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

What is the representation of an element having mass number of 40 and 21 neutrons in it?
  1. ${ }_{18}^{40} \mathrm{X}$
  2. ${ }_{19}^{40} \mathrm{X}$
  3. ${ }_{12}^{40} \mathrm{X}$
  4. ${ }_{20}^{40} \mathrm{X}$

Solution

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) ${ }_{19}^{40} \mathrm{X}$.

Asked in: MHT CET 2024 (03 May Shift 1)

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