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# Helium Atom Has an Atomic Mass of 4 U and Two Protons in Its Nucleus. How Many Neutrons Does It Have? - CBSE Class 9 - Science

ConceptStructure of an Atom - Neutrons

#### Question

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

#### Solution

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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#### APPEARS IN

NCERT Solution for Science Textbook for Class 9 (2018 to Current)
Chapter 4: Structure of the Atom
Q: 2 | Page no. 49
Solution Helium Atom Has an Atomic Mass of 4 U and Two Protons in Its Nucleus. How Many Neutrons Does It Have? Concept: Structure of an Atom - Neutrons.
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