$_8$X$^{2-}$ and $_{10}$Y are

Analyze the question We are given two species: X²⁻ with atomic number 8, and Y with atomic number 10. We need to find how they are related.

Concept

• Isomers: compounds with the same molecular formula but different structures.
• Isotopes: atoms of the same element (same atomic number) with different mass numbers.
• Allotropes: different physical forms of the same element (e.g., diamond and graphite).
• Isoelectronic: different species with the same number of electrons.

Solution

1. X has atomic number 8, so a neutral atom has 8 electrons (it is oxygen).
2. X²⁻ has gained 2 electrons: 8 + 2 = 10 electrons.
3. Y has atomic number 10, so a neutral atom has 10 electrons (it is neon).
4. Both species have 10 electrons, with the configuration 1s² 2s² 2p⁶.

Checking the other options:

• They have different atomic numbers (8 and 10), so they are different elements. They cannot be isotopes or allotropes.
• They are not compounds with the same molecular formula, so they are not isomers.

Since both have the same number of electrons (10), they are isoelectronic.

Exam tip For "isoelectronic" questions, always compute the electron count: for a negative ion, add the charge to the atomic number; for a positive ion, subtract it. Familiar isoelectronic sets with 10 electrons are N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, and Al³⁺.