1. Evolution of Atomic Theory & Counting Atoms
Cover Chapter 3 today
Quiz on
Isotope Lab (you don’t need to wear pants)=

2. Evolution of Atomic Theory
Democritus:
1) Matter consists of very small, indivisible particles, called atoms.

3. Dalton’s Atomic Theory
Elements are made from tiny particles called atoms.
All atoms of a given element are identical.
The atoms of a given element are different to those of any other element.
Atoms of different elements combine to form compounds. A given compound always has the same relative numbers and types of atoms (Law of constant composition).
Different compounds made up of the same elements differ in the # of atoms of each kind that combine. (Law of multiple proportions)
6. Atoms cannot be created or destroyed in a chemical rxn, they are simply rearranged (law of conservation of mass).

4. J. J Thomson
Cathode Ray Experiment

5. Millikan
Oil Drop Experiment

6. Rutherford
Gold Foil Experiment

7. Subatomic Particles Protons: p+ 1) +1 charge, located in the nucleus
2) this is the atomic #
- the atomic # = number of p+
3) also known as Z
4) identifies the element (i.e. you can’t change the # of p+ without changing the ID of the element.

8. Subatomic Particles Neutrons: n0 1) neutral; located in the nucleus
2) A – Z , where A = the mass number of the element
3) Isotopes of the same element have a different # of neutrons.

9. Subatomic Particles Electrons: e-
1) -1 charge; located outside of nucleus in orbitals
2) For neutral atoms, # p+ = # e-

10. Mass # & Avg. Atomic Mass

11. Isotopes Atoms of the same element
Have the same # of ___________, but different #’s of ___________.
Isotopes have different masses, but the same atomic number
aka, “nuclide”: general term for any isotope of any element that is identified by numbers of protons and neutrons.
Hyphen
Notation
Isotope
Notation X A Z 3 1 H
Hydrogen- 3

12. Atomic Mass Units
What unit is appropriate for finding the mass of a car?
What unit is appropriate for finding the mass of a paper clip?
What unit is appropriate for finding the mass of an atom?

13. Atomic Mass Units (cont.)
Just like with all other units, there must be a standard against which to define the amu, that standard is the C-12 nuclide.
C-12 has been assigned exactly 12 amus
1 amu is exactly 1/12 the mass of one C-12 or x 10-27kg

14. Atomic mass numbers that are not whole numbers?
Example: 35.5Cl17
There are not 18.5 neutrons! Where is the .5 coming from?
There are more than one isotope of chlorine that exists in nature,35Cl17 & 37Cl17
How do I figure out the average atomic mass when considering the isotopes present in a natural sample?

15. Average Atomic Mass
Average = (% of each isotope • atomic mass of each isotope)
Atomic Mass
Example: Boron has two isotopes 10B5 and 11B5. They have the abundance 18.7 % and 81.3 % respectively
Average Atomic mass = ((10)(18.7)) + ((11)(81.3))
100
= 10.8 amu
Example 2: In a given sample of carbon there will be 98.90% of C-12, 1.10% of C-13, and a small trace of C-14, what is the average atomic mass?

16. Atoms vs. Ions Atoms are Neutral Ions are Charged atoms There are two kinds of ions
Cations
+ charged ions
# p+ > # e-
Have lost e-
Are metals X+
Anions
- charged ions
# p+ < # e-
have gained e-
Are non-metals X-

17. Mole Concept SI base unit: mole (mol) - amount of substance
The # of molecules in a mole = avogadro’s # = 6.02 x 1023
Just as 1 dozen oranges contains 12 oranges, 1 mole of hydrogen contains 6.02 x 1023 H atoms.

18. Molar Mass
This is the mass of 1 mole of units (atoms, molecules, formulas units, or particles) of a substance.
We can say that
12.01g C = 1mol C
____g H = 1 mol H

19. Putting it all together
1 mol = 6.02 x 1023 atoms, particles, formula units, or molecules
Molar mass = g/mol
Average atomic mass = amu
Example: How many moles of He atoms are in 6.46g of He?
Example 2: How many atoms are in 16.3g of S?

20. 6.02 x 10 23 atoms, molecules, formula units, or particles
Some more examples
Mass of
Element (m)
# of Moles of
Element (n)
# of atoms of
Element (N)
Molar Mass g/mol
6.02 x atoms, molecules, formula units, or particles
Calculate the # of atoms in g of potassium.
Calculate the # of grams of lead in 12.4 moles of lead.
Calculate the # of grams of H2O in 6.5 moles of water.