Alpha and Beta Decay

Nuclear Reactions1. Occur when nuclei emit particles and/or rays.2. Atoms are often converted into atoms of another element.3. May involve protons, neutrons, and electrons4. Associated with large energy changes.5. Reaction rate is not normally affected by temperature, pressure,

or catalysts.

PROPERTIES OF RADIATION1. Alpha ()

42He, helium nuclei

Blocked by paper; 6.64 x 10-24 kg Slow moving due to mass and charge!

2. Beta () 0

-1 or 0-1e, electrons

Blocked by metal foil; 9.11 x 10-28 kg Fast moving Emitted from a neutron of an unstable nucleus Insignificant mass compared with mass of nucleus Greater penetrating power than alpha particles

3. Gamma () 0

0 , photons Not completely blocked by lead or concrete; 0 kg High energy electromagnetic radiation Almost always accompanies alpha and beta radiation

Comparison of Chemical and Nuclear Reactions

Chemical Reactions Nuclear Reactions

Occur when bonds are broken or formed

Occur when nuclei combine, split, & emit radiation

Involve only valence electrons

Can involve protons, neutrons, & electrons

Associated with small energy changes

Associated with large energy changes

Atoms keeps same identity although they may gain, lose, or share electrons, and form new substances

Atoms of one element are often converted into atoms of another element

Temperature, pressure, concentration, and catalysts affect reaction rates

Temperature, pressure, and catalysts do not normally affect reaction rates

Radioactivity• Radioisotopes are isotopes that have an unstable

nucleus. They emit radiation to attain more stable atomic configurations in a process called radioactive decay.

• Radioactivity is the property by which an atomic nucleus gives off alpha, beta, or gamma radiation.

• Marie Curie named the process.• In 1898, Marie & Pierre Curie identified 2 new elements,

polonium & radium.• The penetrating rays and particles emitted by a

radioactive source are called radiation.

Radioactivity (cont)• The presence of too many or too few neutrons, relative

to the number of protons, leads to an unstable nucleus.• The types of radiation are alpha (α), beta (β), or gamma

(γ).• An unstable nucleus loses energy by emitting radiation

during the process of radioactive decay. • Spontaneous and does not require any input of energy.

The Nucleus

• Remember that the nucleus is comprised of protons and neutrons.

• The number of protons is the atomic number.• The number of protons and neutrons together is

the mass of the atom.

Isotopes• Not all atoms of the same element have the same

mass due to different numbers of neutrons in those atoms.

• There are three naturally occurring isotopes of uranium:

• Uranium-234• Uranium-235• Uranium-238

Stable Nuclei•The shaded region in the fig. shows what nuclides would be stable, the so-called belt of stability.•It is the ratio of neutrons to protons that determines the stability of a given nucleus.

Predicting the mode of radioactive decay.

In general:

• neutron-rich nuclei tend to emit beta particles

• proton-rich nuclei tend to either emit positrons or undergo electron capture

• heavy nuclei tend to emit alpha particles.

•The presence of magic numbers of nucleons and an even number of protons and neutrons also help determine the stability of a nucleus.

Radioactive Series• Large radioactive nuclei

cannot stabilize by undergoing only one nuclear transformation.

• They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

• Transmutation = the reaction by which the atomic nucleus of one element is changed into the nucleus of a different element

Nuclear Equations• For a nuclear reaction to be balanced, the sum of all the

atomic numbers and mass numbers on the right must equal the sum of those numbers on the left.

• To figure out the unknown isotope, you need to balance the equation.

Example

Natural Radioactive Decay• Why

• The nucleus has many positively charged protons that are repelling each other.

• The forces that hold the nucleus together can’t do its job and the nucleus breaks apart.

• All elements with 84 or more protons are unstable and will eventually undergo nuclear decay.

• How• Alpha particle emission• Beta particle emission• Gamma radiation emission• Positron emission (less common)• Electron capture (less common)

Alpha radiation• A type of radiation called alpha radiation consists of

helium nuclei that have been emitted from a radioactive source.

• These emitted particles, called alpha particles, contain 2 protons and 2 neutrons and have a double positive charge.

Alpha Radiation (cont)• Because of their large mass and charge, alpha particles

do not tend to travel very far and are not very penetrating.

• They are easily stopped by a piece of paper or the surface of skin.

• Radioisotopes that emit alpha particles are dangerous when ingested.

Alpha Decay= Loss of an -particle (a helium nucleus)

He42

U23892 Th234

90 He42+

Atomic # decreases by 2Mass # decreases by 4# of protons decreases by 2# of neutrons decreases by 2

Alpha radiation occurs when an unstable nucleus emits a particle composed of 2 protons and 2 neutrons. The atom giving up the alpha particle has its atomic number reduced by two. Of course, this results in the atom becoming a different element. For example, Rn undergoes alpha decay to Po.

Beta Particles• A beta particle is essentially an electron that’s emitted

from the nucleus.• A neutron is converted (decayed) into a proton &

electron…so the atomic number increases by 1 and the electron leaves the nucleus.

• Isotopes with a high neutron/proton ratio often undergo beta emission, because this decay allows the # of neutrons to be decreased by one & the # of protons to be increased by one, thus lowering the neutron/proton ratio.

Beta Decay= Loss of a -particle (a high energy electron)

0

−1 e0−1or

I13153 Xe131

54 + e0−1

Atomic # increases by 1# of protons increases by 1# of neutrons decreases by 1Mass # remains the same

Beta radiation occurs when an unstable nucleus emits an electron. As the emission occurs, a neutron turns into a proton.