Ch. 24--Nuclear Chemistry “It’s all about the nucleus!”

Radioactivity Becquerel (1896)--Found that some of his film was exposed inside its envelope after uranium salts were placed near it. Something penetrated the paper that could expose the film!

Radioactivity Marie and Pierre Curie, students of Becquerel, discovered that invisible rays given off by the Uranium were responsible. Marie called the process of giving off these rays radioactivity.

Radioactivity Marie, one of science’s most famous female scientists, went on to “discover” two radioactive elements: Polonium (named for her native Poland) and radium. She is the only person to win Nobel Prizes in two different sciences.

Types of Radiation Three types: alpha, beta, & gamma

Types of Radiation Alpha (
) radiation consists of helium nuclei, emitted by a radioactive source. Alpha particles have 2 protons, 2 neutrons, and thus a +2 charge. Written as or simply


Types of Radiation Beta (
) radiation --consists of fast electrons, made as neutrons decompose: --Beta particles are simply electrons, they have a mass number of zero, and are given an atomic number of -1. --Written as or simply


Types of Radiation Gamma (
) radiation Is energy that is released, there is no particle. – Gamma rays are emitted at the same time as alpha or beta, or sometimes just as a energy release as an excited nucleus comes to its ground state:

Reasons for radioactivity Some isotopes are radioactive; we call these radioisotopes. Radioisotopes have unstable nuclei. This lack of stability leads to a “shuffling” of the nucleus in search of a more stable nucleus.

Why is nucleus unstable? Too many n0/too few p+ Too few n0/too many p+ Nucleus too heavy! (too many n0 and p+)

Decay Reactions An unstable isotope’s nucleus breaks apart, giving off a small particle and forming a new larger particle. Original isotope = Mother New isotope = Daughter

Particles  Alpha
 or Proton Beta  or Positron Not a particle: neutron Gamma  Deutron

Writing Nuclear Equations Like any equation, both sides must be equal! The arrow is our equal sign The mass numbers on each side must be equal The atomic numbers on each side must be equal

Writing nuclear equations If you’re told the type of reaction it is supposed to be, that tells you what particles are to be involved!

Alpha Decay In an alpha decay, the mother breaks into two particles: an alpha particle, and the daughter. Write the mother on the left, draw the arrow, write the alpha particle formula on the right. Balance, and use P. Table to ID the new daughter.

Alpha Decay Write the equation showing the alpha decay of a uranium-233 isotope.

Beta Decay In an beta decay, the mother breaks into two particles: a beta particle, and the daughter. Write the mother on the left, draw the arrow, write the beta particle formula on the right. Balance, and use P. Table to ID the new daughter.

Beta Decay Write the equation showing the beta decay of a iridium-192 isotope.

Other Possibilities “Electron capture” This reaction has an electron () as a reactant. The isotope on the left is gaining the electron instead of shedding it. Show electron capture for La-137

Other Possibilities Positron emission: “To emit” something means “to give off.” Here, a mother gives off a positron ( ) and a daughter. Show the positron emission reaction of zinc-65:

Summing Up If we know our vocab, it’s not too bad! “Decay” or “emission” reactions will have one reactant, two products. “Capture” reactions will have two reactants, one product.

Series reactions In order to become stable, a radioisotope will often have to undergo a number of successive decays.

Series Reactions Show how Fr-222 undergoes successive beta, alpha, alpha, and alpha decays.

Types of nuclear reactions Natural Decay: An atom decays, giving off a beta, alpha, or positron. Induced decay: An atom’s nucleus is “bombarded” with a proton, neutron, or alpha particle, which destabilizes the nucleus and causes a nuclear reaction.

Types of nuclear reactions 3. Electron capture: An electron converts a proton to a neutron 4. Fission Reaction: A large nucleus cleaves into two smaller (yet larger than alpha!) nuclei 5. Fusion: Two smaller nuclei combine to form a larger nucleus.

Half-life The half-life of a radioisotope is the amount of time it takes for 1/2 of a sample to decay. This amount of time is unique for each radioisotope. The time does not change based on the amount of material present!

Half-life A substance with t1/2 = 22 days Time: 0 days 22 days 44 days 66 days # 1/2-lives 1 2 3 Mass: Sample 1 1000 kg 500 kg 250 kg 125 kg Sample 2 5 mg 2.5 mg 1.25 mg 0.625 mg % orig. sample 100% (1) 50% (1/2) 25% (1/4) 12.5% (1/8)

Half-life Things not to forget when working half-life problems: • The amount of time for a half-life never changes; the amount of mass that is decayed in that time does change! • The initial time is always “0.” Zero time has passed, zero half-lifes have passed. •The fraction of the original isotope remaining can be figured out several ways. is one.

Half-life Another way to do half-life problems is to make a 3-column chart, that features number of half-lives, the amount of time that’s passed, and the amount of sample that remains. Let’s consider problem 14 from our HW: Actinium-226 has a half-life of 29 hours. If 100 mg of actinium-226 disintegrates over a period of 58 hours, how many mg of actinium-226 will remain?

# of half-lives Amt of time Amt of 226Ac 1000 mg

# of half-lives Amt of time Amt of 226Ac 1000 mg 1 29 hrs 500 mg

# of half-lives Amt of time Amt of 226Ac 1000 mg 1 29 hrs 500 mg 2 58 hrs 250 mg

We could also solve this problem by recognizing that 2 half lives go by in the 58 hours ( ) and either using our half-life curve to see that 25% of our original sample is left, or by plugging into the equation