radioactivity

Radioactivity

Spontaneous decay of an unstable nucleus with the emission of alpha, beta, or gamma radiation

Radioisotopes

Isotopes with unstable nuclei that spontaneously decay and emit alpha, beta or gamma radiation i.e. radioactive isotopes

Who discovered radioactivity and how?

Becquerel by uranium salts

What did Pierre and Marie Curie discover?

polonium and radium

Alpha particle

Particle that consists of two protons and two neutrons emitted by radioactive isotopes. They're identical to a helium nucleus and have a positive charge (+2). represented by 4 He 2 or alpha symbol

Changes that occur in the nucleus of an atom during alpha decay?

nucleus emits two protons and two neutrons

in radioactivity...

...mass and atomic numbers flipped so mass no on top and atomic on bottom bc physics chapter

alpha decay eq calcs

mass no - 4

atomic no - 2

Q: write a balanced nuclear eq to show alpha decay of americium-241

mass no given in q and atomic no from periodic table

241 Am 95 ----> 237 Np 93 + 4 He 2 i.e. RHS atomic and mass no total must equal LHS atomic and mass nos!

Three properties of alpha particles?

1. positively charged- will deflect towards a negative plate.

2. low penetrating ability- stopped by paper or a few cm of air.

3. strongest ionising ability- cause mutations (cancerous)

Beta particle

Electron emitted by a radioactive isotope. Has a negative charge (-1) represented by 0 e -1 or beta symbol

Changes that occur in the nucleus of an atom during beta decay?

nucleus splits up into a proton and an electron.

Proton remains but electron is emitted.

beta decay eq calcs

mass no stays same and atomic no decreases by 1

Q: write a balanced nuclear eq for the beta decay of iodine-131

131 I 53 ----> 131 Xe 54 + 0 e -1 i.e. RHS atomic and mass no total must equal LHS atomic and mass nos!

Three properties of beta particles?

1. negatively charged- will deflect towards a positive plate.

2. moderate penetrating ability- stopped by aluminium.

3. moderate ionising ability- cause mutations (cancerous)

Gamma radiation

High energy electromagnetic radiation emitted by radioactive isotopes

Change that occur in the nucleus of an atom during gamma decay?

structure of the nucleus remains the same, but it loses energy to become more stable

Three properties of gamma rays?

1. neutral charge- will not deflect

2. very high penetrating ability- stopped by several cm of lead

3. weakest ionising ability- cause mutations (cancerous)

Three differences between a nuclear and chemical reaction?

1. nuclear reactions: involve a change in structure of nucleus.

chemical reactions: involve electrons being transferred or shared.

2. nuclear reactions: new element formed.

chemical reactions: no new element formed.

3. nuclear reactions: no chemical bonds broken or formed.

chemical reactions: chemical bonds broken and formed

Background radiation

Radiation naturally present in the environment

Sources of background radiation?

1. Radon gas emitted from rocks.

2. Food.

3. Cosmic rays from sun.

Instrument used to detect radioactivity?

Geiger-Muller tube

Precautions to be taken when working with radioactive material?

- Material should be stored in a lead container

- Minimise time with material

- Maximise distance from material

- Wear protective clothing

Half-life

Time taken for half of the nuclei to decay

Use of a radioisotope that emits alpha particles?

Americium-241 in smoke detectors

Why is americium-241 in smoke detectors not harmful?

Alpha particles emitted have a low penetrating ability- only travel a few cm in air and cannot penetrate skin.

Should americium-241 in smoke detectors be replaced?

Never. It has a half-life of 432 years.

Use of a radioactive isotope that emits beta particles?

Carbon-14 to determine age of objects.

How can the age of objects be determined by carbon-14 dating?

- When organisms are alive they have a set ratio of carbon-12 to carbon-14 the same as in the atmosphere.

- When it dies the C-12 to C-14 ratio changes as the C-14 decays and decreases.

- New ratio and half-life of carbon-14 used to calculate age.

Use of a radioisotope that emits gamma rays?

Cobalt 60 to kill cancer cells

Alpha particle ionising ability?

Strongest

Beta particle ionising ability?

Moderate

Gamma particle ionising ability?

Weakest

Q: given a half life of 30 days, what fraction of X remained after 90 days?

remember, half life is time taken for half the nuclei to decay.

@ day 0, fraction is 1

@ day 30 fraction is 1/2

@ day 60, fraction is (1/2)/2 = 1/4

@ day 90, fraction is (1/4)/2 = 1/8 i.e. dividing fraction by 2

NOTE: going up days in half life intervals!

Q: given a half life of 5 days and told that 6g remain after 30 days, find original mass

@ day 30, mass = 6g

@ day 25, mass = 12g

@ day 20, mass = 24g

@ day 15, mass = 48g

@ day 10, mass = 96g

@ day 5, mass = 192g

@ day 0, mass = 384g i.e. multiplying masses by 2 bc working backward

NOTE: going down days in half life intervals!

Q: how long will it take for 18g of X to decay and leave a mass of 2.25g considering half life is 1600 years?

@ year 0, mass = 18g

@ year 1600, mass = 9g

@ year 3200, mass = 4.5g

@ year 4800, mass = 2.25g (answer) i.e. dividing masses by 2

NOTE: going up years in half life intervals!

note for decay problems

1. ur going up/down in half life intervals e.g. if half life is 5 days u check @ day 0, day 5, day 10 etc.

2. if asked to find time length or mass/fraction AT END, normal procedure, just go up half life intervals and divide mass/fraction by 2

3. if asked to find original mass or something AT START, reverse procedure, go down half life intervals and multiply mass/fraction by 2