Shape and polarity
by Cheong Mun Yun
1. Valence Shell Electron Pair Repulsion (VSEPR) Theory
1.1. Postulate 1: E pairs in valence shell experience least repulsion when far apart as possible, so they adopt positions that maximise separations to minimise repulsion
1.1.1. New node
1.2. New node
1.3. Postulate 2: lone pair-lone pair repulsion > lone pair-bond pair repulsion > bond pair-bond pair repulsion
1.3.1. because lone pair closer to nucleus as its attracted by 1 +ve nucleus (bond pair attracted by 2), causing distortion in expected shape so bond angle between bond-pair bond-pair e will be smaller
1.4. Postulate 3: Increase in EN of central atom causes greater displacement of electron density of bond pairs towards central atom, so greater repulsion btw bond pairs of e and larger bond angle
2. Drawing dot-and-cross diagrams
2.1. 1. Arrange atoms such that least EN is the central atom while most EN are peripheral
2.2. 2. Count total no. valence e (take into acc e gained/lost if molecule is ion)
2.3. 3. Assign 1 bond pair to every 2 bonded atoms and lone pairs to peripheral atoms (complete octet)
2.4. 4. Remaining e assigned as lone pairs of central atom
2.5. 5. Ensure central atom has complete octet, if not reassign e from peripheral to form multiple bonds with central
2.6. 6. Allocate dots and crosses to represent e of diff atoms
3. New node
4. Polarity
4.1. 1.Existence of polar bonds: non-polar if polar bonds cancel each other, polar if polar bonds do not cancel each other out
4.2. 2. Shape: Polar molecules are not symmetrical in shape
4.3. 3.Lone pairs of e: lone pairs are -ve, form delta- end of dipole, so if central atom has lone pairs on one side, it is polar