Chapter 25 – Nuclear Changes

25.1 - Radioactivity

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.

Nucleus•Composed of protons and

neutrons which account for most of the atom’s mass

Strong force•Causes protons and

neutrons to be attracted to each other in the nucleus

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

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

Isotopes•Atoms of the same element

with varying number of neutrons

•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

Symbol Atomic Number

Number of protons

Mass Number

Number of Neutrons

1H1

1 1 1 0

235U92

92 92 235 235-92 =

143

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 1898Video Clip

In-Class Assignment/Homework

• 25.1 Reinforcement WKT

• Tomorrow – – 25.1 Enrichment WKT– 25.1 & 25.2 Directed Reading WKT