Question

Which among the following pairs are not having the same number of total electrons?

• A. \( \text{Na}^+ \) and \( \text{Al}^{3+} \)
• B. \( \text{O}^{2-} \) and \( \text{F}^- \)
• C. \( \text{Mg}^{2+} \) and Ar
• D. \( \text{P}^{3-} \) and Ar

Hint:

When comparing ions and neutral atoms, count the electrons based on the atomic number and charge of the ion or atom to determine whether they have the same number of electrons.

Answer 1

The Correct Option is A

The number of total electrons in an ion depends on its atomic number and the charge: - \( \text{Na}^+ \) has 10 electrons (since Na has atomic number 11 and loses 1 electron). - \( \text{Al}^{3+} \) has 10 electrons (since Al has atomic number 13 and loses 3 electrons). So, \( \text{Na}^+ \) and \( \text{Al}^{3+} \) both have 10 electrons, which makes this pair correct. Let's check the other pairs: - \( \text{O}^{2-} \) has 10 electrons (O has atomic number 8 and gains 2 electrons), and \( \text{F}^- \) has 10 electrons (F has atomic number 9 and gains 1 electron). Thus, this pair has the same number of electrons. - \( \text{Mg}^{2+} \) has 10 electrons (Mg has atomic number 12 and loses 2 electrons), and Ar has 18 electrons. Therefore, this pair does not have the same number of electrons. - \( \text{P}^{3-} \) has 18 electrons (P has atomic number 15 and gains 3 electrons), and Ar has 18 electrons. This pair has the same number of electrons. Hence, the correct answer is \( \text{Mg}^{2+} \) and Ar.