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
