Science
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Chapter 4: Structure of Atom
Class: IX
Exercise number 49B
Question 1
Name the three sub-atomic particles of an atom.
Answer 1
The three sub-atomic particles of an atom are:
(i) Protons
(ii) Neutrons, and
(iii) Electrons
Question 2
Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Answer 2
It has two neutrons. Because the mass of an atom is the sum of the masses of protons and
neutrons present in its nucleus.
Atmoic mas of helium =4 u (Given)
Given helium atom has two protons, mass contributed by the two protons is (2 × 1) u = 2 u.
Then, the remaining mass (4 − 2) u = 2 u is contributed by 2u /1u = 2 neutrons.
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