Mass Number
Mass number is the total number of protons and neutrons present in the nucleus of an atom.
In one line: Mass number counts the nucleus team: protons plus neutrons.
Practice This ConceptUnderstand it
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
Rule to remember
A = number of protons + number of neutrons; neutrons = A - Z.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
If Z = 15 and A = 31, protons = 15 and neutrons = 31 - 15 = 16.
Method you can reuse
Sketch it in your answer
Use a nucleus diagram with protons and neutrons only highlighted for mass number.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why are electrons not added while finding mass number?
Think first, then show answerHide answer
Common wrong answer
For 11 protons, 12 neutrons, and 11 electrons, writing mass number 34.
Why it loses marks: Electrons are not included.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
Mass number is the total number of protons and neutrons present in the nucleus of an atom.
- 2
Add for 2 marks
Example: An atom with 9 protons and 10 neutrons has mass number 19.
- 3
Add for 3 marks
Mass number is represented by A.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
Help improve this page
Found something confusing, incorrect, or missing?