SP6: Radioactivity Flashcards
SP6a
1) Describe the structure of an atom (in terms of nucleus and electrons).
2) State where most of the mass of an atom is found.
3) State the sizes of atoms and small molecules.
1) The protons and neutrons make up a central nucleus and the electrons orbit the nucleus.
2) Most of the mass is found in the nucleus.
3) The typical size of an atom is 1 x 10 to the power of -10 metres or 0.1 nanometres (nm)
SP6a
Describe how and why our model of the atom has changed over time, including the plum pudding model and the Rutherford’s experiment.
Democritus: he stated that atoms are particles that cannot be broken down.
John Dalton: he state that atoms were different types of solid spheres.
JJ Thompson: he created the plum pudding model. The atom consisted of a positive ball with negative electrons scattered inside it.
Ernest Rutherford: He did an experiment to test the plum pudding model. He directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum. Most of the alpha particles did pass straight through the foil, but a small number of alpha particles were deflected by large angles as they passed through the foil, and a very small number of alpha particles came straight back off the foil.
Rutherford had discovered the nuclear atom, a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom.
Neil Bohr: he discovered that electrons orbited the nucleus on electron shells.
SP6b
1) State what is meant by an isotope.
2) Represent isotopes using symbols.
3) Explain how atoms of different elements are different (in terms of numbers of electrons and protons).
1) Isotopes are forms of an element that have the same number of protons but different numbers of neutrons.
2) On atomic symbols the mass number (big/ top number) will change while the atomic number (small/ bottom number) remains the same.
3) Atoms of different elements have different numbers of protons, electrons and neutrons. Atoms of the same element will have the same number of protons and electrons but different neutrons.
SP6b
1) Recall the charges and relative masses of the three subatomic particles.
2) Explain why all atoms have no overall charge.
3) What is a nucleon number?
1) Protons: +1 charge and relative mass of 1
Neutrons: 0 charge and relative mass of 1
Electrons: -1 charge and relative mass of almost 0
2) Every atom has no overall charge (neutral). This is because they contain equal numbers of positive protons and negative electrons. These opposite charges cancel each other out making the atom neutral.
3) A nucleon number is another name for the mass number of an atom.
SP6c
1) Describe where electrons are found inside atoms (in terms of shells).
2) Describe when electrons can change orbit.
3) Recall what an ion is.
1) Electrons in atoms occupy energy levels, also called electron shells, outside the nucleus. Different shells can hold different maximum numbers of electrons. The electrons in an atom occupy the lowest available energy level first. This is the shell nearest the nucleus.
2) An electron can move into a high energy level and further away from the nucleus by absorbing electromagnetic radiation. An electron can move into a lower energy level and closer to the nucleus by emitting electromagnetic radiation.
3) An ion is an atom or group of atoms with a positive or negative charge, that has lost or gained electrons in order to have a full outer shell.
SP6c
1) Describe how ionisation occurs.
2) Explain the Bohr model of the atom, and explain the energy levels of the orbits
1) The electrical charge of an atom can be changed by ionisation. If an atom loses electrons, then it will turn into a positive ion, and if an atom gains electrons will turn into a negative ions.
2) The Bohr model of an atom has a small, positively charged central nucleus and electrons orbiting in at specific fixed distances from the nucleus. As you get further away from the nucleus, the energy levels (electrons shell orbits) get closer together. So electrons falling to an energy level closer to the nucleus will have a greater change in energy than one that is further away from the nucleus. This means that they release electromagnetic radiation with a higher energy, and so a higher frequency is released when an electron falls to the energy level closer to the nucleus.
SP6d
1) Explain what background radiation is.
2) Describe how radiation measurements need to be corrected for background radiation.
3) List some sources of background radiation.
4) How can a graph be used to estimate the count rate of the background radiation?
1) Background radiation is ionising radiation that is always in the environment. It comes from space and naturally radioactive substances in the environment.
2) When scientist measure the radioactivity of a source, they need to measure the background radiation first by taking several reading and finding the mean. This mean value is then subtracted from the measurement of the radioactive source.
3) Sources of background radiation include:
- Radon gas: produced by rocks that contain small amount of uranium.
- Cosmic rays: high-energy, charged particles that streams out of stars
- Hospital treatments (eg. X-rays and cancer treatments)
- Fallout from nuclear weapons (Fallout is the radioactive particles that fall to earth as a result of a nuclear explosion.)
4) The count rate of background radiation is when the count rate on the graph plateaus.
SP6d
1) Describe how photographic film can be used to detect radioactivity.
2) Describe how a Geiger-Müller tube works
3) Describe how the amount of radioactivity can be measured (in terms of the darkness of photographic film or by attaching a counter to a GM tube).
1) Radioactivity can be measured using photographic film, which becomes darker and darker as more radiation reaches it. However, the film has to be developed in order to measure the amount of radiation (the dose).
2) The radioactivity of a source can be measured using a Geiger-Müller (GM) tube. Radiation passing through the tube ionises gas inside it and allows a short pulse of current to flow.
3) A GM tube can be connected to a counter, to count the pulses of current, or the GM tube may give a click each time radiation is detected. The count rate is the number of clicks per second or minute.
The darker the photographic film, the more radiation it has been exposed to.
SP6e
1) Recall the relative masses and relative electric charges of protons, neutrons, electrons and positrons.
2) List five types of radiation that are emitted in random processes from unstable nuclei, and what type of radiation is it?
3) State the three types of ionising radiation emitted by radioactive decay
1) Proton: mass = 1, electrical charge = +1
Neutron: mass = 1, electrical charge = 0
Electron: mass = almost 0, electrical charge = -1
Positron: mass = almost 0, electrical charge = +1
2) Alpha, beta +, beta -, gamma, neutron. The five types of radiation are ionising radiation.
3) Alpha, beta and gamma radiation
SP6e
1) Describe what alpha and beta particles are
2) Describe the nature of gamma radiation
1) An alpha particle is also a Helium-4 nucleus, so it is written as ⁴₂He (as seen in the periodic table) and is also sometimes written as ⁴₂α.
A beta particle (β) is a fast moving electron or positron, it has a very small mass. An electron has a -1 charge and a positron has a +1 charge.
2) Gamma ray (γ) is a high-energy electromagnetic wave. Gamma rays are caused by changes within the nucleus. They are part of the electromagnetic spectrum and so travel at the speed of light. They have no mass and no charge.
SP6e
1) Compare the penetrating abilities of alpha, beta and gamma radiation
2) Compare the ionisation abilities and the range of alpha, beta and gamma radiation
1) Alpha particles are absorbed by paper. Beta particles can travel through paper, but it is absorbed by aluminium. Gamma rays are highly penetrating, and can travel through paper and aluminium. It is absorbed by lead.
2) Alpha particles have a range of a few cm because they are highly ionising, beta particles are ionising and have a range of a few metres, gamma particles have a range of a few kilometres because they are weakly ionising.
SP6f
1) Describe the process of β– decay.
2) Describe the process of β+ decay.
3) Explain how the proton and mass numbers are affected by different kinds of radioactive decay.
1) During a β– decay, a neutron changes into a proton and an electron. The electron is ejected from the atom. This type of decay is written as 0 mass number, -1 atomic number, and the symbol ‘e’ in a nuclear equation.
2) In a of β+ decay, a proton becomes a neutron and a positron. The positron is ejected from the atom. This type of decay is written as 0 mass number, +1 atomic number, and the symbol ‘e’ in a nuclear equation.
3) In β- decay, the atomic number increases by 1, but there is no change to the mass number.
In β+ decay, the atomic number decreases by 1 but the mass number does not change.
In alpha decay, the atomic number decreases by 2 and the mass number decreases by 4.
In gamma decay, neither the atomic number nor the mass number changes.
In neutron decay, the atomic number does not change, but the mass number decreases by 1.
SP6f
1) Describe what happens during nuclear rearrangement after radioactive decay.
2) How do you balance nuclear equations for mass and charge.
1) When the radioactive decay is emitted, the nucleus loses energy, and the subatomic particles in the nucleus are rearranged.
2) On the left hand side of the arrow, write the formula for the atom before the radioactive decay. On the right hand side of the arrow, write the formula of the atom after radioactive decay, + sign, then the formula of the decay that it emitted.
SP6g
1) Describe how the activity of a substance changes over time.
2) State how half-life can be used to describe the changing activity of a substance.
3) Recall the unit of activity.
4) How does the half life correspond with how unstable the isotope is?
1) Radioactive decay is a random process. It is not possible to say which particular nucleus will decay next but given that there are so many of them, it is possible to say that a certain number will decay in a certain time.
2) Half-life is the time it takes for half of the unstable nuclei in a sample to decay, or for the activity of the sample to halve or the count rate to half.
3) The activity of a sample is measured in bequerels (Bq).
4) The shorter the half life, the more unstable the isotope is. Therefore, it will have more activity because the shorter the half life, the more nuclei that will decay per second.
SP6g
1) Describe how half-life can be used to work out how much of a substance will decay in a certain time.
2) How do you calculate the half life of a sample?
3) Why is the estimate of activity less likely to be accurate after a longer period of time?
4) Describe what the count rate of radioactive decay will be like
1) First find the number of nuclei remaining after the specified number of half lives, then subtract this from the initial number of nuclei to find how much of the substance will decay. For example, to calculate the number of decays after 2 half lives, if the initial sample contains 400 nuclei, after 2 half lives, there is 200 undecayed nuclei remaining. 800 - 200 = 600 decayed nuclei.
2) You need to find the time it take for the count rate to half.
When using a graph: if the initial count rate is 60cps, half of this is 30cps, and you need to see how many seconds 30cps corresponds to on the graph. The number of seconds is the half life.
When given the time: For example, the initial activity is 8800 Bq, and after 6 hours the activity is 1100 Bq. It takes 3 half lives to get from 8800 to 1100. 6 hours is 3 half lives, so 2 hours is 1 half life.
3) Radioactive decay is random, and the effect of randomness on the results will be greater for lower activities. Therefore, a single reading of the count rate does not necessarily indicate that the count rate will not increase again.
4) The process of radioactive decay is unpredictable and occurs randomly. Therefore, the count rate would not be constant, and there will be variations with each reading.
