In chemistry, the weighted average atomic mass of an element as listed on the periodic table is usually expressed in which standard unit?

Introduction / Context:
The concept of weighted average atomic mass is central to understanding the periodic table and chemical calculations. Each element has more than one isotope, and the atomic mass shown on the periodic table is a weighted average that takes into account the relative abundance of each isotope. This value is not just a simple count of protons and neutrons, but an experimentally determined average. The question asks which unit is used for this quantity, which is often called the atomic weight or relative atomic mass in basic chemistry courses.

Given Data / Assumptions:

• The term used is weighted average atomic mass of an element.
• This is the value typically printed under the element symbol on the periodic table.
• Standard conventions in chemistry are assumed.

Concept / Approach:
The weighted average atomic mass is defined relative to a standard, which is one twelfth of the mass of a carbon 12 atom. The unit used is the atomic mass unit, symbol u or amu. While the numerical value of atomic mass in atomic mass units is equal to the numerical value of molar mass in grams per mole, the physical quantities and their units are conceptually different. Atomic mass is the mass of a single atom or the average mass per atom, while molar mass refers to the mass of one mole of atoms. The approach is to recall that periodic table values are quoted in atomic mass units as a microscopic mass unit.

Step-by-Step Solution:
Step 1: Recognise that the quantity described is the average mass of one atom of an element, based on natural isotopic abundances. Step 2: Recall that chemists use a special small mass unit, the atomic mass unit, to express masses at the atomic scale. Step 3: The definition of one atomic mass unit is one twelfth of the mass of a carbon 12 atom, making it convenient for comparing atomic masses. Step 4: Therefore, the weighted average atomic mass of an element is expressed in atomic mass units, amu or simply u. Step 5: Molar mass, on the other hand, uses gram per mole and is related but not the same conceptually.

Verification / Alternative check:
Looking at any modern periodic table, you will see atomic masses listed as numbers like 12.01 for carbon or 35.45 for chlorine. These numbers are understood to be in atomic mass units. Textbooks often write the unit explicitly when introducing the concept. At the same time, they explain that the molar mass in grams per mole has the same numerical value, but this is because one mole contains Avogadro number of atoms. This cross check between atomic mass and molar mass supports the distinction between the amu unit for atomic scale and g/mol for molar quantities.

Why Other Options Are Wrong:
Option B, gram, is too large a unit for the mass of a single atom and is instead used for macroscopic samples. Option C, gram per mole, is the unit of molar mass, not the microscopic atomic mass itself. Option D, per mole, is dimensionally incorrect for a mass quantity. Option E, kilogram per cubic metre, is a density unit, not a mass unit. Only option A correctly identifies atomic mass unit, amu or u, as the standard unit for the weighted average atomic mass printed on the periodic table.

Common Pitfalls:
A frequent confusion is treating atomic mass and molar mass as identical quantities because their numerical values match. Students sometimes write g/mol when asked for the unit of atomic mass. It is important to distinguish between the mass of a single atom, which uses amu, and the mass of a mole of atoms, which uses g/mol. Keeping track of microscopic versus macroscopic quantities helps avoid unit errors in stoichiometry and other calculations.

Final Answer:
The correct unit is Atomic mass unit (amu or u), which is used to express the weighted average atomic mass of an element on the periodic table.