RadioactivityChapter 25

• Nuclear chemistry

• study of the structure of atomic nuclei

• changes they undergo.

Nuclear Radiation

• Wilhelm Roentgen (1845–1923)

• 1895-invisible rays were emitted when electrons bombarded the surface of certain materials.

The Discovery of Radioactivity

• caused photographic plates to darken.

• named the invisible high-energy emissions X rays.

• Henri Becquerel (1852–1908) was studying phosphorescence

• minerals that emit light after being exposed to sunlight

The Discovery of Radioactivity

•phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates.

• Marie Curie (1867–1934) and her husband Pierre (1859–1906) took Becquerel’s mineral sample (called pitchblende) and isolated the components emitting the rays.

The Discovery of Radioactivity

• darkening of the photographic plates was due to rays emitted specifically from the uranium atoms present in the mineral sample.

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.

• isotopes are atoms of the same element that have different numbers of neutrons.

Types of Radiation

• 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.

Why do some atoms decay?

The nucleus contains tightly packed protons and neutrons (nucleons)(nucleons)

The strong nuclear forcestrong nuclear force keeps the nucleons packed together even though protons want to push each other away

Stable atoms have a neutron to neutron to proton ratio of about 1:1proton ratio of about 1:1

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

Band of Stability When the number of

protons and neutrons are plotted, the stable nuclei are found within the “band of stabilityband 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

Basic Assessment QuestionsBasic Assessment Questions

Example 1Topic 26Topic 26

Calculate the neutron-to-proton ratio for .

Basic Assessment QuestionsBasic Assessment Questions

Answer

1.6 : 1

Topic 26Topic 26

Types of Nuclear Radiation

AlphaBetaGamma

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)

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

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

TransmutationTransmutation: changing one element into another through radioactive decay Adding or removing a proton changes the atomic number,

resulting in a different element

Half-Life: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

Half-life

nif mm )( 2

1

mf: final massmi: initial massn: # of half-lives

Half-life Fluorine-21 has a half-life of 5.0 seconds. If you Fluorine-21 has a half-life of 5.0 seconds. If you

start with 25 g of fluorine-21, how many grams start with 25 g of fluorine-21, how many grams would remain after 60.0 s?would remain after 60.0 s?