Atomic Structure Flashcards
Democritus’ theory of atom
- 5th Century BC
- all matter was made up of identical lumps called ‘atamos’
Dalton theory of atom
- 1804
- agreed with Democritus that matter was made up of tiny spheres
- thought each element was made up of different type of ‘atom’
JJ Thomson theory of atom
- 1897
- discovered electrons that could be removed from atoms
- plum pudding model - atom is ball of positive charge with negative electrons embedded throughout
Alpha scattering experiment
- Rutherford - 1909
- fired beam of alpha particles at thing gold foil
- most particles went straight through sheet, some deflected in random directions, few deflected back the way they came
Conclusions of Alpha scattering experiment
- nuclear model of atom
- most mass is concentrated in centre in nucleus
- nucleus is positive - it repelled positive alpha particles
- mostly empty space as most when straight through
Niels Bohr atom theory
Elections orbiting nucleus do so at certain distances on shells/energy levels
James Chadwick atom theory
- 1932
- discovered neutron
- explained imbalance between between atomic and mass numbers
Current model of atom
- small nucleus concentrates most mass in centre, contains positive protons and neutral neutrons
- mostly empty space
- electrons orbit nucleus on shells/energy levels
- equal number of protons and electrons - no overall charge
What happens if electrons absorb EM radiation
They move to higher energy levels
What happens when electrons release EM radiation
They move to lower energy levels
Isotopes
Atoms with the same number of protons but different number of neutrons
Radioactive decay
Unstable isotopes decaying into other elements to become more stable, giving out radiation
Ionising radiation
Radiation that knocks electrons off atoms, creating positive ions
Ionising power
How easily a radioactive source can knock electrons off atoms
Types of radiation
- alpha
- beta
- gamma
What is an alpha particle
- 2 protons and 2 neutrons
- like a helium nucleus
Alpha particles penetration
Low
Alpha particles range
cm in air
Alpha particles ionising power
High
What stops alpha particles
- paper
- skin
Why are alpha particles strongly ionising
They are large
What is a beta particle
- fast moving electron released from nucleus from neutron turns into a proton
- hardly any mass
- charge of -1
Beta particle penetration
Moderate
Beta particle range
Metres in air
Beta particle ionising power
Moderate
What stops beta particles
Around 5mm of aluminum
Uses of alpha radiation
Smoke detectors - ionises air particles causing current to flow, if smoke it binds to air particles, current stops, alarm sounds
Use of beta radiation
Beta emitters - test thickness of sheets of metal as particles are not immediately absorbed by material
What is gamma radiation
Rays of electromagnetic radiation released by nucleus
Gamma radiation penetration
High
Gamma radiation range
Km in air
Gamma radiation ionising power
Low
Why is gamma radiation lowly ionising
- waves tend to pass through rather than collide with atoms
- eventually hit something and do damage
What stops gamma radiation
- sheets of lead
- metres of concrete
Form of nuclear equations
- atom before decay —> atom after decay + radiation emitted
- total mass and atomic numbers must be equal on both sides
Effect of alpha decay on nucleus
- charge decreases by 2
- mass decreases by 4
Effect of beta decay on nucleus
Charge increases by 1 (gained proton)
Effect of gamma rays on nucleus
No effect
What device measures radiation
Geiger-Muller tube and counter
What does Geiger-Muller tube and counter measure
Count-rate
Count-rate
Number of radiation counts reaching Geiger-Muller tube and counter per second
Effect of radioactive decay being random
You can’t predict which nucleus in a sample will decay next and when
Half-life
- time taken for the number of radioactive nuclei in an isotope to halve
- time taken for the activity to fall to half of its initial value
Use of half-life
- can make predictions about radioactive sources
- can find source’s activity
Activity
Rate at such radioactive source decays
What is activity measured in
Becquerels (Bq)
What is a becquerel
Om1 Bq is 1 decay per second
What happens to activity and radioactivity of a source over time
Decreases as older sources emit less radiation
When does activity reach 0
Never
Why is activity higher than count-rate
not all the radiation emitted by the sample is detected because the radiation spreads out
Axis on a half-life graph
- X - activity (Bq)
- Y - time (s)
Risk of using radiation
Ionising radiation can enter living cells and ionise their atoms, damaging (which can cause cancer) or killing them
Irradiation
Exposure to radiation
How to reduce effects of irradiation
- keeping sources in lead-lined boxes
- standing behind barriers when using sources
- keeping source in different room and handling with remote controlled arms
Contamination
Radioactive particles getting onto objects
Dangers of contamination
- contaminating atoms on skin may decay, releasing radiation and causing you harm
- radioactive particles could get inside your body
How to prevent contamination
- using gloves and tongs when handling sources to avoid particles getting stuck under nails
- wearing protective suits to stop breathing in particles
Most dangerous sources of radiation outside body
- beta
- gamma
Why are beta and gamma radiation most dangerous outside the body
They can penetrate the body and get to delicate organs
Why is alpha radiation less dangerous
It can’t penetrate skin and is easily blocked by a small air gap
Most dangerous radiation source inside body
Alpha
Why is alpha most dangerous radiation source in body
- highly ionising - can ionise and damage/kill cells
- do all their damage in localised area
Largest concern when working with alpha sources
Contamination - alpha radiation does damage inside of body
Why are beta sources less damaging in body
Radiation is absorbed over a wider area and some passes out of the body altogether
Least dangerous radiation source inside body
Gamma
Why is gamma least dangerous inside body
- mostly passes straight out
- lowest ionising power
