1. Chapter 25 – Nuclear Changes
Radioactivity

2. Objectives Describe the structure of an atom and its nucleus.
Explain what radioactivity is.
Compare and contrast properties of radioactive and stable nuclei.
Discuss the discovery of radioactivity.

3. Nucleus
Composed of protons and neutrons which account for most of the atom’s mass

4. Strong force
Causes protons and neutrons to be attracted to each other in the nucleus

5. Video clip
The strong force is powerful only when neutrons and protons are close to each other
The protons and neutrons in a large nucleus are held less tightly by the strong force than those in a small nucleus

6. Radioactivity
The nuclear decay that occurs when the strong force is not large enough to hold the nucleus together and the nucleus gives off matter and energy

7. Isotopes
Atoms of the same element with varying number of neutrons

8. A nucleus with too many or too few neutrons compared to protons is radioactive
Atomic number = number of protons in an atom
Mass number = number of protons and neutrons in a nucleus
If an element has a neutron to proton ratio of 1.2 or higher than the element is radioactive

9. 143 Symbol Atomic Number Number of protons Mass Number 1H 1 235U 92
Number of Neutrons 1H 1
235U 92
235 =
143

10. Element Radioactivity
Uranium was discovered in 1896 by Henri Becquerel as a radioactive element
Polonium and Radium were discovered as radioactivity by Marie and Pierre Curie in 1898
Video Clip

11. In-Class Assignment/Homework
25.1 Reinforcement WKT
Tomorrow –
25.1 Enrichment WKT
25.1 & 25.2 Directed Reading WKT