Question

Which of the following is/are isotones?

• A. $146C$ and $147N$
• B. $2311\mathrm{Na}$ and $2412\mathrm{Mg}$
• C. $4020\mathrm{Ca}$ and $3919K$
• D. $3818\mathrm{Ar}$ and $3517\mathrm{Cl}$

Answer 1

Answer: (B), (C)

The correct options are
B $2311\mathrm{Na}$ and $2412\mathrm{Mg}$
C $4020\mathrm{Ca}$ and $3919K$

Isotones are the atoms of different elements, with different atomic number and different mass number, but the same number of neutrons. Among the given options in the form of $AZX$, the value of ($\text{A - Z}$) should be equal.

• $2311\mathrm{Na}$ and $2412\mathrm{Mg}$
  have the same number of neutrons, (23 - 11) = (24 - 12) = 12
• $4020\mathrm{Ca}$ and $3919K$ have the same number of neutrons, (39 - 19) = (40 - 20) = 20