Ch. 25 Nuclear Chemistry

Reactions involving the nucleus of the atom.

Radiation• Radiation: penetrating rays and particles

emitted by a radioactive source.• Radioisotopes: radioactive isotopes• Why is an atom radioactive?

– The nucleus is unstable. Wrong ratio of p:n– Nuclear forces v. Electromagnetic Repulsion

Nuclear Stability Graph

Types of Radiation

• Alpha: Helium nucleus emitted from nucleus. Results in less p and n

• Beta: High speed electron ejected from the nucleus– n → p + e-

• Gamma: high energy photon

Summary of Radiation

How far will it go?

Alpha Decay

Beta Decay

Gamma Radiation

Gamma Ray on Electromagnetic Spectrum

25.2

Half life

Nuclear Stability Graph

Red band=1:1 n:p ratio

Blue band: stable n:p ratio for isotopes

Falling outside this band =unstable

• The n:p ratio determines the type of decay that will occur

• Too many neutrons: beta emission (n→p+e-)

• Too few neutrons: positron emission (p→n)

• Too many p and n: alpha emission

Examples

Half-life

• A half-life (t1/2) is the time required for one-half of the nuclei of a radioactive sample to decay to products

• After each half-life, half of the existing radioactive atoms have decayed into atoms of a new product

• Half-lives can vary in length depending on the isotope. Some last nanoseconds, some last billions of years

• Decay of U-238:

• What determines the type of decay a radioactive isotope will undergo?

• A radioactive sample has a half life of 4 day. How much of a 20 g sample will be left after 4 days? 8 days? 20 days?

• A radioactive sample has a half life of 5 hours. How much of a 150 g sample will be left after 25 hours?

Ch. 25.3

Fission v. Fusion

● Transmutation: the conversion of an atom of one element to an atom of another element○ Can occur by radioactive decay, or

when particles bombard the nucleus of an atom

● Elements in the periodic table with atomic numbers above 92 are called transuranium elements. ○ These all undergo transmutation○ All are man-made (not found naturally)

Fission vs. Fusion

• Splitting of large nuclei into smaller

• Releases lots of energy (106 times a chem rxn)(E=mc2)

• Occurs in nuclear power plants

• Atomic bomb

• Combining smaller nuclei into larger ones

• Releases even more energy than fission (3-4 times greater)

• Occurs in sun/stars• H-bomb,

thermonuclear bomb

Fission reaction

Fusion

Nuclear power plant

Nuclear power plants are sites of controlled nuclear chain reactions!

Nuclear Power Plant

Nuclear Power Plant

• Control Rods: Cadmium (Cd): absorb n0

• Moderator: water or graphite: slow n0

• Fuel: U-235

•Nuclear Waste – Spent rods are stored in water tanks to cool and protect from radiation for many years.

Detection

• Film badge: photographic paper that gets exposed. Used only to monitor, not detect

• Geiger Counter: gas filled tube to detect radiation through audible clicks

• Scintillation Counter: phosphor-coated surface that produces flashes of light when exposed to radiation

Uses of Radiation

• Smoke detectors: Americium-241• Food irradiation: Cobalt-60 (gamma)• Radioactive tracers to:

– Study processes in plants (P-32)– Diagnose medical conditions

• Thyroid disorders: Iodine-131• Bone scans: Te-99

• Medical treatment: cancer