1. Formation of chemical bonds
The role of electrons

2. Learning objectives Describe the octet rule
Predict number of valence electrons on atom
Write Lewis dot structures for atoms and ions
Predict ionic charges
Predict composition of simple ionic compounds
Describe difference between ionic and covalent bonds
Describe differences between ionic and covalent compounds
Predict when ionic or covalent compounds are formed
Describe a polar bond

3. Driving force for bonds
Chemical bonds make atoms more stable than they are if non-bonded
Bond formation involves changes in the electrons on two atoms
Electron transfer
Electron sharing

4. Noble gases provide clues

5. The Octet Rule: Happiness is a filled shell
All elements strive to become a noble gas, at least as far as the electrons are concerned.
Filling the outer shell – 8 electrons
Achieve this by adding electrons
Or taking them away

6. The valence shell Valence electrons: Core electrons:
Only the electrons in the outer shell matter
Core electrons:
Filled inner shells don’t participate
Filled valence shells means no bond formation – noble gases
Unfilled shells mean reactivity

7. Lewis dot model
The nucleus and all of the core electrons are represented by the element symbol
The valence electrons are represented by dots – one for each electron
Number of dots in Lewis model is equal to group number (in 1 – 8 system)

8. Remember this simple rule!!
Number of dots in Lewis model is equal to group number (in 1 – 8 numbering system)

9. Filled shells by gain or loss of electrons
Gain electron – outer shell full
Lose electron – inner shell full

10. Rules to predict ion charge
Cation
Charge = group number
Anion
Charge = - (8 - group number)
Practice with a few examples

11. Cation loses electrons
Ionic bonding
Transfer of electrons
Create one positive ion – cation
One negative ion – anion
Cation loses electrons
Anion gains electrons

12. Sodium chloride provides example
Na loses electron
Cl gains electron

13. Composition depends on ionic charge (group number)
Charges must balance: compounds are neutral – ions are charged
Total cation charges = total anion charges
One Ca two F-

14. A B Determining formulae Overall charge must be neutral
Metal ion first, nonmetal ion second
Coefficient of metal = charge on nonmetal
Coefficient of nonmetal = charge on metal
x and y are shown with lowest common denominator in most cases. Calcium oxide is CaO not Ca2O2 y+ x-
A B x y

15. Properties of ionic compounds
Hard, rigid solids at room temperature
High melting point
Dissolve in polar solvents (if soluble)
Solutions conduct electricity
Melts conduct electricity
Closely packed dense structures

16. The ionic model works well for metals and non-metals
What about compounds between non-metals CO, PCl3 and diatomic elements like H2, N2, O2, F2
Formation of negative ion is favourable
Formation of a positive ion will be very unfavourable (remember Lewis dot structure)

17. Covalent bonding involves electron sharing
Covalent bond is net result of attractive and repulsive electrostatic forces. Nucleus – electron attractions (blue arrows) are greater than nucleus – nucleus and electron – electron repulsions (red arrows).

18. Sharing two electrons effectively doubles the count
Each atom wants 8
Alone each has seven
Together each one has eight
14 electrons appear to become 16
Single covalent bond

19. Covalent bonds between unlike elements
Oxygen requires eight – shares two with H atoms
Hydrogen requires two – each shares one with O

20. Lewis dot structures
In going from G4 – G7, a H atom is replaced by a lone pair of electrons
The total number of electrons is equal to the sum of all the valence electrons
The total number of electrons remains the same – 8
Each atom has a complete octet

21. Multiple bonds are a feature
O2 and N2 do not achieve octets by sharing two
Must share more electrons
O2 has double bond
N2 has triple bond – one of the strongest in chemistry
N2 is very stable and unreactive – also the major product from explosives

22. Bond order increases as electron total decreases
Molecule
Group number
Total number of electrons
Bond order
F-F 7 14 1
O=O 6 12 2
N≡N 5 10 3

23. Properties of covalent compounds
Gases, liquids and solids at room temperature
May be hard or soft (diamond is a covalent solid)
Dissolve in polar and non-polar solvents, depending on molecule’s polarity
Solutions and melts do not conduct electricity
Most covalent compounds are molecular

24. What is this polarity?
The ionic bond and the equally shared covalent bond are the two extremes of chemical bonding
Ionic - complete transfer of charge
Covalent - equal sharing of charge
Many bonds are somewhere between
Atoms of different elements have different attraction for electrons

25. Enter electronegativity
The degree to which an atom attracts electrons towards itself in a bond with another atom
Highly electronegative atoms attract electrons; weakly electronegative atoms do not

26. Table of electronegativity
Most electronegative
Least electronegative

27. Polar bonds and polar molecules
Any bond containing different elements will be polar to some degree
Molecules contain several bonds
Molecular polarity depends on how the bonds are arranged
A molecule may contain polar bonds and be itself non-polar
We need to understand the molecular structure…

28. Diatomic molecules are easy

29. Beyond diatomics
More complicated molecules demand knowledge of the molecular shape
The next frontier…