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Ch.2 - Atoms, Molecules, and Ions
Brown - Chemistry: The Central Science 14th Edition
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Tutti i libri di testoBrown 14th EditionCh.2 - Atoms, Molecules, and IonsProblema 33b
Capitolo 2, Problema 33b

(b) The atomic weight of boron is reported as 10.81, yet no atom of boron has the mass of 10.81 u. Explain.

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1
Understand that the atomic weight of an element is a weighted average of the masses of its isotopes, not the mass of a single atom.
Identify the isotopes of boron: Boron has two stable isotopes, Boron-10 and Boron-11.
Recognize that each isotope has a different mass: Boron-10 has a mass of approximately 10 u, and Boron-11 has a mass of approximately 11 u.
Consider the natural abundance of each isotope: Boron-10 and Boron-11 occur in nature in different proportions, with Boron-11 being more abundant.
Calculate the weighted average: The atomic weight of boron (10.81 u) is calculated by multiplying the mass of each isotope by its relative abundance and summing these values. This results in a value that does not correspond to the mass of any single boron atom but represents the average mass of boron atoms as they occur naturally.

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Atomic Weight vs. Atomic Mass

Atomic weight is a weighted average of the masses of an element's isotopes, reflecting their relative abundances in nature. In contrast, atomic mass refers to the mass of a specific isotope, measured in atomic mass units (u). Since boron has isotopes with different masses, the atomic weight of 10.81 u does not correspond to any single boron atom.
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Atomic Mass

Isotopes

Isotopes are variants of a chemical element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses. Boron has two stable isotopes: boron-10 (10 u) and boron-11 (11 u). The presence of these isotopes contributes to the average atomic weight of boron being 10.81 u, as it accounts for their relative abundances.
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Isotopes

Relative Abundance

Relative abundance refers to the proportion of each isotope of an element found in a natural sample. For boron, the atomic weight of 10.81 u is calculated based on the relative abundances of its isotopes, boron-10 and boron-11. This average reflects the typical composition of boron in nature, rather than the mass of any individual atom.
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Calculating Abundance Example
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