Atomic Mass MCQs Quiz | Class 9

Test your understanding of Class IX Science (Code 086) Unit I: Matter – Its Nature and Behaviour with this specific quiz on Atomic Mass. Covering key concepts like the meaning of atomic mass, the relative mass idea, and qualitative units, this quiz helps reinforce your chemistry foundation. Click “Submit Quiz” to check your score and “Download Answer PDF” for a detailed revision sheet.

Overview of Atomic Mass

In Class 9 Science, Unit I: Matter – Its Nature and Behaviour, the concept of atomic mass is fundamental. Since atoms are extremely small, determining their absolute mass is difficult and often impractical for daily calculations. Instead, scientists use the concept of Relative Atomic Mass.

Key Concepts

  • Reference Standard: Carbon-12 isotope (C-12) is the universally accepted standard reference for measuring atomic masses.
  • Definition: One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of Carbon-12.
  • Relative Atomic Mass: The relative atomic mass of an element is the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.
  • Unit: The symbol used is ‘u’ (unified mass), defined by IUPAC. Previously, it was denoted as ‘amu’.

Common Atomic Masses

Memorizing the atomic masses of common elements helps in solving numerical problems efficiently.

Element Symbol Atomic Mass (u)
HydrogenH1
CarbonC12
NitrogenN14
OxygenO16
SodiumNa23
MagnesiumMg24
SulphurS32
ChlorineCl35.5
CalciumCa40

Why “Relative”?

Imagine a fruit seller selling fruits without standard weights. He takes a watermelon, calls it “12 units,” cuts it into 12 equal slices, and uses one slice as a standard unit to weigh other fruits. Similarly, scientists chose the Carbon-12 atom (mass = 12 u) and defined 1 u as 1/12th of that mass to weigh all other atoms relative to it.

Extra Practice Questions

  1. What is the atomic mass of Aluminium? (Ans: 27 u)
  2. Which organization approves chemical names and units? (Ans: IUPAC)
  3. Why was Oxygen-16 used as a reference before Carbon-12? (Ans: Because it reacted with most elements to form compounds).
  4. Is atomic mass the same as atomic number? (Ans: No, atomic mass is roughly protons + neutrons).
  5. What is the mass of a Sulphur atom relative to C-12? (Ans: 32 u).