Mass Number
Mass number is the total number of protons and neutrons present in the nucleus of an atom.
Practice This ConceptMain explanation
Teacher explanation
Mass number is represented by A. Since protons and neutrons have nearly one unit mass each and electrons have very small mass, mass number mainly counts the heavy particles in the nucleus. It is a whole number for a particular atom and should not be confused with average atomic mass.
Example
An atom with 9 protons and 10 neutrons has mass number 19.
Simple analogy
Mass number counts the nucleus team: protons plus neutrons.
Common confusion
Students often include electrons while calculating mass number.
Exam tip
For school-level calculation, count only protons and neutrons for mass number.
Answer writing and exam use
1-mark use
Write the exact meaning of mass number in one clean line.
2-mark use
Define mass number and add one example or condition.
3-mark use
Explain mass number, show the method or example, and mention the common mistake.
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