Topic 4 - Atomic Structure Flashcards
What can new experimental evidence of the atom lead to?
Scientific models being replaced or developed
Before the discovery of the electron, what were atoms thought to be?
Tiny spheres that could not be divided
What did the discovery of the electron lead to?
The plum pudding model of the atom - suggesting that the atom was a ball of positive charge with negative electrons embedded in it
What did the alpha particle scattering lead to?
The conclusion that the mass of an atom was concentrated at the centre and was charged (nucleus) - this nuclear model replaced the plum pudding model
How did Niels Bohr adapt the nuclear model?
He suggested that electrons orbit the nucleus at specific distances - the theory agreed with experimental observations
What happened after Niels Bohr adapted the nuclear model?
Experiments led to the idea that positive charge of the nucleus can be subdivided into a number of smaller particles each with the same amount (protons)
What did the experimental work of James Chadwick show?
Provided evidence of the existence of neutrons in the nucleus
When did James Chadwick release his experimental evidence?
20 years after the nucleus became an accepted scientific idea
What happened in the alpha scattering experiment?
Alpha particles were fired though a thin sheet of gold:
- 99% went through (most of atom is empty space)
- 1% were deflected (passed near a positive nucleus)
- 1/10,000 bounced straight back (showing size of nucleus [1/10,000])
What happens to an unstable atomic nuclei?
The nucleus gives out radiation as it changes to become more stable (radioactive decay)
What is radioactive activity?
The rate at which a source of unstable nuclei decays
Measure in becquerel (Bq)
What is count-rate?
The number of decays recorded each second by a detector
Give an example of a count-rate detector.
Geiger-Muller tube
What are the three types of nuclear radiation emitted?
Alpha particle (2 protons, 2 neutrons - like a helium nucleus) Beta particle (high speed electron released from nucleus as a neutron turns to a proton) Gamma ray - electromagnetic waves
What are the features of an ALPHA particle?
- 2 protons, 2 neutrons
- charge = +2
- mass = 4
- range = 5-10 cm
- stopped by: thin sheet of paper
- ionisation: (very) will draw electrons - high charge + mass
What are the features of a BETA particle?
- fast moving electron
- charge = -1
- mass = 1/2000
- range = up to 1m
- stopped by: thin sheet of aluminium (5mm)
- ionisation: (low) repels electrons - low charge + mass
What are the features of a GAMMA ray?
- electromagnetic waves
- charge = 0
- mass = 0
- range = infinite
- stopped by: thick lead
- ionisation: won’t ionise unless it collides with an electron - no charge or mass
What do nuclear equations represent?
Radioactive decay
How will an alpha particle be represented in a nuclear equation?
4
(Alpha sign - backwards fish)
2
How will a beta particle be represented in a nuclear equation?
0
e
-1
What emission causes no change to the mass or charge of the nucleus?
Gamma rays
What is the half-life of a radioactive isotope?
The time it takes for the count-rate from a sample containing the isotope to fall to half of its initial level
Is radioactive decay random?
Yes
What does a long half life mean?
The count rate falls more slowly because most of the nuclei don’t decay for a long time
Is a long half life dangerous?
Yes
Nearby areas are exposed to radiation for millions of years
How do you find half life on a graph?
Find the time interval (x-axis) that corresponds to half of the overall activity on the y-axis
What is radioactive contamination?
The unwanted presence of materials containing radioactive atoms
What is the hazard of radioactive contamination?
Due to the decay of the radioactive atoms - the type of radiation emitted affects the level of hazards
What is irritation?
The process of exposing an object to nuclear radiation - the irritated object does not become radioactive
What is the most dangerous radiation outside of the body?
Beta and gamma - can penetrate body and get to delicate organs (alpha can’t and is easily blocked by a small air gap)
Inside the body, what is the most dangerous radiation?
Alpha - they do their damage in a localised area so contamination is the major concern
Why is it important that reader has is published?
Because we understand how radiation affects our bodies, we can better protect ourselves.
Data is peer-viewed and will be quickly accepted leading to improvements in our use of radioactive sources
Where does background radiation come from?
- natural sources (;rocks and cosmic rays from space)
- man-made sources (;fallout from nuclear weapon testing and nuclear accidents)
What affects background radiation dose?
Occupation and/or location
Why do the hazards associated with radioactive material differ according to the half-life involved?
Radioactive isotopes have a wide range of half-life values - some will be long and therefore be exposed for millions of years, causing deformities and mutations in plants and animals
What are nuclear radiations used for in medicine?
- exploration of internal organs
- control or destruction of unwanted tissue
What are the risks and benefits of radioactive
B: -gamma rays are directed carefully and in the right dosage to kill cancer cells without killing too many normal cells.
R: -damage is inevitably done to normal cells, making patient feel very ill
What are the benefits and risks of medical tracers?
B: -can indicate whether internal structures are working as they’re supposed to, and diagnose life threatening conditions
R: - (small risk) cancer
What is nuclear fission?
The splitting of large and unstable nucleus
How often does fission occur?
Rarely - usually for fission to occur the unstable nucleus must first absorb a neutron
What happens in nuclear fission?
The nucleus splits into two smaller nuclei, roughly equal sizes, and emits two or three neutrons plus gamma rays. Energy is released by fission reaction
What do all fission products have?
Kinetic energy
The neutrons may start a chain reaction, what is a chain reaction?
(Caused by neutrons) controlled in a nuclear reactor to control energy released.
E.g. Explosion caused by nuclear weapon is caused by an uncontrolled chain reaction
What is nuclear fusion?
The joining of two light nuclei to form a heavier nucleus
What happens in nuclear fusion?
Some mass may be converted into the energy of radiation
