p7 - radioactivity

Question 1

how do nuclei of radioactive substances become stable?

Answer 1

through radioactive decay whilst emitting radiation

Question 2

why is it not possible to predict when an unstable nucleus will decay?

Answer 2

• it is a random process
• not affected by external conditions

Question 3

define the plum pudding model

Answer 3

an atom consisting of a sphere of positive charge with electrons buried inside, like plums in a pudding

Question 4

define the alpha scattering experiment

Answer 4

devised by rutherford, geiger and marsden
- fired alpha particles at a very thin gold foil
- alpha particles have a positive charge

Question 5

what were the observations in the alpha particle scattering experiment?

Answer 5

• most passed straight through: atom consists of mainly empty space
• some deflected through small angles: nucleus is subcharged and positive charge
• small number of alpha particles were deflected through large angles: nucleus has large mass

Question 6

why did the plum pudding model become outdated?

Answer 6

it couldnt explain results of alpha particle scattering experiment

Question 7

define the ‘nucleur model’

Answer 7

• updated after plum pudding model
• every atom has a positively charged nucleus
• most of mass of atom is concentrated
• electrons orbit nucleus and most of atom is empty space

Question 8

define atomic number

Answer 8

number of protons in an atom

Question 9

define mass number

Answer 9

total number of protons and electrons in an atom is its mass number

Question 10

define isotopes

Answer 10

atoms of same element with different numbers of neutrons

Question 11

what does an alpha particle consist of?

Answer 11

• two protons, two neutrons
• relative mass is 4 and relative charge is +2

Question 12

what happens when an unstable nucleus emits an alpha particle?

Answer 12

atomic number goes down by 2, mass number goes down by 4

Question 13

define a beta particle

Answer 13

• high-speed electron from nucleus
• emitted when a neutron in nucleus changes to a proton and an electron
• relative mass is 0 and relative charge is -1.
• proton stays in nucleus so atomic number goes up by 1, mass number unchanged, electron instantly emitted

Question 14

what happens when a nucleus emits gamma radiation?

Answer 14

no change in atomic number or mass number because a gamma ray has no charge or mass

Question 15

define ionisation

Answer 15

when atoms become positively charged by losing electrons due to radiation knocking them out

Question 16

define irridation

Answer 16

object becomes irridated when exposed to ionising radiation, does not become radioactive

Question 17

define radioactive contamination

Answer 17

unwanted presence of materials containing radioactive atoms on other materials

Question 18

why do alpha particles have lots of collisions with atoms?

Answer 18

• they are relatively large
• the are strongly ionising

Question 19

why don’t alpha particles penetrate far into material?

Answer 19

they have lots of collisions with atoms so can be stopped by a thin sheet of paper, human skin or a few centimetres of air

Question 20

why are beta-particles less ionising?

Answer 20

smaller and faster than alpha particles so penetrate further into a material than alpha particles

Question 21

what can beta particles be blocked by?

Answer 21

thin sheet of aluminium or few metres of air

Question 22

why are gamma waves weakly ionising?

Answer 22

travel a long way through a material before colliding with an atom so very penetrating

Question 23

what is needed to absorb most of the radiation from a gamma source?

Answer 23

several centimetres of lead or several metres of concrete, unlimited range in air

Question 24

define activity of a radiactive source

Answer 24

number of unstable atoms in the source that decay per second

Question 25

give unit for activity

Answer 25

Becquerel (Bq) which is 1 decay per second

Question 26

define the count rate

Answer 26

what we use to measure the radioactivity of a sample of a radioactive material, number of counts per second

Question 27

define the half-life

Answer 27

• measures how quickly the radioactivity decreases
• average time it takes for number of nuclei of the isotope in a sample to halve and for the count rate from isotope to fall to half its initial value

Question 28

what is a random process?

Answer 28

one in which it is not possible to predict when a particular nucleus will decay e.g radioactive decay

Question 29

give equation for half life calculations

Answer 29

initial count rate/2(n) or initial number of unstable nuclei/2(n)