What is the radius of an atom?
Around 10^-10m
What is the basic structure of an atom?
A positively charged nucleus composed of both protons and
... [Show More] neutrons surrounded by negatively charged electrons
What fraction of the atom is the radius of the nucleus?
1/10,000th
Most of the mass of an atom...
...is concentrated in the nucleus
How can electron arrangements change in an atom?
1. Absorption of electromagnetic radiation - move further from nucleus; higher energy level
2. Emission of electromagnetic radiation - closer to nucleus; lower energy level
The number of protons in an atom is equal to...
...the number of electrons in the atom
What do all atoms of the same element have in common?
They have the same number of protons
Mass number
The total number of protons and neutrons
How can atoms be represented?
Mass number on top, atomic number under
What happens if atoms lose one or more outer electrons?
They turn into positive ions
Isotope
An atom of the same element with a different number of neutrons
What may lead to a scientific model being changed or replaced?
New scientific evidence
How has the atomic model developed?
1. Tiny undivisible spheres
2. Plum pudding model
3. Nuclear model
4. Bohr's nuclear model
5. Discovery of protons
6. Discovery of neutrons
Plum pudding model
The atom is a ball of positive charge with negative electrons embedded in it
Nuclear model (alpha scattering experiment)
1. Most particles went straight through - mostly empty space, nucleus small compared to whole atom
2. Some particles deflected several degrees - nucleus positively charged as positive particles were repelled
3. A few particles deflected almost 180 degrees - nucleus contains most of the mass of the atom
Conclusion: the mass of the atom is concentrated at the centre (nucleus) and the nucleus is charged
Niels Bohr's nuclear model
Electrons orbit the nucleus at specific distances - his theoretical calculations agreed with experimental observations
Proton discovered
Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles which each had the same amount of positive charge
Neutrons discovered
The experimental work of James Chadwick provided the evidence to show the existence of neutrons
How long after the nucleus became an accepted scientific idea were neutrons discovered?
20 years
Radioactive decay
A random process in which an unstable atomic nucleus give out radiation as it changes to become more stable
Activity
The rate at which a source of unstable nuclei decays, measured in bequerel (Bq)
Count-rate
The number of decays recorded each second by a detector (eg Geiger-Muller tube)
Alpha particle
Two neutrons and two protons, the same as a helium nucleus
Beta particle
A high speed electron ejected from the nucleus as a neutron turns into a proton
Gamma ray
Electromagentic radiation from the nucleus
What are the 4 types of nuclear radiation?
Alpha particle, beta particle, gamma ray, neutron
Alpha radiation - absorber materials, range in air, ionising power
1. Thin sheet of paper
2. About 5cm
3. Most ionising power
Beta radiation - absorber materials, range in air, ionising power
1. Aluminium sheet (5mm), lead sheet (2 - 3mm)
2. 1m
3. Medium ionising power
Gamma radiation - absorber materials, range in air, ionising power
1. Thick lead sheet (several cm), concrete (more than 1m)
2. Long range - spreads in air without being absorbed
3. Least ionising power
Which radiation is used in smoke alarms and why?
Alpha radiation - ionises the air so there is a current; smoke causes a drop in current, triggering the alarm
Beta and gamma radiation do not have enough ionising power to make the air conduct electricity
Which radiation is used in metal foil production and why?
Beta radiation - detector measure radiation passing through foil; if the foil is too thick, the detector reading drops
Alpha would be stopped by the foil, gamma would pass through the foil unaffected
The emission of the different types of nuclear radiation may cause...
...a change in mass and/or the charge of the nucleus
Radioactive decay is...
...random
Half-life (2)
1. The time it takes for the number of nuclei in a sample for halve
2. The time it takes for the count rate/activity from a sample containing the isotope to fall to half its initial level
Equation for the final count rate (remaining number of unstable nuclei)
count rate (number of unstable nuclei after n half lives) = initial count rate (number of unstable nuclei)/2^n
Radioactive contamination
The unwanted presence of materials containing radioactive atoms on other materials
What is the hazard of contamination from?
The decay of the contaminating atoms
What effects the level of hazard?
The type of radiation emitted
Irradiation
The process of exposing an object to nuclear radiation; the irradiated object does not become radioactive
Why is radiation dangerous?
Radiation causes ionisation which can damage or kill the cell, causing mutations and possibly cancer
How can workers reduce their exposure to ionising radiation? (3)
1. Keep as far away from source of radiation -e.g. by using special handling tools with long handles
2. Spend as little time as possible in at-risk areas
3. Shield themselves by staying behind thick concrete barriers and/or using thick lead plates
Why is it important for the findings of studies into the effects of radiation on humans to be published and shared with other scientists?
So that the findings can be checked by peer review
Background radiation is around us...
...all the time
What 2 sources does background radiation come from?
1. Natural sources such as rocks, cosmic rays from space
2. Man-made sources such as the fallout from nuclear weapons testing and nuclear accidents
What may the level of background radiation and radiation dose be affected by?
occupation and/or location
sieverts (Sv)
The unit of radiation dose (do not need to recall for exam)
1000 millisieverts (mSv) =
1 sivert (Sv)
Radioactive isotopes have a ... range of half-life values
very wide
Why do hazards associated with radioactive materials differ according to their half life? (2)
1. Materials with a longer half life have the potential to be more dangerous as they will be emitting radiation for a long time
2. For example iodine, which emits gamma radiation, is used as a medical tracer as it has a half life of 8 days - lasts long enough for test but decays into a stable product after a few weeks so radiation does not kill cells
What are nuclear radiations used in medicine for? (2)
1. Exploration of internal organs.
2. Control or destruction of unwanted tissues
Describe the 4 uses of nuclear radiation
1. Iodine tracers
2. Gamma cameras
3. Beams of gamma radiation to cause cancerous tumours
4 Radioactive implants to destroy cancer cells
Describe the use of nuclear radiation to explore internal organs - iodine
1. Tracers are used to track the flow of a substance through an organ
2. Patient drinks water containing radioactive iodine. For a blocked kidney the count rate reading stays up as the iodine stays in the kidney
3. Iodine is used for this as it has a half life of 8 days (long enough for test, but decays after a few weeks), emits gamma radiation (can be detected outside body) and decays into a stable product
Describe the use of nuclear radiation to explore internal organs - gamma cameras
1. Patient injected with a solution containing a gamma-emitting radioactive isotope which is absorbed by the organ
2. Detector signals are used to build up an image of where the isotope is located in the organ
3. Isotope must be a gamma emitter with a long enough half life to give useful image, but short enough so that it is mostly decayed after picture
Describe the use of nuclear radiation to control and destroy unwanted tissue - beams of gamma
1. Gamma radiation in a beam destroys cancerous tumours
2. Isotope of cobalt with 5 year half life
3. Gamma is used as it can penetrate deeper into the body
Describe the use of nuclear radiation to control and destroy unwanted tissue - radioactive implants
1. Small seed/rod implants irradiate tumour
2. Beta or gamma radiation
3. Half lives long enough to irradiate tumour over a given time, but short enough that most of the nuclear will decay soon after
Risks of using radiation to treat patients
Radiation can ionise both healthy and cancerous cells, causing the patient to feel ill
Nuclear fusion and fission
don't forget to revise this if you're doing triple science!! (sorry I ran out of time to make it)
What is nuclear fusion?
The splitting of a large and unstable nucleus (e.g. uranium or plutonium)
What must usually happen for fission to occur and why?
Usually the unstable nucleus must first absorb a neutron as spontaneous fission is rare
Describe what happens in a fission reaction
1. Nucleus splits into 2 smaller nuclei roughly equal in size
2. Emits 2 or 3 neutrons plus gamma rays
3. Energy is released
What do all of the fission products have?
Kinetic energy
What may happen to the neutrons?
They may go on to start a chain reaction
What is controlled in a nuclear reactor and why?
The chain reaction is controlled in a nuclear reactor to control the energy released
How are chain reactions controlled in a nuclear reactor?
1. Water acts as a moderator to slow down fission neutrons
2. Control rods whose depths can be adjusted absorb surplus neutrons
Why is there an explosion caused by a nuclear weapon?
Due to an uncontrolled chain reaction
Nuclear fission diagram
Draw at least 2 levels! with e neutrons emittes b
What is nuclear fusion?
Nuclear fusion is the joining of 2 light nuclei to form a heavier nucleus. In this process some of the mass may be converted into the energy of radiation. [Show Less]