Chemistry · Chemical Bonding · NEET
Isoelectronic means SAME number of electrons. Isostructural means SAME shape (geometry). They are two different tests. Two species can be isoelectronic but have different shapes, or have the same shape but different electron counts. A NEET question that says 'isoelectronic AND isostructural' needs BOTH to be true for the same pair.
Count the total electrons in each. For a neutral atom, electrons = atomic number (Z). For a cation, subtract the charge; for an anion, add the charge. Example: Na+ = 11 - 1 = 10 electrons; F- = 9 + 1 = 10 electrons. Same count (10), so they are isoelectronic. NCERT gives O2-, F-, Na+, Mg2+ as a classic 10-electron set.
Add up the electrons of every atom, then adjust for the charge. NO3-: N(7) + 3 O(8 each = 24) = 31, then add 1 for the negative charge = 32 electrons. CO3 2-: C(6) + 3 O(24) = 30, add 2 = 32. Both have 32, so they are isoelectronic. Tip: for shape questions, counting only VALENCE electrons is faster and gives the same conclusion.
No. Same electron count does not force the same shape, especially for atoms and ions from Structure of Atom (like K+ and Ca2+ are just spheres). But for MOLECULES/POLYATOMIC IONS built from similar central atoms, isoelectronic pairs very often ARE isostructural because they have the same number of bond pairs and lone pairs. That is why NEET pairs the two ideas.
Both have 22 valence electrons, so they are isoelectronic. Each has a central atom with 2 bond pairs and 3 lone pairs (AB2E3). Three lone pairs sit in the equatorial plane and the two bonds go straight up and down, giving a LINEAR shape. Same electrons + same shape = isoelectronic and isostructural. This is the answer to a real NEET 2016/2017 question.
Not exactly, but they are linked. Isoelectronic diatomic species usually have the SAME bond order because they fill the same molecular orbitals. Example: CN- and CO both have 14 electrons and both have bond order 3. So if two diatomic species are isoelectronic, expect the same bond order and often the same magnetic behaviour.
Which of the following pairs of ions is isoelectronic and isostructural?
Which of the following pairs of compounds is isoelectronic and isostructural?
Which one of the following pairs of species have the same bond order?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
O2-, F-, Na+ and Mg2+ all have 10 electrons, so they are isoelectronic. Their sizes still differ because their nuclear charges are different, but the electron count is the same.
Yes. N2 (7+7=14), CO (6+8=14) and CN- (6+7+1=14) all have 14 electrons. They are isoelectronic and all have bond order 3, which is why they behave similarly in NEET MO questions.
Yes. Isoelectronic only means equal electron count, not the same element. Ca2+, Ar, K+ and Cl- are all different species but each has 18 electrons, so they are isoelectronic.
H2O has 10 electrons, so it is isoelectronic with Ne, HF, NH3 and CH4 in terms of electron count (all 10). But their shapes differ, so they are isoelectronic without being isostructural. This shows the two tests are separate.
Because it forces you to use two chapters together: electron counting (Structure of Atom) and shape from VSEPR (Chemical Bonding). Doing only one test makes you fall for a trap option, so combining them tests deeper understanding.
