Isoelectronic species have the same number of

Step 1: Analyze the question The question asks what isoelectronic species have in common. Look closely at the word itself.

Step 2: Explain the concept "Isoelectronic" breaks into two parts:

• iso- means "same"
• electronic refers to electrons

So isoelectronic species are atoms, ions, or molecules that have the same number of electrons (and usually the same electron configuration), even though they may be different elements.

Step 3: Worked example Consider these species:

| Species | Protons | Electrons | |---|---|---| | O²⁻ | 8 | 8 + 2 = 10 | | F⁻ | 9 | 9 + 1 = 10 | | Ne | 10 | 10 | | Na⁺ | 11 | 11 − 1 = 10 | | Mg²⁺ | 12 | 12 − 2 = 10 |

All of them have 10 electrons and the configuration 1s² 2s² 2p⁶. Their numbers of protons differ, so they are different species, but they are isoelectronic.

Step 4: Check the other options

• A. ions: Not a defining property. Some isoelectronic species, like Ne, are not ions.
• C. protons: The number of protons is the atomic number, which identifies the element. Isoelectronic species usually have different proton numbers.
• D. neutrons: This has nothing to do with being isoelectronic.

Step 5: Answer The correct option is B. electrons.

Exam tip: Break the prefix down. Iso- = same, so:

• Isoelectronic = same electrons
• Isotopes = same protons, different neutrons
• Isobaric = same mass number

Also remember that for isoelectronic ions, the one with more protons is smaller, because the nucleus pulls the same electrons more strongly (Mg²⁺ < Na⁺ < F⁻ < O²⁻). This is a common follow-up question.