1. Section 2: Masses of Atoms
Chapter 17: Properties of Atoms

2. Warm-Up:
Which of the following can change within an element and not change the identity of that element?
Proton
Neutron
Electron

3. Learning Goals Compute the atomic mass and mass number of an atom.
Identify the components of isotopes.
Interpret the average atomic mass of an element.

4. Atomic Mass The nucleus contains most of the mass of the atom.
Protons and neutrons are far more massive than electrons
The mass of a proton and a neutron is almost 1 atomic mass unit (amu)

5. Atomic Mass
Different elements are distinguished by the number of protons that their atoms have
Every carbon atom has six protons
Every oxygen atom has eight protons
Atomic Number: the number of protons in an atom

6. Atomic Mass
Mass Number: the sum of the number of protons and the number of neutrons in a single atom
If you know the mass number and the atomic number, you can calculate the number of neutrons
# of neutrons = mass number - atomic number

8. Atomic Mass
Example: An atom of potassium (K) has a mass number of 40. How many neutrons does it have?

9. Isotopes Not all atoms of an element have the same number of neutrons.
Isotopes: atoms of the same element that have different numbers of neutrons

10. Isotopes Suppose you have a sample of boron (B):
Naturally occurring atoms of boron have mass numbers of 10 or 11. How many neutrons are in each isotope of boron?

12. Isotopes
Because most elements have more than one isotope, each element has an average atomic mass.
Average atomic mass: the weighted average of mass of the mixture of its isotopes
The average atomic mass for each element is listed in the periodic table!

13. Isotopes
Four out of every five atoms of boron are boron-11 and one out of every five are boron-10. 11 11 11 11 10
4/5(11 amu) + 1/5(10 amu) = 10.8 amu

14. Check-In:
What is the mass number of an isotope of neon (Ne) that has 11 neutrons? What is the average atomic mass of neon?