MHT CET · Chemistry · Structure of Atom

What is the representation of an element having mass number of 40 and 21 neutrons in it?

A \({ }_{18}^{40} \mathrm{X}\)
B \({ }_{19}^{40} \mathrm{X}\)
C \({ }_{12}^{40} \mathrm{X}\)
D \({ }_{20}^{40} \mathrm{X}\)

Verified Solution

Answer & Solution

Correct Answer

(B) \({ }_{19}^{40} \mathrm{X}\)

Step-by-step Solution

Detailed explanation

3. Protons \((Z)\) : The number of protons can be determined by subtracting the number of neutrons from the mass number:
\(Z=A-N=40-21=19\)
So, the element has 19 protons.
4. Element symbol: The element with 19 protons is potassium (K), which has the atomic number 19.
Thus, the correct representation of the element is K-40 or \({ }_{19}^{40} \mathrm{~K}\).
Therefore, the correct answer is:
(B) \({ }_{19}^{40} \mathrm{X}\).

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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