1. Chemical Bonds & Molecular Attractions
Chapter 12

2. Student Learning Objectives
Determine the type of chemical bond and the chemical formula
Describe molecular attractions.

3. Student Learning Objectives
Determine the type of chemical bond and the chemical formula
Describe molecular attractions.

4. What is a chemical bond? How Atoms Bond
A chemical bond is an attraction between atoms.
When two substances chemically react, electrons are transferred or shared.
The chemical bond puts each atom in a lower energy state.
Water H2O
Ibuprofen C13H18O2

5. What is conserved in chemical reactions?
Mass and charge are conserved in chemical reactions.
Formula mass is the sum of atomic masses.
The relative mass of one type of atom compared to another is always the same for a particular substance. (law of definite proportions)
H2O

6. Calculating Formula Masses
Find the formula mass (FM) of lead chromate, PbCrO4 – used for yellow lines on streets
Using the Periodic Table, look up the atomic masses of Pb, Cr, and O
Pb (207.2 u), Cr (52.0 u), O (16.0 u)
Formula Mass = u u + (4 × 16 u)
FM of PbCrO4 = u
Section 12.2

7. Law of Definite Proportions
Different samples of a pure compound always contain the same elements in the same proportion by mass.
For Example:
9 g H2O = 8 g Oxygen + 1 g Hydrogen
18 g H2O = 16 g Oxygen + 2 g Hydrogen
36 g H2O = 32 g Oxygen + 4 g Hydrogen
In each case the ratio (or proportion) by mass of Oxygen to Hydrogen is 8 to 1
Section 12.2

8. Calculating Percentage by Mass of an Element
Section 12.2
H2O for example:
(the remainder)

9. Calculating Percentage by Mass for CO2
“Dry Ice” is solid CO2
AM (atomic mass) of C = 12.0 u & O = 16.0 u
FM (formula mass) of CO2 =
12.0 u + (2×16.0 u) = 44.0 u
% mass of C = (Mc/FMCO2)×100 = ???%
% mass of C = (12.0 u/44.0 u)×100 = 27.3%
Since the % mass of C = 27.3%
\ the % mass of O = 72.7%
Section 12.2

10. Calculating Percentage by Mass for Al2O3
Mineral corundum (ruby & sapphire) is Al2O3
AM (atomic mass) of Al = 27.0 u & O = 16.0 u
FM (formula mass) of Al2O3 =
(2×27.0 u) + (3×16.0 u) = u
Total mass of O = 3×16.0 = 48.0 u
Total mass of Al = 2×27.0 = 54.0 u
% mass of O = (MO/FMAl2O3)×100 = ???%
% mass of O = (48.0 u/102.0 u)×100 = 47.1%
Since the % mass of O = 47.1%
\ the % mass of Al = 52.9%
Section 12.2

11. Definite Proportions
When a compound is broken down, its elements are found in a definite proportion by mass
Also, when the same compound is formed, the elements will combine in that same proportion by mass
Section 12.2

12. Limiting & Excess Reactants
If constituent elements are not mixed in the correct proportions then
One of the elements will be used completely up and is called the limiting reactant
And one of the elements will only partially be used up and is called the excess reactant
Let’s look at an example …
The science of macaroni salad: What's in a mixture? - Josh Kurz
Section 12.2

13. Practice
Problems will be presented in class.

14. How does an electron dot structure compare to the shell model?
The Lewis electron dot symbols represent the valence electrons of the main group elements.
The electron dot structure is based on the octet rule.

15. Practice
Problems will be presented in class.

16. Outer Shell Electrons Ion
What is an ionic bond?
Ionic bonds form between positive and negative ions.
Electric force establishes bond
Strongest of all chemical bonds
Charges must balance to give a net charge of zero when atoms bond.
Outer Shell
Electrons
Ion
Mostly Full
Gained
Negative
Mostly Empty
Lost
Positive

17. Example: Na + Cl
Practice
Problems will be presented in class.

18. What is a metallic bond?
Metallic bonds form in metals as all electrons are shared among all the nuclei in the metal.
e e e
e e e e e
e e e
e e e e e
Sharing of all electrons establishes bond
Results in conductivity and malleability of metals
Weakest of all chemical bonds
Metal ions are homogeneously distributed in an alloy. Cu Cu Ni Ni

19. What is a covalent bond?
Covalent bonds are formed between atoms that share pairs of electrons in the inter-nuclear region (between the atoms).
The science of snowflakes - Maruša Bradač

20. Sharing of electron pairs establishes the covalent bond
There can be 1, 2, or 3 pairs of electrons shared.
Polar Molecules exhibit uneven sharing of electron pairs.
In general, the farther apart (left and right) two atoms are on the periodic table, the more polar the bond will be.
H – H e single bond
C  O e double bond
N  N e triple bond

21. Practice
Problems will be presented in class.

22. Molecular Attractions
Molecular attractions occur as a result of polarity.
Ion-dipole
Dipole-dipole
Dipole-induced dipole
Induced dipole-induced dipole
Molecular attractions result in surface tension.
What is the shape of a molecule? - George Zaidan and Charles Morton

23. Hydrogen bonding is a special form of a dipole-dipole molecular attraction.
H is part of the bond pair
Very uneven sharing of e−
Results in high Lf and Lv for water
A hydrogen bond is the attraction between the positively charged hydrogen and the negatively charged lone pair electrons.
Why does ice float in water? - George Zaidan and Charles Morton