Question

question 2. if z is the symbol for an element, which pair among the following represents isotopes of the same element?
$_{25}^{51}z$ and $_{26}^{51}z$
$_{25}^{50}z$ and $_{27}^{50}z$
$_{25}^{51}z$ and $_{25}^{50}z$
$_{26}^{51}z$ and $_{25}^{50}z$

Explanation:

Step1: Recall isotope definition

Isotopes of the same element have the same atomic number (lower - left number) and different mass numbers (upper - left number).

Step2: Analyze each option

For the first option, the atomic numbers are different ($25$ and $26$). For the second option, the atomic numbers are different ($25$ and $27$). For the third option, the atomic number is $25$ for both, and the mass numbers are $51$ and $50$ which is correct for isotopes. For the fourth option, the atomic numbers are different ($26$ and $25$).

Answer:

$\ce{^{51}_{25}Z}$ and $\ce{^{50}_{25}Z}$