3.6
原子重量
质子和中子的质量大致相同，约为1.67 × 10-24克。科学家们将这个质量定义为一个原子质量单位（amu）或一个道尔顿。电子的质量比质子小得多，重量仅为9.11 × 10-28 克，或大约原子质量单位的1/1800。因此，它们对元素的总原子质量贡献并不大。这意味着，在考虑原子质量时，通常会忽略任何…
元素的原子量，有时称为原子质量，约等于质子数与中子数之和。原子量以原子质量单位表示，简称 amu 或道尔顿（Daltons），在元素周期表上通常标注在其化学符号附近。
但是，大多数元素都有几种同位素，它们的中子数不同，因此质量也不同。
对于这些元素，原子量是该元素所有同位素质量的平均值，根据它们在自然界中的相对丰度或每种同位素所占的百分比进行加权计算。
例如，氯有两种天然存在的同位素。氯-35 的质量为 34.97 amu，氯-37 的质量为 36.97 amu。
将氯-35（75.77%）和氯-37（24.23%）的相对丰度分别乘以其质量数，再相加，即可得到最终的原子量35.45 amu。
由于氯-35在自然界中的丰度高于氯-37，因此氯的原子量接近35 amu。
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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.