1. Ch. 3 Atoms
3.3 Counting Atoms

2. Relative Atomic Mass
since masses of atoms are so small, it is more convenient to use relative atomic masses instead of real masses
to set up a scale, we have to pick one atom to be the standard
since 1961, the carbon-12 nuclide is the standard and is assigned a mass of exactly 12 amu

3. Relative Atomic Mass
atomic mass unit (amu)- one is exactly 1/12th of the mass of a carbon-12 atom
mass of proton= amu
mass of neutron= amu
mass of electron= amu

4. Relative Atomic Mass
the mass number (A) and the relative atomic mass are very close but not the same because
relative atomic mass includes electrons
the proton and neutron masses aren’t exactly 1 amu

5. Average Atomic Mass
weighted relative atomic masses of the isotopes of each element
each isotope has a known natural occurrence (percentage of that elements’ atoms)

6. Calculating Average Atomic Mass
Naturally occurring copper consists of:
69.17% copper-63 ( amu)
30.83% copper-65 ( amu)
( x )+( x )
=63.55 amu

7. Calculating Average Atomic Mass
An element has three main isotopes with the following percent occurances:
#1: amu, 90.51%
#2: amu, 0.27%
#3: amu, 9.22%
Find the average atomic mass and determine the element.

8. Calculating Average Atomic Mass
Neon

9. Calculating % Naturally Occurring Isotopes
An element Nitrogen has two main isotopes with the following atomic masses:
N-15: amu
N-14: amu
The average atomic mass for Nitrogen is amu
Find the % of each isotope as it naturally occurs.

10. Calculating Isotope %
N-14 = 99.63%
N-15 = 0.37%

11. The mole
a unit for measuring a very large amount- like number of atoms or molecules in a sample
like one dozen (1 dozen = 12 things)
except bigger: mole = 6.022x1023 things
Why 6.022x1023 ?
6.022x1023 is the number of atoms in exactly 12 g of carbon-12

13. The mole
6.022x1023 is called Avogadro’s Number in honor of all of his contributions to chemistry
can be used as a conversion factor between a number of things and mole

14. Molar Mass the mass of one mole of pure substance in grams per mole
numerically equal to average atomic mass
under the symbol on the periodic table
can be used as a conversion factor between moles and grams

15. Conversion Factors # Atoms Grams Moles Use Molar Mass:
grams per mole
Use Avog.’s Number:
atoms per mole

16. Gram  Moles use molar mass Ex. 32.3 g Na = ? mol Na
Ex mol Fe = ? g Fe

17. # Atoms  Moles use Avogadro’s Number Ex: 1.40 mol Na = ? Na atoms
Ex: 3.4x1023 atoms Fe = ? mol Fe

18. Grams  # Atoms Ex: 0.0326 g N = ? atoms of N
use both: Avogadro’s # and molar mass
Ex: g N = ? atoms of N
Ex: 2.01x1041 atoms of H = ? g H