've got it
- isotopes = same element, same $Z$, different $A$ (different neutrons)
- written $^{A}_{Z}\text{X}$ (e.g. $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$)
- same chemical properties (same electrons); different physical (mass → density)