Chemistry - Radioactivity

who discovered radioactivity?

Becquerel:

• left some uranium on photographic plate covered w/ black paper.

• area of photographic plate near uranium became darker even though it had not been exposed to sunlight

• uranium was emitting some sort of radioactivity here

radioactivity

the spontaneous disintegration of the nucleus with the emission of alpha, beta or gamma radiation (α,β or γ)

i.e. there is no way of knowing when an alpha particle or beta or gamma particle will be emitted

(you don’t get all 3 particles being emitted)

alpha particles (α)

charge - positive particles

structure - consists of two protons and two neutrons (hence same as helium nucleus)

penetrability - not very penetrating, but elements that emit alpha particles are strongly ionising and very dangerous if ingested (causes mutations in cells)

alpha emmitter

elements that emit alpha particles

beta particles (β)

charge - negatively charged particles

structure - high-speed electrons

penetrability - moderately penetrating (more that alpha particles) as they are lighter

properties of beta particles

• mass of 1/1840 a.m.u

• Formed when there are too many neutrons in the nucleus to be stable. A neutron changes into a proton and an electron. The electron is ejected from the nucleus, as it would require too much energy to stay in the nucleus. The ejected electron is the beta particle. Hence the element changes completely

• uses: carbon-14 dating & used to kill cancerous skin cells

gamma rays (γ)

charge - neutrally charged

structure - high-energy electro-magnetic radiation

penetrability - extremely penetrating

e.g. cobalt-60

* the loss of gamma radiation does not give rise to a new element → it’s simply the loss of energy from the nucleus

* can cause cancer as they change structure of our cells, but are also used to treat cancer to kill cancerous cells

nuclear reaction

involves a change in the nucleus of an atom and a new element is usually formed as a result

chemical reaction

involves electrons being shared or transferred from one atom to another → nucleus of atom remains the same

half-life

the time taken for half the nuclei in a given sized sample to decay (i.e. emit radiation)

the number of nuclei left undecayed governs the overall rate of decay

the time taken for half of the atoms in a sample of a radioisotope to decay is constant for that particular isotope

radioisotopes

an isotope of an element that undergoes radioactive decay

uses of radioisotopes

1.       Nuclear power – heat generated in nuclear power stations by nuclear reactions is used to produce electricity.

2.       Cancer treatment – gamma rays of cobalt-60 are used to kill cancerous cells inside the body. Beta rays are used to treat skin cancers, as they are less penetrating.

3.       Sterilisation – radiation is used to sterilise medical equipment. It is also used to kill bacteria to give food a longer shelf life.

4.       Archaeological uses – The ratio of carbon -12 to carbon – 14  remains constant when the organism is alive. When a plant/animal dies the carbon-14 already in the plant/animal continues to decay very slowly and    therefore the ratio of carbon-14 to carbon-12 changes in the organism. By measuring the RATIO of carbon-12 to carbon-14 and knowing the half-life of carbon-14 the time of death of the organism can be calculated.

nuclear fission

the splitting of a nucleus into two roughly equal fragments

nuclear fusion

the joining of two smaller nuclei to form a larger one → energy is given out

chemical vs nuclear reactions

chemical:

• involves the loss, gain or sharing of valence electrons

• no change in the nucleus

• never forms a new element

nuclear:

• doesn’t involve valence electrons

• always a change in the nucleus

• usually forms a new element