Chemistry 103

Lecture 12

Outline

I. Covalent Bonding Lewis Dot Diagrams/Nomenclature (in review) Bond/Molecular Polarity

II. Counting in Chemistry (CH8) The Mole/Avogadro’s Number

Ionic vs. Covalent

NaCl (sodium chloride) CH4 (methane)

4

Learning Check

Name each of these compounds.

A. SO3

B. MnCl2

C. (NH4)3PO4

D. Cu2CO3

E. N2O4

Bonds and reality

Ionic Covalent

(e-transfer) (e- shared)

NaCl H2

Bonds and reality

Ionic uneven e- sharing covalent

(e-transfer) (e- shared)

NaCl HCl H2

Even vs. Uneven sharing of bonding electrons

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Bond Polarity

Bond Polarity: a measure of the degree of inequality in the sharing of electrons between two atoms in a chemical bond

The positive end (or pole) in a polar bond is represented + and the negative pole -. The negative end is toward the atom with the higher

electronegativity Example: HCl

H Cl

Polar v. Nonpolar Bonds

Polar covalent bonds (example: HCl) A covalent bond in which there is unequal sharing

of electrons between two atoms.

Nonpolar covalent bonds (example: H2) A covalent bond in which there is equal sharing of

electrons between two atoms

Electronegativity

A measure of the relative attraction that an atom has for the shared electrons in a bond Electrons are pulled closer to the atom with the

higher electronegativity

Cl-

Trends in Electronegativity

Electronegativity Trends in electronegativity

Electronegativity values increase from left to right on the periodic table

Electronegativity values increase from the bottom to the top of the periodic table

Range of Bond Types

Some Rules of Thumb about Bond Polarity If a bond is between two of the same kinds of atoms:

It will be NONpolar

If a bond is between two different atoms: It will be polar to some degree (more so the further apart the two

atoms are on the periodic table) An important exception: C-H bonds are nonpolar

Some Rules of Thumb about Bond Polarity

RULES: Electronegativity difference between the two

elements in a bond is 0.0 to 0.4 nonpolar covalent. (your book’s convention)

Electronegativity difference between the two elements that make a bond is 1.8 or greater, bond considered ionic

Polar Bonds?

Example: NaF

Example: O2

Example: NH3

Example: HCl

Molecular Polarity

A measure of the degree of inequality in the attraction of bonding electrons to various locations within a molecule

Polar Molecule: A molecule in which there is an unsymmetrical distribution

of charge

Why do we care if the molecule is polar? Polar molecules dissolve in polar solvents (like water or

blood) Nonpolar molecules dissolve in nonpolar solvents (like

hexane or cell membranes)

Molecular Shape and Molecular Polarity For a molecule to be polar:

1. It must contain polar bonds 2. The molecular geometry must not

cancel out the effect of the polar bonds (through vector addition)

Polar Bonds and Nonpolar Molecules

For example, the bond dipoles in CO2 cancel each other because CO2 is linear.

Polar Bonds and Polar MoleculesIn water, the molecule is not linear

and the bond dipoles do not cancel each other.Therefore, water is a polar molecule.

Chemical Quantities

Atomic Mass

Atomic mass is the Mass of a single atom

in atomic mass units (amu)

Mass of an atom compared to a 12C atom

Number below the symbol of an element

Periodic Table and Atomic Mass

Ag has atomic mass = 107.9 amu

C has atomic mass = 12.01 amu

S has atomic mass = 32.07 amu

Molecular and Formula Mass Atomic mass unit (amu) = the unit of mass for

atoms Molecular mass is the sum of the masses of

all atoms in a molecule Formula mass is the sum of the masses of

all atoms in a formula unit of an ionic compound (or molecule - used interchangeably)

Formula MassThe formula

mass is The mass in

amu of a compound

The sum of the atomic masses of the elements in a formula

Calculating Formula MassTo calculate formula mass of Na2SO4, Multiply the atomic mass of each element by its subscript Total the masses

2 Na x 22.99 amu = 45.98 amu Formula mass

1 Na Na2SO4

1 S x 32.07 amu = 32.07 amu 142.05 amu1 S

4 O x 16.00 amu = 64.00 amu1 O

Atomic masses in Grams

107.9g of Ag. How many Ag atoms?

12.01g of C. How many atoms of C?

32.07g of S. How many S atoms?

The “gram” mass of certain elements from the Periodic Table

Counting Particles by Mass

Experimentally know conversion:

1 amu = 1.66053 x 10-24 g

Let’s say we want to use the atomic mass number in the periodic table in a way useful for experimentation in the lab - grams units

Counting Particles by Mass

1 amu = 1.66053 x 10-24 g

C (ave atom) 12.01g - how many atoms of C are in this gram quantity using the conversion above?

Counting Particles by Mass

1 amu = 1.66053 x 10-24 g

C (ave atom) 12.01g - how many atoms of C are in this gram quantity using the conversion above?

12.01g C x ( 1 amu___ ) x ( 1 ave C atom) 1.66053x10-24 g 12.01 amu

= 6.022 x 1023 C atoms

The MOLE

Chemists’ counting unit

6.022 x 1023 “anything”

Avogadro’s Number

Solution = 6.022 x 1023

Avogadro’s number is equal

to 1 mole Makes working with large numbers

easier

Familiar Collection Terms

A collection term statesa specific number of

items. 1 dozen donuts

= 12 donuts 1 ream of paper

= 500 sheets 1 case = 24 cans

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Avogadro’s number 6.022 x 1023 can be written as an

equality and two conversion factors.

Equality:1 mol = 6.022 x 1023 particles

Conversion Factors:

6.022 x 1023 particles and 1 mol___ 1 mol 6.022 x 1023 particles

Avogadro’s Number

Calculations with Avogadro’s Number

A Tums tablet has 7.224 x 1023 atoms of Ca. How many moles is this?

Subscripts State Atoms and Moles

1 mole aspirin 9 mol C 8 mol H 4 mol O

Parts of the Whole

How many “C” atoms are in 125 molecules of C2H2?

How many moles of “C” atom are in 3.0 moles of C2H2?

How many individual “C” atoms are in 3.0 moles of C2H2?

Molar Mass from Periodic TableMolar mass • Is the atomic

mass expressed in grams

Molar Mass

The molar mass • Is the mass of one mole of

an element or compound• Is the atomic mass

expressed in grams

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Give the molar mass for:

A. 1 mol K atoms = 39.10 g

B. 1 mol Sn atoms = 118.7 g

Solution

Molar Mass of a Compound

The molar mass of a compound is the sum of the molar masses of the elements in the formula.

Example: Calculate the molar mass of CaCl2.

Element Number of Moles

Atomic Mass Total Mass

Ca 1 40.08 g/mol 40.08 g

Cl 2 35.45 g/mol 70.90 g

CaCl2 110.98 g

Molar Mass of K3PO4

Calculate the molar mass of K3PO4.

Element Number of Moles

Atomic Mass Total Mass in K3PO4

K 3 39.10 g/mol 117.3 g

P 1 30.97 g/mol 30.97 g

O 4 16.00 g/mol 64.00 g

K3PO4 212.3 g

Some One-Mole Quantities

32.07 g 55.85 g 58.44 g 294.20 g 342.30 g

Molar mass factors are used to convert between the grams of a substance and the number of moles.

Calculations Using Molar Mass

Grams Molar mass factor Moles

Molar mass conversion factors: • Are written from molar mass• Relate grams and moles of an element or compound.

Example: Write molar mass factors for methane CH4 used

in gas cook tops and gas heaters.

Molar mass:

1 mol CH4 = 16.04 g

Conversion factors:

16.04 g CH4 and 1 mol CH4

1 mol CH4 16.04 g CH4

Molar Mass Factors

Aluminum is often used to build lightweight bicycle frames. How many grams of Al are in 3.00 mol Al?

Moles to Grams

Aluminum is often used to build lightweight bicycle frames. How many individual Al atoms are in 3.00 mol Al?

Moles to Individual Particles

Subscripts State Atoms and Moles

1mole aspirin 9 mol C 8 mol H 4 mol O

Learning Check

Calculate the number of moles of aspirin (C9H8O4) in 52.1 g of aspirin (C9H8O4).