The atomic mass of an element is usually expressed in which of the following units?

Question

Examoogle · Accepted Answer

The correct answer is Option D: Atomic mass units (amu). The atomic mass of an element is a physical constant that represents the weighted average mass of the atoms of an element. It is commonly expressed in 'atomic mass units' (amu), also known as the unified atomic mass unit (u) or Dalton (Da).

Step-by-Step Solution & Concepts:
The atomic mass of an element is a physical constant that represents the weighted average mass of the atoms of an element. It is commonly expressed in 'atomic mass units' (amu), also known as the unified atomic mass unit (u) or Dalton (Da).

Working Principle: An atomic mass unit (amu) is defined as one-twelfth ($1/12$) of the mass of a single carbon-12 atom. Because atoms are extremely small, using kilograms or grams is impractical for individual atoms; hence, this relative scale provides a standardized way to compare the masses of various isotopes and elements.

Key Formulas: $1 	ext{ amu} = 1.660539 	imes 10^{-27} 	ext{ kg}$ — conversion factor to SI units; $1 	ext{ u} = \frac{1}{12} 	imes 	ext{mass of one } ^{12}	ext{C atom}$ — definition based on Carbon-12

Key Points: Atomic mass is a relative mass unit, not an absolute mass in kilograms.; The value is calculated based on the abundance and mass of all naturally occurring isotopes of an element.; While molar mass is expressed in $g/mol$, atomic mass for a single atom is expressed in $amu$.; Carbon-12 is the international standard reference for this scale.

Advantages: Simplifies calculations by avoiding extremely small decimal numbers in scientific notation.

Disadvantages: Requires knowledge of isotopic abundance to determine the atomic weight for a specific element.

Applications: Stoichiometric calculations in chemical reactions.; Determination of molecular weights for chemical compounds.

Additional Info: Grams per mole (g/mol) is used for molar mass, which refers to the mass of one mole ($6.022 	imes 10^{23}$ particles) of an element.. Pounds and kilograms are units of mass for macroscopic objects, not individual atoms.

Answer: D is correct — Atomic mass is standardly expressed in atomic mass units (amu) to reflect the relative mass of an atom compared to the carbon-12 isotope.

Exam Tip: Always distinguish between 'Atomic Mass' (measured in amu for individual atoms) and 'Molar Mass' (measured in g/mol for Avogadro's number of atoms).

Examoogle · Answer

Option A: Grams per mole (g/mol)

Examoogle · Answer

Option B: Pounds

Examoogle · Answer

Option C: Kilograms

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