Chemistry · Chemical Bonding · NEET
Oxygen in H2O has 4 electron pairs: 2 bond pairs (to the two H) and 2 lone pairs. So the electron-pair geometry is tetrahedral (like CH4). But the two lone pairs are invisible — you only see the O and the 2 H atoms. Two bonds coming out of a tetrahedron give a BENT shape. The lone pairs also push the bonds closer, so the angle drops from 109.5 degrees to about 104.5 degrees. This is the AB2E2 type.
Electron-pair geometry counts ALL pairs — bond pairs AND lone pairs — to decide the basic arrangement (linear, tetrahedral, octahedral, etc.). Molecular shape counts ONLY the atoms you can see, because lone pairs are not atoms. Example: XeF4 has 6 pairs (4 bonds + 2 lone pairs), so electron geometry is octahedral, but the SHAPE is square planar. In NEET, read the question carefully — some ask geometry, some ask shape.
Sulphur in SF4 has 5 electron pairs: 4 bond pairs (to F) + 1 lone pair. That is trigonal bipyramidal electron geometry (sp3d). The single lone pair goes to an EQUATORIAL position (more room, less repulsion). Removing one equatorial atom from a trigonal bipyramid leaves a see-saw shape. This is AB4E.
Chlorine in ClF3 has 5 pairs: 3 bond pairs + 2 lone pairs (AB3E2, sp3d). Both lone pairs sit in equatorial positions of the trigonal bipyramid to avoid repulsion. The 3 F atoms are left in a T arrangement — two axial and one equatorial. So it is T-shaped, NOT flat trigonal planar. Trigonal planar (like BF3) has NO lone pair on the central atom.
Both N in NH3 and C in CH4 have 4 electron pairs, so both are basically tetrahedral in electron geometry. But NH3 has 1 lone pair (AB3E) and 3 bonds — the lone pair is invisible, so the 3 H atoms form a PYRAMID (pyramidal, angle ~107 degrees). CH4 has 4 bonds and NO lone pair, so all 4 H are visible and it is a full tetrahedron (109.5 degrees).
Step 1: count bond pairs (B) = number of atoms bonded. Step 2: count lone pairs (E) on the central atom = (valence electrons of central atom minus electrons used in bonds) divided by 2. Step 3: total pairs (B + E) gives the electron geometry, then delete the lone-pair positions to read the shape. Memory: AB2E = bent, AB3E = pyramidal, AB2E2 = bent, AB4E = see-saw, AB3E2 = T-shape, AB5E = square pyramidal, AB4E2 = square planar.
In the structure of ClF3, the number of lone pairs of electrons on the central atom Cl is:
Among the following, which one is a wrong statement?
Match List-I with List-II. A. NH3; B. BrF5; C. XeF4; D. SF6 with I. Trigonal pyramidal; II. Square planar; III. Octahedral; IV. Square pyramidal.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Bent (H2O, SO2), trigonal pyramidal (NH3), see-saw (SF4), and T-shape (ClF3). These come from AB2E/AB2E2, AB3E, AB4E and AB3E2 respectively.
Yes. Hybridisation counts total electron pairs = bond pairs + lone pairs. For example NH3 has 3 bonds + 1 lone pair = 4 pairs = sp3. But the SHAPE ignores the lone pair, so NH3 is pyramidal, not tetrahedral.
Equatorial positions have more space (only two neighbours at 90 degrees vs three for axial). A lone pair needs the most room to reduce repulsion, so it prefers equatorial. This is why SF4 is see-saw and ClF3 is T-shaped. (Covered in the next topic: axial vs equatorial lone pair.)
Yes, for main-group molecules like XeF4. XeF4 is AB4E2: 4 bonds + 2 lone pairs = 6 pairs (octahedral geometry). The two lone pairs sit opposite each other, leaving the 4 F atoms in one plane = square planar.
Lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. This order is why lone pairs shrink bond angles and decide where lone pairs sit. It was directly asked in NEET 2016.
