Atomic Structure Flashcards
Give an approximation for the radius of an atom.
1x10⁻¹⁰ metres
What are the three subatomic constituents of an atom?
proton, neutron, electron
Where is most of the mass of an atom located?
inside the nucleus
Approximately what proportion of the total radius of an atom is the radius of the nucleus?
1/10,000
Describe the arrangement of protons, neutrons and electrons in an atom
● The protons and neutrons are found in the atom’s nucleus
● The electrons are found in discrete energy levels around the nucleus
What type of charge does the nucleus of an atom have ? Why?
● Positive charge
● The nucleus contains protons and neutrons
● Protons have a positive charge
● Neutrons have no charge
Give two ways that an atom’s electron arrangement can be changed.
- absorbing electromagnetic radiation
- emitting electromagnetic radiation
What is EM radiation?
energy waves
Explain how an atoms electron arrangement changes when it absorbs EM radiation
● Electrons move further away from the nucleus
● They move to a higher energy level
Explain how an atom’s electron arrangement changes when it emits EM radiation
● Electrons move closer to the nucleus
● They move to a lower energy level’s
How does the ratio of electrons to protons in an atom result in the atom having no overall charge?
● The number of protons is equal to the number of electrons
● Protons and electrons have equal and opposite charges, so charge cancels
What do all forms of the same element have in common?
They all have the same number of protons.
What is the name given to the number of protons in an atom?
atomic number
What is the atomic mass number?
Number of protons and neutrons in an atom
What is an isotope of an atom?
An atom of an element that has a different number of neutrons, but the same number of protons.
How do atoms turn into positive ions?
● They lose one or more of their outer electrons
● Electrons are negatively charged, so the resultant charge of the atom is positive
What may lead to a scientific model being changed or replaced?
The discovery of new experimental evidence which doesn’t agree with the existing theory.
How did the plum-pudding model describe the atom?
A ball of positive charge, with negatively charged electrons distributed evenly throughout it.
Prior to the discovery of the electron what was believed about the atom?
The atom was believed to be indivisible.
Which experiment led to the plum-pudding model being discarded?
Rutherford’s alpha-scattering experiment
What is the name given to the currently accepted model of the atom?
The Bohr nuclear model.
State the conclusions of the Alpha-Scattering experiment.
● Most of the mass of the atom is concentrated at the centre in the nucleus
● The nucleus is positively charged
What reinforces a scientific theory?
When experimental results agree with the hypothesised theoretical calculations and theories.
What did James Chadwick’s experiments on the atom prove?
the existence of neutrons
Why do unstable nuclei give out radiation?
● Unstable nuclei undergo decay to become more stable
● As they release radiation their stability increases
What is radioactive decay?
The process in which unstable isotopes decay into other elements and emit radiation as they attempt to become more stable
Define the activity of an unstable nucleus
Activity is the rate of decay of a source of unstable nuclei.
What is the unit of radioactivity?
Becquerel (Bq)
What is count-rate?
The number of radioactive decays per second for a radioactive source.
Give an example of a detector that may be used to measure count-rate
Geiger-Muller tube
State four types of nuclear radiation.
Alpha particles, beta particles, gamma rays and neutrons
What are the constituents of an alpha particle?
two protons and two neutrons - the same as a helium nucleus
What is the range of an alpha particle through air?
A few centimetres (normally in the range of 2-10cm).
What will stop beta radiation from passing through a point?
● A thin sheet of aluminium
● Several metres of air
What will stop gamma radiation from passing through a point?
● Several centimetres of lead
● A few metres of concrete
Which type of radiation is the most ionising?
alpha radiation
Which type of radiation is the least ionising?
gamma radiation
State any changes to mass or charge that occur due to the emission of a gamma ray.
Both mass and charge remain unchanged.
Describe the nature of radioactive decay
● Random
● Which nuclei decays and when is determined only by chance
● It is impossible to predict which nuclei will decay and when
Define the half-life of a radioactive isotope.
● The time it takes for the number of unstable nuclei in a substance to halve
● The time it takes for the count rate from a sample to fall to half its initial level
What is radioactive contamination?
The presence of unwanted radioactive nuclei on other materials.
What is irradiation?
The process of exposing an object to nuclear radiation. The irradiated object does not become radioactive
Why is it important for the results of studies on the effects of radiation to be published and shared with other scientists?
● To allow the findings to be independently checked
● This is known as peer review
Give 4 sources of background radiation
- rocks
- cosmic rays from space
- nuclear weapon testing
- nuclear accidents
How should background radiation be dealt with in calculations?
The background count should be subtracted from any readings before calculations (half life etc.) are attempted.
What is the unit used to measure radiation dosage?
Sieverts (Sv)
How many millisieverts (mSv) make up 1 sievert (Sv)?
1000 millisieverts (mSv) = 1 sievert (Sv)
Why might the radiation dosage that different people experience differ?
● Some occupations involve working with radiation
● Background radiation differs with location due to things such as the locality of nuclear power stations or radiation related testing
What factors determines how dangerous a particular radioactive isotope is?
The half-life of the isotope
Why are isotopes with long half-lives particularly harmful?
● They remain radioactive for much longer periods of time
● They must be stored in specific ways to avoid humans and the environment from being exposed to radiation for too long
State two uses of nuclear radiation in the field of medicine
- examining internal organs
- controlling and destroying unwanted tissue
How is radiation used in sterilisation?
Gamma emitters are used to kill bacteria/parasites on equipment.
Explain the process of radiotherapy
● Gamma emitters direct gamma rays at the cancerous cells
● The cancerous cells absorb the radiation and are killed
How are medical tracers chosen?
They should have a short half life and decay into a stable isotope which can be excreted.
They should only release gamma radiation since it is weakly ionising and can easily pass through body tissue without damaging it.
What is nuclear fission?
The splitting of a large, unstable nuclei to form smaller more stable nuclei
Give an example of fissionable isotope
Uranium-235
What usually needs to happen to induce fission?
● The unstable nuclei must absorb a neutron
● Spontaneous fission (where no neutron absorption occurs) is rare
Alongside two smaller nuclei, what else is emitted in a fission reaction?
● Two or three neutrons
● Gamma rays
● Energy
What forms of energy do all fissions products have?
Kinetic energy
What takes place during a chain reaction in a nuclear reactor?
● An unstable nucleus absorbs a neutron
● The nucleus undergoes fission and releases 2 or 3 further neutrons
● These induce more fission, which results in a chain reaction
What is the consequence of an uncontrolled chain reaction?
● The rate of fission events becomes to high and results in the production of too much energy
● This can lead to a nuclear explosion
What are the three main components of the core a nuclear reactor?
fuel rods, control rods, moderator
What is the role of the moderator in a nuclear reactor?
To slow down the neutrons so they are travelling at speeds which allow them to be absorbed by fissile nuclei and cause fission.
How is the chain reaction in a fission reactor kept under control?
● Control rods are positioned in between the fuel rods
● The rate of fission is controlled by moving these rods up and down
● The lower the rods are inserted, the slower the rate of fission
What is fissile nuclei?
Nuclei in which fission can be induced through the absorption of slow neutrons
What is nuclear fusion?
The joining of two light nuclei to produce a heavier nuclei and release energy
Name two isotopes of hydrogen which are commonly used in nuclear fusion
deuterium and tritium
Which releases more energy, fusion or fission?
nuclear fusion
Explain the difficulty of generating energy through nuclear fusion.
Fusion requires very high temperatures which in itself requires large quantities of energy and also requires casing which can withstand them.
Explain why nuclear fusion is currently not a viable way to produce energy on Earth.
With current equipment/techniques, the energy required is greater than the energy produced, resulting in a net energy loss.
