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.

RadioactivityMarie, 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

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24 He

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.

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01n → 1

1p + -10e

--Written as or simply

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-10e

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?

1) Too many n0/too few p+

2) Too few n0/too many p+

3) 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

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24 He

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-10e

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+10e

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01n

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12H

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11H

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.

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92233U →

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24 He

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+ 90229

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Th

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.

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77192Ir →

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-10e

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+ 78192

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Pt

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

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57137La

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+ -10e →

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56137

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Ba

Other Possibilities

• Positron emission: “To emit” something means “to give off.” Here, a mother gives off a positron ( ) and a daughter.

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+10e

Show the positron emission reaction of zinc-65:

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+10e

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3065 Zn →

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+ 2965

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Cu

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.

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87222Fr → -1

0e + 88222Ra

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88222Ra → 2

4He + 86218Rn

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86218Rn → 2

4He + 84214Po

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84214 Po → 2

4He + 82210Pb

Types of nuclear reactions

1. Natural Decay: An atom decays, giving off a beta, alpha, or positron.

2. 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-lifeA substance with t1/2 = 22 days

Time: 0 days 22 days 44 days 66 days

# 1/2-lives

0 1 2 3

Mass:Sample 1

1000 kg 500 kg 250 kg 125 kg

Mass: 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-lifeThings 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.

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amount remaining = 1

2

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⎝ ⎜

⎞

⎠ ⎟# half -lives