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CAIE A-Level Chemistry 2.1.1 Unified Atomic Mass Unit

Practise stating the exact carbon-12 reference used for atomic, isotopic, molecular and formula masses.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • define one unified atomic mass unit as one twelfth of the mass of one carbon-12 atom
  • distinguish the mass unit from the dimensionless relative-mass quantities Ar and Mr
  • use the same carbon-12 reference when defining relative isotopic, atomic, molecular or formula mass

2.1.1—Unified atomic mass unit question 1

[Maximum number: 1]

Calcium, magnesium and radium are Group 2 elements. Radium follows the same trends as the other members of Group 2.

A sample of magnesium contains three isotopes, 25Mg,26Mg{ }^{25} \mathrm{Mg},{ }^{26} \mathrm{Mg} and X.

The percentage abundance of the three isotopes is shown in Table 1.1.

Table 1.1

Table 1.1

The relative atomic mass, ArA_{r}, is calculated by comparing the average mass of the isotopes of an element to the unified atomic mass unit.

Define the unified atomic mass unit.

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