NCERT Questions

Class 9
Chapter 4 Class 9 - Structure of Atom

## (i) Atomic number

• It is the number of protons present in the nucleus of an atom
• It is denoted by Z

## Example - 1

• For Hydrogen, Z = 1
• It means that in a hydrogen atom, there are 1 proton present in the nucleus.

## Example - 2

• For Carbon, Z = 6

It means that in a carbon atom, there are 6 protons in the nucleus.

## ii) Mass number

• Mass number is the total number of protons and neutrons present in the nucleus of an atom.
• It is denoted by A .

## Example - 1

• Carbon atom has 6 protons and 6 neutrons
• So, Mass number is 6 + 6 = 12
• A = 12

## Example - 2

• An aluminium atom has 13 protons and 14 neutrons
• So, Mass number = 13 + 14 = 27
• A = 27

## iii) Isotopes

• Isotopes are elements having the same atomic number but different mass numbers.
• That is, they’ve equal number of protons/electrons but different numbers of neutrons.
• Because they’ve same number of electrons, their electronic configuration is the same.
• Example, and .
• Atomic number of both the elements is 17 but mass numbers are different.

## iv) Isobars

• Isobars are elements having different atomic numbers but same mass numbers.
• That is, they’ve same number of nucleons (protons+neutrons) but different numbers of protons/electrons.
• Because they’ve different numbers of electrons, their electronic configuration is different.
• For example, and  .
• Mass number of both the elements is 40 but atomic numbers are different.

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