Topic 6: Radioactivity Flashcards
6.1 describe an atom as a positvely charged nucleus, consisting of protons and neutrons, surrounded by negativey charged electrons, with the nuclear radius much smaller than that of the atom and with almost all of the mass in the nucleus.
the centre of an atom is a small positively charged nucleus.
This is mad eup of protons and neutrons and is the source of most of the atoms mass.
Most of the atom is empty space.
Electrons orbit the nucleus in set energy levels.
6.2 recall the typical size (order of magnitude) of atoms and small molecules
atoms = 1x10^-10m (atom is 100,000 times bigger than the nucleus inside it)
6.3 Describe the structure of the nuclei of isotopes of using the terms atomic number and mass nuber and using symbols
(see image for clarification)
two isotopes of the same element will always have the same atomic (proton) number, but they can have different mass (nucleon) numbers if they contain different numbers of neutrons. ie, can get 13C 6 and 12C 6
6.4 Recall different element characteristics
the nucleus of each element has a characteristic positive charge, but that isotopes of an element differ in mass by having different numbers of neutrons
6.5 Recall the relative masses and relative electric charges of protons, neutrons, electrons and positrons
proton = +1
neutron = 0
electron = -1
positron = +1
6.6 Recall the charge of atoms and why this is
in an atom the number of protons equals the number of electrons and is therefore neutral
6.7 Recall what electrons do in an atom
in each atom its electrons orbit the nucleus at different set distances from the nucleus called energy levels
6.8 Explain information about electron orbits
electrons change orbit when there is absorption or emission of electromagnetic radiation (if an atom absorbs energy, an electron can move to a ‘higher orbit’.
when an electron returns to a lower orbit the atom emits energy as visible light).
6.9 Explain how atoms may form positive ions by ionising radiation
if an atom gains enough energy (from electromagnetic radiation) one or more of the electrons can escape from the atom.
an atom that has lost or gained electrons is called an ion and this radiation is called ionising radiation.
an atom has the same number of protons and electrons so overall it has no change. if an atom loses an electron, it then has one more proton than it has electrons, therefore it now has an overall positive change
6.10 Recall the different types of ionising radiation
alpha, β- (beta minus), β+ (positron), gamma rays and neutron radiation are emitted from unstable nuclei in a random process
6.12 Explain what is meant by background radiation
low levels of ionising radiation from space and naturally radioactive substances, which we are exposed to but are harmless
6.13 Describe the origins of background radiation from Earth and space
radon gas contributes to about 50% of background radiation.
it is produced by rocks that contain a small amount of uranium.
it diffuses into the air from rocks and soil and can build up in houses, especially where there is poor ventilation (rock types vary around the country)
some food (bananas) naturally contain small amounts of radioactive substances. x rays, gamma ray scans and cancer treatments also contribute
high energy, charged particles stream out of the sun and other stars (cosmic rays). many are stopped in the upper atmosphere
6.14 Describe methods for measuring and detecting radioactivity limited to photographic film and a Geiger-Müller tube
- use photographic film, which becomes darker and darker as more radiation reaches it. however it has to be developed in order to measure the amount of radiation (dose)
- film badges (dosimeters) can be used to check how much radiation a person has been exposed to, and they do not need to be developed to change colour
- can use a Geiger-Müller tube (GM tube). radiation passing through the tube ionises gas inside it and allows a short pulse of current to flow. can click each time radiation is detected (count rate is number of clicks per second/ minute)
6.15 Recall that an alpha particle is equivalent to a helium nucleus, a beta particle is an electron emitted from the nucleus and a gamma ray is electromagnetic radiation
an alpha particle is equivalent to a helium nucleus (relative mass of 4 and have a change of +2, 4He 2),
a beta particle is an electron emitted from the nucleus (therefore they have a charge of -1 and a mass of 0, 0e -1) and a gamma ray is electromagnetic radiation (no electrical charge)
6.16 Compare alpha, beta and gamma radiations in terms of their abilities to penetrate and ionise
alpha particles = travel a few centimetres in the air, very ionising (have a high relative mass, therefore transfer a lot of energy), can be stopped by a sheet of paper ((not penetrating) - this is because each time they ionise an atom they lose energy)
beta particles = travel a few metres in the air, moderately ionising, can be stopped by 3mm thick aluminium (moderately penetrating)
gamma rays = will travel a few kilometres in the air, weakly ionising, thick lead or several metres of concrete to stop them
6.17 Describe how and why the atomic model has changed over time including reference to the plum pudding model.
used to think that atoms were hard spheres
JJ Thomson made a new model describing the atom as a positive sphere (‘pudding’), with negatively charged electrons (‘plums’) scattered through it - explains an atom as neutral, but doesnt state about an atom having a nucleus.
6.17 Describe how and why the atomic model has changed over time including rutherford’s model
rutherford carried out an experiment, in which alpha particles were fired at a thin sheet of gold foil. alpha particles are positive, so a lot were expected to bounce back, as the plumb pudding model said that an atom is a positive sphere (with negative electrons dotted inside). most particles went straight through, showing atoms are mainly empty space. some particles were deflected, because the positive alpha particles went too close to the positive nucleus, showing there is a concentrated positive space, instead of the whole atom being made of a positive sphere. some particles bounced back, explaining there is a tiny, dense nucleus, with electrons moving around it.
6.17 Describe how and why the atomic model has changed over time including bohr’s model
Bohr amended the model by suggesting that electrons can only be in certain fixed orbits (electron shells) around the nucleus (meaning they cannot be between two orbits)
6.18 Describe the process of β- decay (how a beta particle forms)
can be released from an unstable nucleus (are like electrons however found in the nucleus). they are released if there is an unstable n:p ratio (beta minus are released if there are too many neutrons)
a neutron changes into a proton plus an electron (the electron is ejected from the atom)
6.19 Describe the process of β+ decay (how a beta particle forms)
can be released from an unstable nucleus. they are released if there is an unstable n:p ratio (beta plus are released if there are too many protons)
a proton becomes a neutron plus a positron
6.20 Explain the effects on the atomic (proton) number and mass (nucleon) number of radioactive decays (α, β, γ and neutron emission)
α (alpha - 4He 2) - mass number of the nucleus goes down by 4 and the atomic number goes down by 2
β+ (beta plus - 0e -1) - the atomic number goes down by 1 but the mass number does not change
β- (beta minus - 0e +1) - the atomic number increases by 1 but there is no change to mass number
γ (gamma) - no electrical charge or relative mass
neutron (n)- the atomic number remains the same but the mass number decreases by 1
6.21 Recall information about nuclei and radioactive decay
nuclei that have undergone radioactive decay often undergo nuclear rearrangement with a loss of energy as gamma radiation - it is a random process (we cannot predict when it will happen)
