1. Electronegativity + – 0 0 H Cl H H

2. The basic units: ionic vs. covalent
Ionic compounds form repeating units.
Covalent compounds form distinct molecules.
Consider adding to NaCl(s) vs. H2O(s): Cl Na Cl H O Na Cl Cl Na Na H O H O
NaCl: atoms of Cl and Na can add individually forming a compound with million of atoms.
H2O: O and H cannot add individually, instead molecules of H2O form the basic unit.

3. I’m not stealing, I’m sharing unequally
We described ionic bonds as stealing electrons
In fact, all bonds share – equally or unequally.
Note how bonding electrons spend their time: H2
HCl
LiCl H Cl
[Li]+
[ Cl ]– H + – 0 + –
covalent (non-polar)
polar covalent
ionic

4. I’m not stealing, I’m sharing unequally
Point: the bonding electrons are shared in each compound, but are not always shared equally.
The greek symbol  indicates “partial charge”.

5. Electronegativity
Electronegativity is “a number that describes the relative ability of an atom, when bonded, to attract electrons”.
The periodic table has electronegativity values.
We can determine the nature of a bond based on EN (electronegativity difference).
EN = higher EN – lower EN
Example
NBr3: EN = 3.0 – 2.8 = 0.2 (for all 3 bonds).

6. Electronegativity a EN Again, it is a grey scale not black and white
below 0.5 = covalent
= polar covalent
above 1.7 = ionic
Again, it is a grey scale not black and white
Determine the EN and bond type for these: HCl, CrO, Br2, H2O, CH4, KCl

7. Electronegativity Answers
HCl: 3.0 – 2.1 = polar covalent
CrO: 3.5 – 1.6 = 1.9 ionic
Br2: 2.8 – 2.8 = 0 covalent
H2O: 3.5 – 2.1 = 1.4 polar covalent
CH4: 2.5 – 2.1 = 0.4 covalent
KCl: 3.0 – 0.8 = 2.2 ionic

8. Homework
Read pg in textbook