# Lecture 17: Electron Affinity, Electronegativity, Polarity

• Electron Affinity - (Of an atom) the amount of energy released when an electron is added to a neutral atom to form a negative ion
• ﻿$x + e^- \rightarrow x^-$﻿ + energy
• Electronegativity - The ability of an atom in a molecule to attract the bonded pair of electrons towards itself
• Usually atoms want to add electrons to reach octate
• More electronegative means it can attract a bonded pair stranger to itself
• When one end of the molecule is more negative and the other is more positive, the molecule is polar. The shared pair of electrons is more attracted to one atom than the other
• ﻿$O, F$﻿ are the most electronegative ions
• ﻿$C-O$﻿ molecule is an exception
• Carbon is more negative than oxygen
• In this molecule explained by molecular orbital theory
• For a molecule to be polar
• The connected atoms should have a difference in electronegativity (﻿$M - Cl$﻿)
• Magnitude and Direction
• Shape should have a distinct positive and negative ends
• Look at the net dipole of the molecule
• A bond can be polar but the whole molecule is not
• For example ﻿$CCl_4$﻿ or ﻿$CH_4$﻿
• ﻿$MP$﻿ and ﻿$BP$﻿ (melting and boiling point)
• Bromine has a lower ﻿$BP$﻿ than Iodine. Why?
• ﻿$Br - Br$﻿ ﻿$\rightarrow$﻿ Has a weaker bonder than 1-1 bond
• Bond energy of a ﻿$Br_2$﻿ is 193 and ﻿$I_2$﻿ is 151
• Boiling point and melting point does not depend on bond energy
• It depends on the inter molecular forces
• How the molecules are linked to each other
• Water is an exception because of hydrogen bonding
• ﻿$F$﻿ being a very small atom and the non-bonding electrons repel and hence ﻿$F-F$﻿ bonds is weak
• But in other cases, ﻿$Cl_2$﻿, ﻿$Br_2$﻿ and ﻿$I_2$﻿ as the size of the atoms increases the bond also becomes weak
• In the case of ﻿$HX$﻿ as the size of the halogen atom increases the bond also becomes weaker
• When the ﻿$F$﻿ atoms share the electron, they come closer, but because they are so small each one has 8 electrons in the outermost shell (when bonded)
• Then they start to repel and the bond is weaker
• When the atoms are so small, they bond to reach the octate, but the size is so small that they repel each other

### Bond Polarity

• If ﻿$|x_a - x_b|$﻿
• ﻿$= 0$﻿ Pure covalent (2 identical non-metals)
• ﻿$>0, <1$﻿ Polar covalent (2 different non-metals)

### Molecular Polarity

• Dipole Moment
• Diatomic Molecules
• Polyatomic Molecules
• Molecular Dipole and Molecular Shape
• Molecular Dipole and Boiling Point
• When 2 atoms of different electronegativity bond the bond is polar
• The molecule is a dipole, 2 equal and opposite charges separated
• The dipole moment is a measure of the magnitude of the charge separation ﻿$(N = q*r)$﻿
• Polar = Non-Symmetrical
• When looking at ﻿$HCl$﻿ and ﻿$HBr$﻿ we predict that ﻿$HCl$﻿ is more polar than ﻿$HBr$﻿ because ﻿$Cl$﻿ is more electronegative than ﻿$Br$﻿ and has a higher dipolement