1. Section 24.2 Radioactive Decay
Explain why certain nuclei are radioactive.
radioactivity: the process by which some substances spontaneously emit radiation
Apply your knowledge of radioactive decay to write balanced nuclear equations.
Solve problems involving radioactive decay rates.
Section 24-2

2. Section 24.2 Radioactive Decay (cont.)
transmutation
nucleon
strong nuclear force
band of stability
positron emission
positron
electron capture
radioactive decay series
half-life
radiochemical dating
Unstable nuclei can break apart spontaneously, changing the identity of atoms.
Section 24-2

3. Protons and neutrons are referred to as nucleons.
Nuclear Stability
Except for gamma radiation, radioactive decay involves transmutation, or the conversion of an element into another element.
Protons and neutrons are referred to as nucleons.
All nucleons remain in the dense nucleus because of the strong nuclear force.
Section 24-2

4. Nuclear Stability (cont.)
The strong nuclear force acts on subatomic particles that are extremely close together and overcomes the electrostatic repulsion among protons.
Section 24-2

5. Nuclear Stability (cont.)
As atomic number increases, more and more neutrons are needed to produce a strong nuclear force that is sufficient to balance the electrostatic repulsion between protons.
Neutron to proton ratio increases gradually to about 1.5:1.
Section 24-2

6. Nuclear Stability (cont.)
The area on the graph within which all stable nuclei are found is known as the band of stability.
All radioactive nuclei are found outside the band.
The band ends at Pb-208; all elements with atomic numbers greater than 82 are radioactive.
Section 24-2

7. Types of Radioactive Decay
Atoms can undergo different types of decay—beta decay, alpha decay, positron emission, or electron captures—to gain stability.
Section 24-2

8. Types of Radioactive Decay (cont.)
In beta decay, radioisotopes above the band of stability have too many neutrons to be stable.
Beta decay decreases the number of neutrons in the nucleus by converting one to a proton and emitting a beta particle.
Section 24-2

9. Types of Radioactive Decay (cont.)
In alpha decay, nuclei with more than 82 protons are radioactive and decay spontaneously.
Both neutrons and protons must be reduced.
Emitting alpha particles reduces both neutrons and protons.
Section 24-2

10. Types of Radioactive Decay (cont.)
Section 24-2

11. 1+ Positron (+) Types of Radioactive Decay (cont.)
Nuclei with low neutron to proton ratios have two common decay processes.
Positron emission is a radioactive decay process that involves the emission of a positron from the nucleus.
A positron is a particle with the same mass as an electron but opposite charge.
Positron (+)
positron
foil 1+
Section 24-2

12. Types of Radioactive Decay (cont.)
During positron emission, a proton in the nucleus is converted to a neutron and a positron, and the positron is then emitted.
Electron capture occurs when the nucleus of an atom draws in a surrounding electron and combines with a proton to form a neutron.
Section 24-2

13. Types of Radioactive Decay (cont.)
Section 24-2

14. Types of Radioactive Decay (cont.)
Section 24-2

15. Numbers must balance!! Alpha Emission parent nuclide daughter nuclide
Writing and Balancing Nuclear Equations
Nuclear reactions are expressed by balanced nuclear equations.
In balanced nuclear equations, mass numbers and charges are conserved.
Alpha Emission
parent
nuclide
daughter
nuclide
alpha
particle
Numbers must balance!!
Section 24-2

16. Beta Emission
electron
Positron Emission
positron

17. Electron Capture
electron

18. Radioactive Series
A series of nuclear reactions that begins with an unstable nucleus and results in the formation of a stable nucleus is called a radioactive decay series.
Section 24-2

19. Radioactive Decay Rates
Radioactive decay rates are measured in half-lives.
A half-life is the time required for one-half of a radioisotope to decay into its products.
N is the remaining amount.
N0 is the initial amount.
n is the number of half-lives that have passed.
t is the elapsed time and T is the duration of the half-life.
Section 24-2

20. mf = mi(1/2)n
Mf = final mass
Mi = initial mass
N = # of half-lives

21. Fluorine-21 has a half-life of 5. 0 seconds
Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s?

22. Radioactive Decay Rates (cont.)
Section 24-2

23. Radioactive Decay Rates (cont.)
Section 24-2

24. Radioactive Decay Rates (cont.)
The process of determining the age of an object by measuring the amount of certain isotopes is called radiochemical dating.
Carbon-dating is used to measure the age of artifacts that were once part of a living organism.
Section 24-2

25. An element has a half-life of 4 minutes
An element has a half-life of 4 minutes. If you start with 100 grams of the element, how much will you have after 20 minutes?

26. A B C D Section 24.2 Assessment
The process of converting one element into another by radioactive decay is called ____.
A. half-life
B. nuclear conversion
C. transmutation
D. trans-decay A B C D
Section 24-2

27. A B C D Section 24.2 Assessment
An unknown element has a half-life of 40 years. How much of a 20.0g sample will be left after 120 years?
A. 0.00g
B. 2.50g
C. 5.00g
D. 7.50g A B C D
Section 24-2