radioactivity nuclear chemistry. discovery of radioactivity wilhelm roentgen discovered x-rays in...

Radioactivity

Nuclear Chemistry

Discovery of RadioactivityWilhelm Roentgen discovered x-rays in

1895.Henri Becquerel discovered that

uranium salts produced similar emissions.

Marie and Pierre Curie isolated the source on the emissions in Becquerel's sample—uranium.

Marie Curie named the process of emitting rays—radiation.

Changes in the Nucleus

A change in the nucleus of an atom is called a nuclear reaction.

These reactions are associated with radioactivity – the emission of radiation in the form of rays and particles.

Nuclear reactions occur because of nuclei that are unstable.

Unstable isotopes are referred to as radioisotopes.

When is a nucleus stable?

The strong nuclear force holds the protons and neutrons together in the nucleus of an atom.

The electrostatic charge of the protons is repulsive, but the presence of the neutrons adds a stronger attractive force.

To be stable, a nucleus must have from 1 to 1.5 neutrons for each proton. Atoms with more protons need the higher number of neutrons.

TransmutationThe changes in the nucleus can sometimes

transform an element into a different element The result = change in atomic number

An element is considered radioactive because it releases

Energyduring transmutation

Artificial TransmutationChange in nucleus occurs when

bombarded by an alpha particle or neutron

Natural TransmutationChange in nucleus occurs spontaneouslyThe ONLY reactant is the atom undergoing

transmutation

Ca-44 + n Ca-4510

U-238 Th-234 +

Stability of NucleiMost nuclei are stable ex. C-12As atoms increase in size the ratio of

neutrons to protons increases the larger the ratio the more unstable the atom ex. C-14

All nuclei with atomic numbers > 83 are unstable and radioactive

Unstable nuclei will spontaneously decay in order to form a

stable nucleus

Types of RadiationName Symbol Mass

(amu)Charge Description

Alpha 4 2+ a helium nucleus

Beta 1/1840 1- electrons

Gamma 0 0 short wavelength electromagnetic waves

Radioactive Shielding

Alpha Particles

or Hehelium nucleus

HeaviestTravels slowest & least amount of distance

Poor penetrating power MOST DAMAGING

42

Alpha DecayA type of radioactive decay where

an atomic nucleus emits (releases) an alpha particle (2 p+ and 2 no).

The atom reduces its atomic number by 2 and its mass number by 4. This transforms the atom into a different element.

238 92U

234 90Th

4 2He

Alpha Decay

U238 92

He

Th

42

234 90

Alpha Particle is one productmass and charge

must be conserved

Beta Particles

ß-

Electrons

travel near the speed of lightmoderate penetrating power

can be stopped by aluminum

e 0

-1

Beta DecayA type of radioactive decay where

a beta particle (electron or positron) is emitted.

This causes a neutron to be turned into a proton.

Beta Decay

Gamma Radiation

No mass no charge

similar to x-rays

Fastest type of radiationStrongest penetrating powernot very dangerous

Where did they come from?Uranium DecayAtoms of U-238 go through a series of

decays to make Thorium Th Radon Rn Radium Ra Lead Pb

Example of natural decay

ß ß

ß ß

ß ß

Radioactive Decay RatesOver time, radioisotopes will decay

into stable atoms. The time this takes is measured in half-lives.

A half-life is the time it takes one-half of radioisotope to turn into stable atoms.

Radioactive Decay Rates (cont’d)

Example: 20 g of the strontium-90 will decay into 10 g over 29 years.

Amount remaining = (Initial amount)(½)t/T

t = total time T = half-life

0 1 2 3 4Number of half-lives

Rad

iois

otop

e re

mai

ning

(%

)

100

50

25

12.5

Initial amountof radioisotope

t1/2

t1/2

t1/2

After 1 half-life

After 2 half-lives

After 3 half-lives

Half-life of Radiation