Mass Number
Master Mass Number in Minutes
When you see an atom written in the form of an element symbol with two numbers beside it, those numbers are not there just for decoration. They tell us important things about the atom. One tells us its atomic number, while the other tells us its mass number.
The mass number is connected to the nucleus. It tells us the total number of protons and neutrons present there. Once this idea is clear, finding the number of neutrons or reading an atom’s notation becomes much easier.
1.0Learning Outcomes
- Understand what the mass number of an atom represents.
- Identify which particles are included in the mass number.
- Calculate mass number when the numbers of protons and neutrons are given.
- Calculate the number of neutrons when atomic number and mass number are known.
- Identify the mass number in the standard representation of an atom.
- Distinguish between atomic number and mass number.
- Understand why isotopes of an element can have different mass numbers.
- Solve basic Class 9 questions based on mass number.
2.0Introduction
An atom has a tiny nucleus at its centre. This nucleus contains protons and neutrons, while electrons are found outside it. The number of these particles is different for different atoms.
Suppose you are told that the nucleus of an atom contains 6 protons and 6 neutrons. Instead of mentioning both numbers every time, we can give their total. Here, the total is 12. This total is called the mass number of the atom.
So, mass number is simply a count of the protons and neutrons present in the nucleus.
Electrons are not included in this count. Their mass is extremely small compared with that of a proton or neutron, so their mass is ignored when we calculate the mass number at the Class 9 level.
3.0What is Mass Number?
The mass number of an atom is the total number of protons and neutrons present in its nucleus. It is represented by the letter A.
The relationship can be written as:
A = Number of protons + Number of neutrons
Protons and neutrons are collectively called nucleons because both are present in the nucleus.
For example, suppose an atom has 5 protons and 6 neutrons. Its mass number will be:
[A = 5 + 6 = 11]
So, the mass number is 11.
One thing to keep in mind is that mass number is a count of particles. It is not the actual mass of the atom measured in kilograms or grams. The name can be a little confusing at first, but thinking of it as the total number of nucleons makes the idea easier to remember.
What Contributes to Mass Number?
Only protons and neutrons are counted when we find the mass number.
Why are electrons left out? The mass of an electron is much smaller than that of a proton or neutron. Therefore, for the calculation used in Class 9, the electron’s mass is considered negligible.
Take an atom containing:
- 8 protons
- 8 neutrons
- 8 electrons
The mass number is:
[A = 8 + 8 = 16]
We do not add the 8 electrons.
Another useful point is that both protons and neutrons are located inside the nucleus. That is why the mass number gives us information about what is present in the nucleus rather than about all the particles in the entire atom.
Calculating Mass Number
Finding mass number is usually a straightforward addition when the numbers of protons and neutrons are given.
For example, an atom contains 12 protons and 13 neutrons. What is its mass number?
We simply add them:
[A = 12 + 13]
[A = 25]
Therefore, the mass number of the atom is 25.
If a question also gives the number of electrons, do not include it in this calculation. First identify what the question is asking and then use only the particles needed for that calculation.
Another Example
An atom has 9 protons and 10 neutrons.
Mass number:
[A = 9 + 10 = 19]
So, its mass number is 19.
This is the basic calculation behind many questions on the topic.
Finding the Number of Neutrons
What if the question gives the mass number but does not tell you how many neutrons are present?
You can find it if the atomic number is also given.
Remember:
A = Protons + Neutrons
The atomic number tells us the number of protons. Therefore, subtracting the atomic number from the mass number gives the number of neutrons:
Number of neutrons = A − Z
Here, A is the mass number and Z is the atomic number.
Example
An atom has a mass number of 23 and an atomic number of 11.
Since its atomic number is 11, it has 11 protons.
Now:
Number of neutrons = 23 − 11
= 12
The atom therefore has 12 neutrons.
This is a calculation worth practising because questions often give the two numbers and ask you to work out the missing particle.
4.0Mass Number in the Representation of an Atom
There is a standard way of writing an atom using its symbol, atomic number and mass number.
For example, sodium can be represented as:
²³₁₁Na
The two numbers have different positions:
- 23 is the mass number.
- 11 is the atomic number.
- Na is the chemical symbol for sodium.
The upper number tells us the total number of protons and neutrons.
Since sodium has an atomic number of 11, it has 11 protons. We can then find its neutrons:
[23 - 11 = 12]
So this atom has 11 protons and 12 neutrons.
If the sodium atom is neutral, it will also have 11 electrons.
When solving questions based on this notation, don’t rush to add the two numbers. First identify which one is the mass number and which one is the atomic number.
Mass Number and Atomic Number
Mass number and atomic number are related, but they give us different information.
For example:
¹⁶₈O
The 8 at the bottom is the atomic number. It tells us that the oxygen atom has 8 protons.
The 16 at the top is the mass number. It gives the total number of protons and neutrons.
So:
Neutrons = 16 − 8 = 8
The nucleus therefore contains 8 protons and 8 neutrons.
A simple way to keep the two terms apart is:
Atomic number → tells us the number of protons.
Mass number → tells us the number of protons and neutrons together.
Mass Number and Isotopes
The mass number also helps us understand isotopes.
Atoms belonging to the same element always have the same number of protons. However, they may not always have the same number of neutrons. Atoms of the same element that have different numbers of neutrons are called isotopes.
Since the number of protons stays the same but the number of neutrons changes, their mass numbers are different.
Hydrogen is a familiar example. Its isotopes all have one proton, but the number of neutrons can differ. As a result, their mass numbers are different.
This gives us an important distinction:
- Same atomic number → same element
- Different number of neutrons → different mass number
So, when you come across isotopes later, the connection with mass number becomes quite easy to see.
5.0Supporting Study Materials
This Class 9 Science study material on Mass Number covers the basic idea of mass number, the role of protons and neutrons, simple calculations, standard atomic notation and the connection between mass number and isotopes. It can be used along with NCERT-based study material for revising the numerical and conceptual questions from the topic.
6.0Sample Questions & Answers: Mass Number
Q1. What is mass number?
Answer: Mass number is the total number of protons and neutrons present in the nucleus of an atom. It is represented by A.
Q2. An atom has 6 protons and 8 neutrons. What is its mass number?
Answer:
[A = 6 + 8 = 14]
The mass number is 14.
Q3. Why are electrons not included in the mass number?
Answer: Electrons have a very small mass compared with protons and neutrons. Their mass is therefore ignored when calculating the mass number at the Class 9 level.
Q4. An atom has atomic number 17 and mass number 35. Find the number of neutrons.
Answer:
The atomic number gives the number of protons.
So,
Neutrons = A − Z
= 35 - 17 = 18
The atom has 18 neutrons.
Q5. What are nucleons?
Answer: Protons and neutrons are together called nucleons. Both are present in the nucleus of an atom.
Q6. What do the numbers in ²³₁₁Na represent?
Answer: The upper number, 23, is the mass number and the lower number, 11, is the atomic number. Therefore, the sodium atom has 11 protons and:
[23 - 11 = 12]
So it has 12 neutrons.
Q7. Can atoms of the same element have different mass numbers?
Answer: Yes. Atoms of the same element can have different numbers of neutrons. These atoms are called isotopes, and their different neutron numbers give them different mass numbers.
7.030-Second Review: Mass Number
- Mass number (A) is the total number of protons and neutrons in the nucleus.
- Protons and neutrons together are called nucleons.
- Formula: A = protons + neutrons.
- Number of neutrons = A − Z.
- Electrons are not included when calculating mass number.
- Atomic number (Z) gives the number of protons.
- In standard atomic notation, the mass number is written above the atomic number.
- In ²³₁₁Na, 23 is the mass number and 11 is the atomic number.
- Isotopes have the same atomic number but different mass numbers because their numbers of neutrons are different.
8.0Recommended Next Topics