Nuclear transformations

Nuclear transformations

Nuclear transformations: reactions that occur in the nucleus of an atom.

Nuclear transformation change the number of protons and neutrons, that is the nature of the element.

Some are exothermic (release a huge amount of energy).

Nuclear transformations are used for the generation of electricity.

Nuclear transformations

Nuclear stability: state of the nucleus in which the nuclear force is bigger than the electrical repulsion force between protons.

Number of neutrons directly related to stability of the nucleus. (see Fig. 4.25 on page 125)

Nuclear force decreases as the size of the nucleus increases.

Nuclear transformations

Radioactivity:

Naturally occurring nuclear transformation. An unstable atom spontaneously transforms

into a more stable or several more stable atoms.

It transforms while releasing energy in the form of radiation.

Nuclear transformations

Radioactivity (ionizing radiation)

Alpha (α) particlesBeta (β) particlesGamma (γ) rays

(See fig 4.26 on page 127 for more details)

Nuclear transformations

Radioactivity (ionizing radiation)

Alpha (α) particles

Positive particles (deflected towards negative pole of electrical field)

Made of two protons and two neutrons Relatively large and heavyCan be stopped with a sheet of paper

Nuclear transformationsRadioactivity (ionizing radiation)

Beta (β) particles

Negative particles (deflected towards positive pole of electrical field)

Made of an electron (or a positron if a positive beta particle) Lighter than alpha particles More penetrating power than alpha particles Can be stopped with a sheet of foil 3 mm minimum

Nuclear transformations

Radioactivity (ionizing radiation)

Gamma (γ) rays

Neutral (no deflected by an electrical field)Made of only energyGreatest penetrating power of the three radiationsCan be stopped with a very dense material (lead or

high density concrete)

Nuclear transformationsAdvantages of Radioactivity

(ionizing radiation)

Improve quality of materials.

Increase shelf life of fresh products (food).

In medicine to treat cancer, or to obtain images of organs.

Disadvantages of Radioactivity (ionizing radiation)

Can be harmful to living organisms.

Alters DNA cells leading to cancer development.

Nuclear transformationsRadioactivity

Random and spontaneous process. Impossible to predict which atoms will decay. Impossible to predict when they will decay.

What is possible to predict is the half life of a sample of atoms.

Half-life: time required for half of the nuclei in a sample of radioactive material to decay.

(See Fig. 4.28 on page 128 for half-life of C14)(See Fig. 4.29 on page 129 for half-life values)

Nuclear transformationsHow does half-life works?

_Take carbon-14 in figure 4.28. _Half-life is 5770 years

Meaning, if you have two grams of carbon-14: After 5770 years you would still have one gram. After 5770 more years (11540 years) you would still have half a gram. After 5770 more years (17310 years) you would have a quarter of a gram. After 5770 more years (23080 years) you would still have an eighth of a gram. After 5770 more years (28850 years) you would still have a sixteenth of a gram. And so on…until is completely gone.

The longer the half-life of an element or isotope, the longer it takes to completely eliminate it from the environment!

Nuclear transformations

Nuclear fission (humans): nuclear reaction where the nucleus of an atom is split to form two or more lighter atomic nuclei.

(See Fig. 4.30 on page 130 for U-235 nuclear fission)

Nuclear transformationsNuclear fission:

3 neutrons produced that can react with as many U-235 nuclei to continue the chain reaction (atomic bomb).

Slowing the neutrons produced, is the way to control these reactions.

Then nuclear fission can be used to generate electricity. Some of the waste of nuclear reactors is used in medicine, the

rest is buried!

Nuclear transformations

Nuclear fusion (stars): nuclear reaction in which two small atomic nuclei join together to form one heavier nucleus.

(See Fig. 4.31 on page 131 for U-235 nuclear fission)

Nuclear transformations

Nuclear fusion:

This type of reaction can only be started and maintained at temperatures of million degrees Celsius.

Few applications (H bomb is one of the few, in it nuclear fission is used to start nuclear fusion).

More energy efficient than nuclear fission. Produces less radioactive waste.

Assessment:

Questions 24 – 28 on page 134 Textbook

Homework:

Questions 1 – 4 on page 70 Workbook