Ch 21: Nuclear Chemistry
Section 21.1 - Radioactivity
Radioactivity• Wilhelm Roentgen made a big discovery in
1895. He found that invisible rays were emitted when electrons bombarded materials.
• He named these rays, X-rays.• At the same time, Henri Becquerel was
studying minerals that emitted light after being exposed to sunlight, a phenomenon called phosphorescence.
Radioactivity• Marie Curie and her husband Pierre were
working with Becquerel and took his mineral sample and were able to isolate the components emitting the rays.
• Marie named the process by which materials give off such rays Radioactivity.
• Radiation: the penetrating rays and particles emitted by a radioactive source.
Radioactivity
• Marie Curie was the first woman to win the Nobel Prize and is the only person to receive Nobel Prizes in two different sciences--physics and chemistry!
Nuclear Reactions vs. Normal Chemical Changes
• Marie Curie discovered that: Chemical Reactions were affected by Pressure and Temperature, while Nuclear Reactions are not.
• Nuclear Reactions involve the nucleus, which changes the type of element.
• Chemical reactions involve electrons, not protons and neutrons.
• When a substance emits radiation, it changes its identity.
• A radioactive element has an unstable nucleus.
• Isotopes are atoms of the same element that have different numbers of neutrons and mass number.
• Radioisotopes: isotopes of atoms with unstable nuclei
Nuclear Stability and Decay• Nuclear Force: the attractive force that acts
between all nuclear particles that are extremely close together, such as neutrons and protons in a nucleus.
• Band of Stability: the location of stable nuclei on a neutron vs. proton plot.
Types of Radiation• The three types of nuclear radiation are
alpha radiation, beta radiation, and gamma radiation.
• There is also positron emission • They can be separated by an electric field, as
shown below.
Alpha Radiation• Alpha Particle: a positively charged
helium isotope. • Written with the symbol: • It contains 2 protons and 2 neutrons
and has a 2+ charge
He42
21084 Po Pb82
206 + 42 He
Beta Radiation•
146 C N7
14 +0-1e
Positron Emission•
15 8 O N7
15 +0+1e
Gamma Radiation• Gamma Rays: high energy
electromagnetic radiation. • The emission of gamma rays does not
change the atomic number or mass number of a nucleus.
• Used to destroy tumors.
21084 Po Po84
210 + 00 γ
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Section 21.2 – Penetrating Abilities and Half-Life
Penetrating Abilities• Alpha: stopped by piece of paper.• Beta: stopped by thin metal• Gamma: stopped by thick lead and
concrete.
Half-Life• Half-life: the time required for one-half of
the nuclei of a radioisotope sample to decay to products.
• After each half-life, half of the existing radioactive atoms have decayed into atoms of a new element.
Half-Life Calculation• Carbon-14 emits beta radiation and decays
with a half-life of 5730 years. Assume you start with a mass of 2040 g of Carbon-14.
a. How long is three half-lives?3 x 5730 years = 17,190 years
b. How many grams of the isotope remain at the end of three half-lives?
2040 2 2 2 = 240 gramsOr
2040 23 = 240 grams
Half-Life Calculation• The half-life of Zn-71 is 2.4 minutes. If one
had 100.0 g at the beginning, how many grams would be left after 9.6 mins elapses?
a. Figure out how many half-lives went by.9.6 2.4 = 4 half-lives
b. Then divide beginning amount by 2 to the number of half-lives.
100 24 = 6.25 grams*Don’t worry about significant figures, just write
whatever you get on your calculator.
Half-Life Calculation• Os-182 has a half-life of 21.5 hours. How
many grams of a 10.0 g sample would have decayed after exactly two half-lives?
a. Half-life calculations tell us how much remains, not how much is gone. Solve for remains first, and subtract that from the initial amount.
10.0 22 = 2.5 grams remains10.0 - 2.5 = 7.5 g decayed
