6 Radioactivity

Question 1

What does a radioactive substance contain?

Answer 1

an unstable nuclei that becomes stable after emitting radiation

Question 2

What are the three main types of radiation from radioactive substances?

Answer 2

• alpha
• beta
• gamma

Question 3

What is radioactive decay known as?

Answer 3

a random event

we cannot predict it or influence when it will happen

Question 4

What can measure background radiation and where does it come from?

Answer 4

a Geiger counter…

• in the environment (e.g. in the air or the ground or in building materials)
• from space (cosmic rays)
• from devices such as x-ray tubes

Question 5

What was the ‘plum pudding model’?

Answer 5

some scientists thought that an atom was like a plum pudding with:

• the positively charged matter in the atom evenly spread out
• electrons buried inside

Question 6

What was the set up of Ernest Rutherford’s radioactivity investigation?

Answer 6

• used a gold leaf (1 layer of gold atoms) and passed alpha particles through it
• known as scattering experiment to see if the alpha particles would bounce back

Question 7

What did Ernest Rutherford find in his investigation?

Answer 7

• most of the alpha particles passed straight through the metal foil
• the number of alpha particles deflected per minute decreased as the angle of deflection increased
• about 1 in 10 000 alpha particles were deflected by more than 90degrees

Question 8

What did Rutherford deduce from his scattering experiment results?

Answer 8

he knew that alpha particles are positively charged so he deduced that there is a nucleus at the centre of every atom that is…

• positively charged because it repels alpha particles (like charges attract/unlike charges repel)
• much smaller than the atom because most alpha particles pass through without deflection
• where most of the mass of the atom is located

Question 9

Why did Rutherford’s nuclear model of the atom become accepted?

Answer 9

• it agreed exactly with the measurements made in the experiment (found the diameter of the nucleus was about 100 000 times smaller than atom)
• it explained radioactivity in terms of changes that happen to an unstable nucleus when it emits radiation
• predicted the existence of the neutron which was later discovered

Question 10

Why was the plum pudding model abandoned?

Answer 10

could not explain why some alpha particles were scattered through large angles

Question 11

What is the atomic (proton) number?

Answer 11

• the number of protons in the nucleus

- symbol: Z

Question 12

What are isotopes?

Answer 12

• atoms of the same element with different numbers of neutrons
• (same proton number, different neutron number)

Question 13

What is the mass number?

Answer 13

• the number of protons and neutrons in a nucleus

- symbol: A

Question 14

What is radioactive decay?

Answer 14

when an unstable nucleus becomes more stable by emitting an alpha or beta particle or by emitting gamma radiation

Question 15

What does an alpha particle consist of?

Answer 15

2 protons
2 neutrons
+2 relative charge
( a helium nucleus)

Question 16

What happens when an unstable nucleus emits an alpha particle?

Answer 16

• atomic number goes down by 2 and mass number by 4
• equation:
  A A-4 4
  Z X –> Z-2 Y + 2 alpha particle

Question 17

What does a beta particle consist of?

Answer 17

an electron (high speed)
(emitted by nucleus with too many neutrons compared to protons- a neutron in the nucleus changes into a proton and a beta particle which is instantly emitted at high speed from the nucleus)

Question 18

What happens when an unstable nucleus emits a beta particle?

Answer 18

• atomic number of nucleus goes up by 1 but its mass number stays the same (because neutron changes into proton)
• equation:
  A A 0
  Z X –> Z+1 Y + -1 beta particle

Question 19

How is gamma radiation emitted by some unstable nuclei?

Answer 19

(high energy EM wave)

• after an alpha or beta particle has been emitted
• gamma radiation is uncharged and has no mass
• does not change the number of protons or neutrons in a nucleus

Question 20

What is the penetration power of alpha particle radiation?

Answer 20

• can be absorbed by a thin sheet of paper

- has a range of around 5 cm in air

Question 21

What is the penetration of beta particle radiation?

Answer 21

• can be absorbed by an aluminium sheet about 5 mm thick and a lead sheet about 2-3 mm thick
• has a range of around 1 m in air

Question 22

What is the penetration power of gamma radiation?

Answer 22

• can be absorbed by a thick lead sheet about several cm thick and concrete more than 1 m thick
• has an unlimited range in air

Question 23

What can be used to separate a beam of alpha, beta and gamma radiation?

Answer 23

a magnetic or electric field

shows properties as the charges influence it…

Question 24

What is ionisation?

Answer 24

when atoms become charged because they lose or gain electrons

Question 25

Which radiation types can ionise and how strongly?

Answer 25

They ionise substances they pass through which in living cells can damage or kill them... - alpha: yes, very strongly - beta: yes, weaker - gamma: yes, very weakly

Question 26

What is the half-life of a radioactive isotope?

Answer 26

the average time it takes: - for the number of nuclei of the isotope in a sample (and therefore the mass of parent atoms) to halve - for the count rate of the isotope in a sample to fall to half its initial value

Question 27

What is the activity of a radioactive source?

Answer 27

the number of atoms/nuclei that decay per second

Question 28

What can we use the Geiger counter for?

Answer 28

- to monitor the activity of a radioactive sample | - can measure a count rate

Question 29

What does the use we can make of a radioactive isotope depend on?

Answer 29

- its half life | - the type of radiation it gives out

Question 30

Give an example of how a radioactive substance can be useful for monitoring?

Answer 30

Kidney monitoring (radioactive iodine water substance goes into kidney- if functioning reading will go up then down where it flows straight through, if blocked then reading on detector stays up)

Question 31

What makes a radioactive substance good at monitoring?

Answer 31

- if it has a long enough half-life but will then decay and be gone - if it emits gamma radiation as it can be detected outside the body - if it decays into a stable product

Question 32

What are the 2 types of radioactive dating?

Answer 32

- carbon dating: used to find the age of organic material (half-life around 5600 years) - uranium dating: used to find age of igneous rocks (half-life around 4500 million years)

Question 33

What do we need for a radioactive dating sample?

Answer 33

a radioactive isotope that is present in the sample which has the same half life as the age of the sample