Chemistry · Chemical Bonding · NEET
Do 3 quick steps. Step 1: count how many atoms are bonded to the central atom (call it B). Step 2: count lone pairs on the central atom (E). Formula for lone pairs: E = (central atom valence electrons - electrons used in bonds) / 2, adjusting for charge. Step 3: total pairs = B + E decides the base shape. If E = 0, the shape IS the base shape. If E is not 0, lone pairs push the atoms and give a bent, pyramidal, T-shape, see-saw, or square shape. Example: BF3 has B=3, E=0, so it is trigonal planar. This is the exact skill NEET tests every year in match-the-column questions.
These are the 5 clean cases you must memorise: AB2 = Linear (180°), example BeF2, CO2. AB3 = Trigonal planar (120°), example BF3. AB4 = Tetrahedral (109.5°), example CH4, NH4+. AB5 = Trigonal bipyramidal, example PCl5. AB6 = Octahedral (90°), example SF6. These 5 are the backbone of all molecular shapes for NEET, so learn them first before the lone-pair cases.
Electron pair geometry counts ALL pairs (bond pairs + lone pairs) around the center. Molecular shape counts only the ATOMS you can see (bond pairs only). For CH4 both are tetrahedral because there are no lone pairs. But for H2O, the electron pair geometry is tetrahedral (2 bonds + 2 lone pairs), while the actual molecular shape is bent (angular), because you cannot see the lone pairs. NEET keys sometimes ask for one, sometimes the other, so read the word 'geometry' vs 'shape' carefully.
Carbon in CH4 has 4 bond pairs and 0 lone pairs. Electron pairs repel each other and want to sit as far apart as possible. In a flat square, the angle would be 90°, but in 3D tetrahedral the angle is 109.5°, which is larger and means less repulsion. So the molecule prefers the tetrahedral shape. Rule to remember: electron clouds always spread out to the maximum angle they can.
These come up almost every year. PCl5 = trigonal bipyramidal (sp3d, AB5, no lone pair). XeF4 = square planar (sp3d2, AB4E2, two lone pairs). XeF6 = distorted octahedral (sp3d3, AB6E1, one lone pair). Also learn: BrF5 and XeOF4 = square pyramidal (AB5E1), ClF3 = T-shaped (AB3E2), XeF2 = linear (AB2E3). NEET loves matching these xenon and interhalogen compounds to shapes.
Lone pairs take a spot in the geometry but are invisible, so they 'bend' the visible shape. Tetrahedral base: 0 lone pairs = tetrahedral (CH4), 1 lone pair = trigonal pyramidal (NH3), 2 lone pairs = bent (H2O). Trigonal bipyramidal base: 1 lone pair = see-saw (SF4), 2 = T-shape (ClF3), 3 = linear (XeF2). Octahedral base: 1 lone pair = square pyramidal (BrF5), 2 = square planar (XeF4). Lone pairs also repel more, so they shrink the bond angles.
Match List-I with List-II. List-I: (a) PCl5, (b) SF6, (c) BrF5, (d) BF3. List-II: (i) Square pyramidal, (ii) Trigonal planar, (iii) Octahedral, (iv) Trigonal bipyramidal.
Match List-I with List-II. List-I (Compound): A. NH3, B. BrF5, C. XeF4, D. SF6. List-II (Shape): I. Trigonal pyramidal, II. Square planar, III. Octahedral, IV. Square pyramidal.
Identify the correct statement about ClF3 from the following options:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The number of electron pairs (bond pairs plus lone pairs) around the central atom decides the shape. This is the core idea of VSEPR theory. The pairs spread out to be as far apart as possible, and lone pairs bend the visible shape.
Linear = 180°, trigonal planar = 120°, tetrahedral = 109.5°, and octahedral = 90°. Trigonal bipyramidal has two angles, 120° in the plane and 90° up and down. Learn these standard angles because NEET directly asks them.
Common linear molecules are BeF2, BeCl2, CO2, and HgCl2 (all AB2 with no lone pair). XeF2, I3-, and IBr2- are also linear but as AB2E3, where three lone pairs push the two atoms to a straight line.
No. BF3 is trigonal planar (flat, AB3, no lone pair). NH3 is trigonal pyramidal (AB3E1, one lone pair on nitrogen), so it is a 3D pyramid, not flat. This one lone pair difference is a classic NEET point.
Almost every year NEET asks a match-the-column question linking a molecule to its shape or hybridisation, especially xenon and interhalogen compounds. Knowing shapes also helps you predict dipole moment, polarity, and bond angle, which are separate scoring questions.
