Chemistry · Chemical Bonding · NEET
An octet means 8 electrons around each atom. Electrons around atoms come in pairs (2 per bond, 2 per lone pair). If a molecule has an ODD total number of valence electrons, one electron is always left over and cannot be paired. That leftover single electron means at least one atom ends up with 7 electrons (an incomplete octet). So NO and NO2 physically cannot give every atom a full octet.
Add the valence electrons of each atom. Nitrogen (group 15) has 5 valence electrons. Oxygen (group 16) has 6. For NO: 5 + 6 = 11 electrons. 11 is an odd number, so NO is an odd-electron molecule. You cannot split 11 into complete pairs, so one electron stays unpaired.
NO2 = 1 nitrogen + 2 oxygen. That is 5 + 6 + 6 = 17 valence electrons. 17 is odd, so NO2 is also an odd-electron molecule. The single unpaired electron sits mainly on the nitrogen atom, which is left short of a full octet.
Yes. A molecule with one unpaired (single) electron is called a free radical. NO and NO2 both have one unpaired electron, so they are free radicals. This is why they are chemically reactive - the lone unpaired electron wants to pair up with another electron.
Yes. Any species with one or more unpaired electrons is paramagnetic (weakly attracted by a magnet). Because NO and NO2 each have one unpaired electron, both are paramagnetic. This links directly to the idea NEET tests: unpaired electrons = paramagnetic.
No. NCERT lists THREE types of exceptions: (1) incomplete octet - central atom has fewer than 8 electrons (like BeH2, BCl3, BF3, AlCl3); (2) odd-electron molecules (like NO and NO2); (3) expanded octet - central atom has more than 8 electrons (like PF5, SF6). Odd-electron molecules are the middle category.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Molecules that have an odd (uneven) total number of valence electrons. Because electrons pair up, an odd total leaves one electron unpaired, so the octet rule cannot be satisfied for every atom. NO (11 electrons) and NO2 (17 electrons) are the standard NCERT examples.
NO2 has 17 valence electrons (5 from N + 6 + 6 from two O). 17 is odd, so one electron stays unpaired on nitrogen, leaving N with fewer than 8 electrons. This makes NO2 an odd-electron exception to the octet rule.
NO is paramagnetic because it has one unpaired electron. Any molecule with an unpaired electron is attracted by a magnetic field and is called paramagnetic. NO2 is paramagnetic for the same reason.
1) Incomplete octet (BeH2, BCl3, BF3, AlCl3 - central atom below 8 electrons); 2) Odd-electron molecules (NO, NO2); 3) Expanded octet / hypervalency (PF5, SF6 - central atom above 8 electrons).
Yes, because their unpaired electron makes them reactive. NO2 pairs up its odd electrons to form N2O4 (dinitrogen tetroxide), which has all electrons paired and is therefore diamagnetic. This dimerisation is nature's way of getting rid of the unpaired electron.
