The atomic number of an element is $21$ and the number of neutrons in its atom is $18$ . Find the mass number of the element?

40

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35

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39

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41

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Solution

The correct option is C $39$
Atomic number or number of protons $= 21$
Number of neutrons $= 18$
Mass number $(A) = N u m b e r o f p r o t o n s + N u m b e r o f n e u t r o n s$
Mass number $(A) = 21 + 18$
Mass number $(A) = 39$

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