Chemistry - Mole Concept Question with Solution | TestHub

Explain the question: Check which option correctly defines the given species.

Concept: This question is based on isotopes, isotones, isobars, and isoelectronic species.

Solution: We must check the definition for each option:

1. Isotopes (${}_{20}^{40}Ca$, ${}_{19}^{40}K$): Incorrect. Isotopes have the same atomic number (Z). Here, Z is 20 and 19. These are isobars (same mass number A = 40).

2. Isotones (${}_{14}^{30}Si$, ${}_{15}^{31}P$, ${}_{16}^{32}S$): Correct. Isotones have the same number of neutrons ($N=A-Z$). For Si: $N=30-14=16$. For P: $N=31-15=16$. For S: $N=32-16=16$. All have 16 neutrons.

3. Isobars (${}_8^{16}O$, ${}_8^{17}O$, ${}_8^{18}O$): Incorrect. Isobars have the same mass number (A). Here, A is 16, 17, and 18. These are isotopes (same $Z=8$).

4. Isoelectronic ($N^{3-}$, $O^{2-}$, $C{r}^{3+}$): Incorrect. Isoelectronic species have the same number of electrons. N has $7+3=10$ e⁻. O has $8+2=10$ e⁻. Cr has $24-3=21$ e⁻. They do not all have the same electron count.

Final answer: B