Изотопы
Введено| English | Значение |
|---|---|
| isotope/ˈaɪsətəʊp/ | изотоп |
| nucleon number/ˈnjuːklɪən ˈnʌmbə/ | массовое число |
| density/ˈdensɪti/ | плотность |
| relative atomic mass/ˈrelətɪv əˈtɒmɪk mæs/ | относительная атомная масса |
Different masses can belong to the same element
- Isotopes 同位素 are atoms with the same number of protons but different numbers of neutrons. The proton number identifies the element; changing neutrons does not create a new element.
- Keep isotope identity separate from ion charge. A nucleus can remain the same while electrons are gained or lost.
Лаборатория изотопов
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.
Read the notation without mixing the numbers
- In $^{A}_{Z}\mathrm{X}$, $A$ is the nucleon number 核子数 and $Z$ is proton number. Neutrons $=A-Z$. A charge written at the upper right changes the electron count, not these nuclear counts.
- $^{35}_{17}\mathrm{Cl}$ and $^{37}_{17}\mathrm{Cl}$ both have 17 protons but have 18 and 20 neutrons. Cl⁻ formed from either has 18 electrons; ionisation does not turn one chlorine isotope into the other.
The changed neutron count explains the changed nuclear mass.
Explain the property, rather than naming mass
- Neutral isotopes of an element have the same electron configuration, so the same chemical properties in the syllabus model. A different neutron number is not a different outer-electron arrangement.
- Their masses differ. Comparing the same physical form under the same conditions, the heavier isotope can give a higher density 密度 because similar volumes contain a greater mass. The assessed physical differences here are mass and density.
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.
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.
Neutral atoms of two isotopes have different masses. Explain why they have the same chemical properties in this syllabus model, and why equal-volume samples of the same physical form can have different densities.
They have the same proton number and neutral electron configuration, so the same outer-electron arrangement determines their chemical behaviour. More neutrons increase atomic mass without adding positive nuclear charge. For comparable volumes under the same conditions, a sample enriched in the heavier isotope can have greater mass and therefore greater density.
Separate an isotope comparison from an ion comparison
- Compare neutral atoms of two isotopes when explaining the same chemistry. Comparing an atom with its ion changes the electron arrangement, so it is a different question.
- More neutrons do not mean a stronger positive nuclear charge: neutrons have no charge. This is why isotope mass alone is not a reason for a large change in attraction to the outer electrons.
State both parts of an isotope definition: same protons, different neutrons. “Different mass” alone also describes atoms of different elements, so it is not precise enough.
Compare ⁶³₂₉Cu⁺ and ⁶⁵₂₉Cu⁺ by proton, neutron and electron counts. Then explain what would change if ⁶³₂₉Cu⁺ gained one electron.
Both ions have 29 protons and 28 electrons. Their neutron counts are 34 and 36. Gaining one electron gives a neutral ⁶³Cu atom with 29 electrons; it does not change the 29 protons or 34 neutrons. The isotope remains copper-63.
Use the right abundance total
- Relative atomic mass 相对原子质量 is a weighted mean of an element's isotopic masses, compared with one twelfth of the mass of a carbon-12 atom. Multiply each isotopic mass by its abundance, add and divide by the abundance total for that element.
- Worked teaching sample: 75 parts of mass 35 and 25 parts of mass 37 give $(35\times75+37\times25)\div100=35.5$. If the supplied data include another element, first select the isotopes of the element whose mean is wanted.
This calculation also supports Topic 2.1; it does not replace the nuclear definition of an isotope.
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.
Teaching data for an alloy: another element has abundance 40, while isotopes of element Q with relative isotopic masses 63 and 65 have abundances 45 and 15. Calculate Q’s relative atomic mass and explain which abundance total you used.
Use only Q’s isotopes: total Q abundance = 45 + 15 = 60. The weighted mean is (63 × 45 + 65 × 15) ÷ 60 = 3810 ÷ 60 = 63.5. Dividing by 100 would incorrectly include the other element. The result lies between 63 and 65.
You've got it
- Same proton number and different neutron numbers define isotopes.
- A changed ion charge changes electrons; a changed isotope changes neutrons.
- The same neutral electron arrangement explains the same chemical properties; mass explains the assessed physical differences.
- For a weighted mean, use only the relevant element's abundance total. Check that the answer lies between its isotopic masses.