3.6
原子量
陽子と中性子はほぼ同じ質量を持ち、約1.67×10-24グラムです。科学者は、この質量を1つの原子量単位（amu）または1ダルトンとして定義しています。電子は陽子よりもずっと質量が小さく、9.11×10-28グラム、つまり原子量単位の1/1800程度しかありません。そのため、元素の全体的な原子量には…
元素の原子量は、原子質量と呼ばれることもあり、陽子と中性子の数の合計にほぼ等しくなります。これは原子質量単位で表され、アムまたはダルトンと略され、周期表の化学記号の近くで表されます。
しかし、ほとんどの元素には、中性子の数や質量が異なるいくつかの同位体があります。
このような元素の場合、原子量は、元素のすべての同位体の平均質量であり、それらの相対的な存在量または自然界に見られる各同位体の割合に従って重み付けされます。
たとえば、塩素には2つの天然同位体があります。塩素-35の質量は34.97 amu、塩素-37の質量は36.97 amuです。
それらの相対的な存在量である塩素-35の75.77%と塩素-37の24.23を掛け合わせると、最終的な原子量は35.45 amuになります。
塩素-35は塩素-37よりも自然界に豊富に存在するため、塩素の原子量は35 amuに近いです。
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Q1: What is the difference between atomic weight and atomic mass?
Atomic weight and atomic mass are closely related terms. Atomic mass refers to the sum of protons and neutrons in a single atom, expressed in atomic mass units (amu) or Daltons. Atomic weight is the weighted average mass of all naturally occurring isotopes of an element, accounting for their relative abundance in nature. For example, chlorine's atomic weight of 35.45 amu reflects the average of its two isotopes.
Q2: Why don't electrons contribute significantly to an element's atomic weight?
Electrons are much smaller than protons and neutrons, weighing only about 1/1800 of an atomic mass unit. Since protons and neutrons each weigh approximately 1.67 × 10⁻²⁴ grams, their combined mass dominates the atom's total mass. Therefore, scientists ignore electron mass when calculating atomic weight and base it solely on the number of protons and neutrons.
Q3: How is atomic weight calculated for elements with multiple isotopes?
Atomic weight is calculated by summing the mass of each isotope multiplied by its natural fractional abundance. For chlorine, chlorine-35 (34.97 amu at 75.77% abundance) and chlorine-37 (36.97 amu at 24.23% abundance) combine to yield an atomic weight of 35.45 amu. This weighted average reflects the isotope composition found in nature.
Q4: What are atomic mass units and why are they used?
Atomic mass units (amu), also called Daltons, are standardized units for measuring atomic and subatomic particle masses. Scientists arbitrarily defined one amu as the mass of a proton or neutron, approximately 1.67 × 10⁻²⁴ grams. This convenient scale allows chemists to express atomic weights as simple whole numbers or decimals rather than using unwieldy scientific notation.
Q5: How does isotope abundance affect an element's atomic weight on the periodic table?
The periodic table displays atomic weight as a weighted average based on isotope abundance. Since chlorine-35 is more abundant than chlorine-37 in nature, chlorine's listed atomic weight of 35.45 amu is closer to 35 than to 37. Elements with more evenly distributed isotopes show atomic weights further from whole numbers, reflecting their natural isotopic composition.
Q6: What is the relationship between atomic weight and molar mass?
A mole of an element has a mass in grams equal to its atomic weight. For compounds, molecular weight is the sum of atomic weights of all component atoms. Glucose (C₆H₁₂O₆) has a molecular weight of 180.156 grams per mole, calculated by summing six carbons (72.066 g), twelve hydrogens (12.108 g), and six oxygens (95.982 g).
Q7: Why is understanding isotope composition important for determining atomic weight?
Most naturally occurring elements consist of multiple isotopes with different masses and abundances. Ignoring isotope composition would yield inaccurate atomic weights. By considering both the mass and natural fractional abundance of each isotope, chemists obtain precise atomic weights that reflect real elemental composition, essential for accurate stoichiometric calculations in chemical reactions.