1. Distinguishing Between Atoms
Section 5.3

2. Atomic Number- number of protons in the nucleus of the atom
In neutral atoms, the protons equal the electrons

3. Mass Number – total number of protons and neutrons
# of Neutrons = mass # - atomic #
Written two ways:
1) 197Au nuclear symbol 79
2) Gold-197

4. How many electrons, protons, and neutrons are in each?
Beryllium-9
Sodium-23

5. Isotopes- atoms that have the same # of protons but different # of neutrons.
Can you write the complete nuclear symbols for Hydrogen-1 and Hydrogen-2?

6. Atomic Mass Actual mass of protons and neutrons are very small
Use atomic mass unit (amu) instead
- one twelfth the mass of carbon-12 atom

7. In nature, most elements occur as a mixture of two or more isotopes.
Atomic mass – is the weighted average mass of the atoms in naturally occurring sample of the element.

8. You can calculate the atomic mass based on relative abundance.
To do this you need to know:
Number of isotopes
Mass of each
Natural % abundance

9. A chemistry student’s grade is weighted
A chemistry student’s grade is weighted. Tests are worth 50%, labs are 25%, and homework is 25%. A students test average is 85.0, lab average is 77.0 and homework is 91.0.
1. What is the student’s average?

10. Sample Problem
Element X has two natural isotopes. The isotope with mass amu has an abundance of 19.91%. The isotope with mass of amu has a relative abundance of 80.09%. Calculate the atomic mass of an element.

11. Iron has four isotopes. Isotope Abundance Mass Iron-54 5.90% 53.94
Estimate the average mass.
Calculate the average atomic mass of iron.