p7 - radioactivity Flashcards
how do nuclei of radioactive substances become stable?
through radioactive decay whilst emitting radiation
why is it not possible to predict when an unstable nucleus will decay?
- it is a random process
- not affected by external conditions
define the plum pudding model
an atom consisting of a sphere of positive charge with electrons buried inside, like plums in a pudding
define the alpha scattering experiment
devised by rutherford, geiger and marsden
- fired alpha particles at a very thin gold foil
- alpha particles have a positive charge
what were the observations in the alpha particle scattering experiment?
- 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
why did the plum pudding model become outdated?
it couldnt explain results of alpha particle scattering experiment
define the ‘nucleur model’
- 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
define atomic number
number of protons in an atom
define mass number
total number of protons and electrons in an atom is its mass number
define isotopes
atoms of same element with different numbers of neutrons
what does an alpha particle consist of?
- two protons, two neutrons
- relative mass is 4 and relative charge is +2
what happens when an unstable nucleus emits an alpha particle?
atomic number goes down by 2, mass number goes down by 4
define a beta particle
- 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
what happens when a nucleus emits gamma radiation?
no change in atomic number or mass number because a gamma ray has no charge or mass
define ionisation
when atoms become positively charged by losing electrons due to radiation knocking them out
define irridation
object becomes irridated when exposed to ionising radiation, does not become radioactive
define radioactive contamination
unwanted presence of materials containing radioactive atoms on other materials
why do alpha particles have lots of collisions with atoms?
- they are relatively large
- the are strongly ionising
why don’t alpha particles penetrate far into material?
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
why are beta-particles less ionising?
smaller and faster than alpha particles so penetrate further into a material than alpha particles
what can beta particles be blocked by?
thin sheet of aluminium or few metres of air
why are gamma waves weakly ionising?
travel a long way through a material before colliding with an atom so very penetrating
what is needed to absorb most of the radiation from a gamma source?
several centimetres of lead or several metres of concrete, unlimited range in air
define activity of a radiactive source
number of unstable atoms in the source that decay per second
give unit for activity
Becquerel (Bq) which is 1 decay per second
define the count rate
what we use to measure the radioactivity of a sample of a radioactive material, number of counts per second
define the half-life
- 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
what is a random process?
one in which it is not possible to predict when a particular nucleus will decay e.g radioactive decay
give equation for half life calculations
initial count rate/2(n) or initial number of unstable nuclei/2(n)
