相対原子質量：各同位体の存在比で重み付けして加重平均する
導入済み| English | 意味 |
|---|---|
| relative atomic mass/ˈrelətɪv əˈtɒmɪk mæs/ | 相対原子質量 |
| isotope abundance/ˈaɪsətəʊp əˈbʌndəns/ | 同位体存在比 |
What would explain this observation?
- A periodic-table mass need not be a whole number. It describes an average over the natural mixture of isotopes rather than a nucleus containing a fraction of a neutron.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Relative atomic mass 相对原子质量, Ar, is an average that accounts for isotope abundance 同位素丰度. For GCSE calculations using supplied isotope mass numbers, multiply each mass by its percentage abundance, add the products and divide by 100. More abundant isotopes contribute more to the mean. If frequencies rather than percentages are supplied, divide the weighted sum by total frequency.
- relative atomic mass: The abundance-weighted average atomic mass relative to one twelfth of carbon-12; isotope abundance: The proportion of atoms belonging to a particular isotope in a sample.
加重平均がより豊富に存在する同位体の質量に近いのはなぜか？
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加重平均は、各同位体が寄与する原子の数に基づいて重み付けを行うため、 abundant（多存在）な同位体の質量に平均値が引き寄せられます。両方の同位体が計算に含まれますが、分数個の陽子は存在しません。
各専門用語を正確な意味と対応させよ。
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関連する物理量やプロセスを区別するために定義を使用せよ。
Choose evidence that can test it
- For a model chlorine sample containing 75% chlorine-35 and 25% chlorine-37, Ar=(35×75+37×25)/100=35.5. The mean is closer to 35 because that isotope is more abundant. An ordinary unweighted mean would give 36 and incorrectly treat both abundances as equal. Ar is a relative quantity without a gram unit; it is not the mass number of an individual atom.
- Check that percentage abundances total 100 before using the percentage formula. A missing second abundance can be found as 100 minus the first only when the sample has exactly two stated isotopes. Estimate where the answer should lie between the isotope masses, then calculate without premature rounding. A bead model with labelled masses illustrates weighted counting without implying all natural samples have the chosen fictional proportions.
この場合、調査やモデルをより妥当にする2つの習慣は何か。
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条件や仮定を明記することで他者が正確に再現でき、根拠となる証拠を記録することで結論を検証できます。相反する結果を隠すと、誤った結論を疑う証拠を失いますし、単一の測定でより広い主張を成立させることはできません。
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a fictional magnesium sample contains 80% mass-24, 10% mass-25 and 10% mass-26. Weighted sum=24×80+25×10+26×10=2,430. Divide by 100 to obtain Ar=24.3. It lies between 24 and 26 and close to the dominant mass-24 isotope. No atom in this model has mass number 24.3.
A fictional sample has 60% mass-10 and 40% mass-11. Calculate its relative atomic mass. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
架空のサンプルは質量60%が質量10、質量40%が質量11である。相対原子量を求めよ。
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結果は10.4です。既知：60% 質量-10 と 40% 質量-11。加重平均を用います：Ar = (10 × 60 + 11 × 40) / 100 = (600 + 440) / 100 = 10.4。単一の原子に質量10.4があるわけではありません。これはサンプル全体の平均値です。
Check the conclusion and its limits
- A percentage is divided by 100 once, not twice. Isotope abundance is not the proportion of neutrons inside a nucleus. A changing mean in different samples can reflect a different isotope mixture; it does not show that a single atom acquired a fractional neutron.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Every atom of an element has mass number equal to the decimal value on its periodic-table entry. This claim is false: The periodic-table value is an abundance-weighted average of the isotope masses. Individual atoms always have whole-number mass numbers; the decimal belongs to the mixture, not to any atom.
Relative atomic mass: weight each isotope by its abundance: For a model chlorine sample containing 75% chlorine-35 and 25% chlorine-37, Ar=(35×75+37×25)/100=35.5. The mean is closer to 35 because that isotope is more abundant. An ordinary unweighted mean would give 36 and incorrectly treat both abundances as equal. Ar is a relative quantity without a gram unit; it is not the mass number of an individual atom.
ある元素のすべての原子は、周期表の値にある小数点以下の値と等しい質量数を持つ。
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周期表の値は、同位体の質量の存在量加重平均です。個々の原子の質量数は常に整数ですが、小数点以下の値は混合物自体のものであり、特定の原子のものではありません。
炭素-12 の十二分の一に対する相対的な豊度加重平均原子質量を記述してください。
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相対原子質量とは、炭素-12 の十二分の一に対する豊度加重平均原子質量を指します。