1. Radioactivity
Chapter 25

2. Nuclear Radiation Nuclear chemistry
study of the structure of atomic nuclei
changes they undergo.

3. The Discovery of Radioactivity
Wilhelm Roentgen (1845–1923)
1895-invisible rays were emitted when electrons bombarded the surface of certain materials.
caused photographic plates to darken.
named the invisible high-energy emissions X rays.

4. The Discovery of Radioactivity
Henri Becquerel (1852–1908) was studying phosphorescence
minerals that emit light after being exposed to sunlight
phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates.

5. The Discovery of Radioactivity
Marie Curie (1867–1934) and her husband Pierre (1859–1906) took Becquerel’s mineral sample (called pitchblende) and isolated the components emitting the rays.
darkening of the photographic plates was due to rays emitted specifically from the uranium atoms present in the mineral sample.

6. The Discovery of Radioactivity
Marie Curie named the process by which materials give off such rays radioactivity
the rays and particles emitted by a radioactive source are called radiation.

7. Types of Radiation
isotopes are atoms of the same element that have different numbers of neutrons.
Isotopes of atoms with unstable nuclei are called radioisotopes
emit radiation to attain more stable atomic configurations in a process called radioactive decay
lose energy by emitting one of several types of radiation.

8. Why do some atoms decay?
The nucleus contains tightly packed protons and neutrons (nucleons)
The strong nuclear force keeps the nucleons packed together even though protons want to push each other away
Stable atoms have a neutron to proton ratio of about 1:1

9. As atomic number increases, more neutrons are required to have enough of a strong force to keep the protons pushed together
The neutron to proton ratio for stable atoms increases to 1.5:1

10. Band of Stability
When the number of protons and neutrons are plotted, the stable nuclei are found within the “band of stability”
Radioactive isotopes are outside the band of stability
They will undergo nuclear reactions to become more stable
All elements higher than atomic# 83 are radioactive

12. Example 1 Calculate the neutron-to-proton ratio for . Topic 26
Basic Assessment Questions
Topic 26
Example 1
Calculate the neutron-to-proton ratio for

13. Basic Assessment Questions
Topic 26
Answer
1.6 : 1

14. Types of Nuclear Radiation
Alpha
Beta
Gamma

15. Alpha Radiation Release of 2 protons and 2 neutrons
Equivalent to a He nucleus
Charge of 2+
Mass = 4 amu
Largest and slowest
Least penetrating  can be stopped by paper
Changes to a different element with a lower atomic mass and lower atomic number
Example: Polonium-212 (atomic# 84) is converted to Lead-208 (atomic# 82)

17. Beta Radiation Decay of a neutron into a proton and electron
Electron is emitted, proton stays
Forms a new element b/c of addition of proton
Decay of the proton into a neutron and positron (like a positive electron)
The positron is emitted as a beta particle
Faster than alpha particles  can be stopped by aluminum foil

19. Gamma Radiation Not a particle
Electromagnetic wave with short wavelength and high frequency & energy
No mass, no charge
Very fast  speed of light
Stronger than X-ray
Stopped by several centimeters of lead

22. Transmutation: changing one element into another through radioactive decay
Adding or removing a proton changes the atomic number, resulting in a different element
Half-Life: amount of time for half of a sample of a radioactive element to decay into something else
Can range from a fraction of a second to billions of years
Amount remaining=initial amount(1/2)t/T
t=total time
T=half-life

24. Half-life
mf: final mass
mi: initial mass
n: # of half-lives

25. Half-life
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?