Question

NCERT CLASS IX - Science Textbook
Chapter 4 - Structure of the Atom

Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Answer 1

mass number of an element is equal to the sum of number of neutrons and protons in its atom.

Given that atomic mass= 4u
number of proton = 2
number of neutron = 4 - 2 = 2

Answer 2

☆$\red {HELLO\:DEAR}$☆

Helium atom has two neutrons.

The mass of an atom is the sum of the masses of protons and neutrons present in its nucleus.

Since 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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