Are you preparing for competitive exams like SSC, Banking, RRB NTPC, UPSC, or any State-level exam? Then learning about the Atomic Mass Number is super important! This topic comes under General Science Chemistry, and it’s one of the most common questions in exams.
In this blog, you’ll find simple notes on Atomic Mass Number explained in easy to understand language. We’ve also included the best MCQs (Multiple Choice Questions) to help you practice and score better.
You’ll learn what atomic mass number means, how to calculate it, and why it matters in science. Whether you’re just starting out or revising before the exam, these notes and questions will make your learning fun and easy.
Quick Definition: What is Mass Number?
- The mass number (symbol A), also known as the atomic mass number or nucleon number, is the total count of protons and neutrons found within the nucleus of an atom.
- So, Mass Number is the total number of protons and neutrons in the nucleus of an atom.
- ✅ Mass Number (A) = Total number of Protons + Neutrons in an atom’s nucleus.
Basic Structure of an Atom
Before we understand Mass Number, let’s recall what an atom looks like:
| Subatomic Particle | Charge | Location in Atom |
|---|---|---|
| Proton | Positive (+1) | Inside the Nucleus |
| Neutron | Neutral (0) | Inside the Nucleus |
| Electron | Negative (-1) | Outside the Nucleus |
➡️ The nucleus of the atom contains protons and neutrons. These two particles contribute to the mass of the atom.
➡️ Electrons are very light, so they are not included in the mass number.
Simple Formula of Mass Number
👉 Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N)
Where:
- A = Mass Number
- Z = Atomic Number (Number of Protons)
- N = Number of Neutrons
Example of Mass Number
Let’s take the example of Carbon (C):
- Atomic Number (Z) of Carbon = 6
- Number of Neutrons in most Carbon atoms = 6
- So, Mass Number = 6 (Protons) + 6 (Neutrons) = 12
✅ Therefore, Carbon has a mass number of 12.
Mass Number of Some Common Elements
| Element | Symbol | Atomic Number (Protons) | Neutrons | Mass Number |
|---|---|---|---|---|
| Hydrogen | H | 1 | 0 | 1 |
| Helium | He | 2 | 2 | 4 |
| Lithium | Li | 3 | 4 | 7 |
| Carbon | C | 6 | 6 | 12 |
| Oxygen | O | 8 | 8 | 16 |
📝 Note: Some elements may have different numbers of neutrons, which we call isotopes. But for exams, focus on the most common mass number.
Difference Between Mass Number and Atomic Number
| Mass Number | Atomic Number |
|---|---|
| Total of protons + neutrons | Only the number of protons |
| Symbol: A | Symbol: Z |
| Helps find the mass of atom | Helps find the identity of atom |
| Not always unique (due to isotopes) | Unique for every element |
Why is Mass Number Important in Exams?
- Mass Number is often used to calculate the number of neutrons.
- Questions like: “An atom has a mass number of 23 and atomic number 11. Find the number of neutrons?”
→ Answer: 23 – 11 = 12 neutrons
✅ Knowing how to apply the formula quickly helps you score better in exams!
Fun Fact: What are Isotopes?
Isotopes are atoms of the same element that have the same number of protons, but different number of neutrons.
Example:
- Carbon-12 (6 protons + 6 neutrons = 12)
- Carbon-14 (6 protons + 8 neutrons = 14)
Both are Carbon, but their mass numbers are different.
📗Read more in-depth notes and exam-focused points on Isotopes
Memory Tip to Remember
🔢 “Mass = Protons + Neutrons”
➡️ Just think of MASS as adding together the weighty parts of the atom (protons + neutrons).
🧠 Use the mnemonic:
“Mass Adds Stuff Securely” → Mass = Add (Protons + Neutrons)
15 MCQs on Mass Number
1. What does the mass number of an atom represent?
a) Number of electrons
b) Number of neutrons
c) Number of protons + electrons
d) Number of protons + neutrons
✅ Answer: d) Number of protons + neutrons
➡️ Mass number = Protons + Neutrons (found in the nucleus)
2. If an atom has 6 protons and 6 neutrons, what is its mass number?
a) 6
b) 12
c) 18
d) 3
✅ Answer: b) 12
➡️ Mass Number = 6 + 6 = 12 (For example Carbon)
3. An atom has a mass number of 23 and an atomic number of 11. How many neutrons does it have?
a) 12
b) 10
c) 11
d) 13
✅ Answer: a) 12
➡️ Neutrons = Mass number – Atomic number = 23 – 11 = 12 (for example Sodium (Na))
4. Which particle does not contribute to the mass number of an atom?
a) Proton
b) Neutron
c) Electron
d) Both a and b
✅ Answer: c) Electron
➡️ Electrons have negligible mass and are not included in the mass number.
5. Which of the following is true about mass number?
a) It is always less than atomic number
b) It is always more than atomic number
c) It can never be a whole number
d) It includes electrons
✅ Answer: b) It is always more than atomic number
➡️ Because it includes neutrons too.
6. The mass number of an atom is determined by counting:
a) Only neutrons
b) Neutrons and electrons
c) Protons and neutrons
d) Protons and electrons
✅ Answer: c) Protons and neutrons
7. What is the mass number of Oxygen if it has 8 protons and 8 neutrons?
a) 14
b) 16
c) 8
d) 18
✅ Answer: b) 16
8. An atom of Carbon has a mass number of 14 and an atomic number of 6. How many neutrons does it have?
a) 8
b) 6
c) 7
d) 14
✅ Answer: a) 8
➡️ Neutrons = 14 – 6 = 8
9. Which symbol is generally used to represent mass number?
a) Z
b) N
c) M
d) A
✅ Answer: d) A
➡️ A = Mass Number, Z = Atomic Number, N = Neutrons
10. An isotope differs in which of the following?
a) Number of protons
b) Mass number
c) Atomic number
d) Number of electrons
✅ Answer: b) Mass number
➡️ Isotopes have the same atomic number but different mass numbers due to a different number of neutrons.
11. What is the mass number of an atom having 15 protons and 16 neutrons?
a) 30
b) 29
c) 31
d) 32
✅ Answer: c) 31
12. Which two particles are present inside the nucleus of an atom?
a) Electrons and protons
b) Protons and neutrons
c) Neutrons and electrons
d) Only neutrons
✅ Answer: b) Protons and neutrons
13. What is the number of neutrons in an atom of Lithium (Mass number = 7, Atomic number = 3)?
a) 4
b) 3
c) 7
d) 10
✅ Answer: a) 4
➡️ Neutrons = 7 – 3 = 4
14. What will be the mass number of an atom if it has 20 protons and 20 neutrons?
a) 20
b) 30
c) 40
d) 25
✅ Answer: c) 40
15. If an atom has 17 protons and its mass number is 35, how many neutrons does it have?
a) 17
b) 18
c) 35
d) 52
✅ Answer: b) 18
➡️ Neutrons = 35 – 17 = 18
Final Recap
Let’s sum it up quickly:
- Mass Number = Protons + Neutrons
- It tells us the total mass inside the nucleus of an atom.
- Mass Number is always a whole number.
- It is important for solving basic science questions in exams.
Keep Learning, Keep Growing!
Understanding simple concepts like Mass Number builds a strong base for topics like Atoms, Molecules, and Nuclear Physics. These concepts are common in competitive exams and help you answer questions faster and more accurately.
Stay curious, stay confident—and remember, small concepts lead to big success! 💪📘
