Topic 6- Radioactivity Flashcards
What did J.J. Thomson discover about electrons in 1897
He discovered that electrons could be removed from atoms, so atoms must be made up of smaller bits
What did J.J. Thomson suggest about atoms
He suggested the ‘plum-pudding’ model - that atoms were spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding
How long did the “plum pudding” model theory last
Until 1909
Who disproved the “plum pudding” model and how
Rutherford and Marsden tried firing a beam of alpha particles at thin gold foil. From the plum-pudding model, they expected the particles to pass straight through the gold sheet, or only be slightly deflected
But although most of the particles did go straight through the sheet, some were deflected more than expected, and few were deflected straight back the way they had come - something the plum-pudding model couldn’t explain
Rutherford realised this meant that most of the mass of the atom was concentrated at the centre in a tiny nucleus
He also realised that most of an atom is empty space, and that the nucleus must have a positive charge, since it repelled the positive alpha particles
This lead to the creation of the nuclear model of the atom
Who tweaked Rutherford’s idea a few years later and what did he propose
Niels Bohr and he proposed a model where the electrons were at fixed orbits at set distances from the nucleus. These distances were called energy levels
What did Neil Bohr suggest about the electron and whatsthe model called
He suggested that electrons can only exist in these fixed orbits (shells),and not anywhere in between and the model is called the Bohr model
What’s our current model of the atom like
Positively charged nucleus surrounded by negatively-charged electrons
Virtually all the mass of the atom is in the nucleus. The nucleus is tiny - about 10000 Tim’s smaller than the whole atom. The nucleus contains positively charged protons and neutrally charged neutrons. The rest of the atom is mostly empty space
The negative electrons whizz around the outside of the nucleus in fixed orbits called energy levels or shells. They give the atom its overall side of around 1x10^-10m
The atoms are neutral so the number of protons = number of electrons. This is because protons and electrons have an equal but opposite relative charge.
What happens if an atom looses an electron
It becomes a positive ion
What happens if an atom gains an electron
It becomes a negative ion
What do atoms join together to form
A molecule
What is a protons relative mass and relative charge
Relative mass is 1
Relative charge is +1
What is an electrons prelative mass and relative charge
Relative mass is 0.0005
Relative charge is -1
What is a neutrons relative mass and relative charge
Relative mass is 1
Relative charge is 0
How do electrons in an atom sit
On different energy levels (shells)
Is each energy level a different distance away from its nucleus
Yes
How can an inner electron move up to a higher energy level
If it absorbs a enough Electromagnetic (EM) radiation with the right amount of energy
What happens when an inner electron moves up an energy level
It moves to an empty space or partially filled space and is said to be ‘exited’
What happens after the electron falls back into its original energy level
It will emit(lose) the same amount of energy it absorbed. The energy is them carried away by EM radiation
As you move further out from the nucleus what happends to the energy levels
They come closer together (so the difference in energy between two levels next to each other gets smaller)
Would an exited electron release more or less energy when falling from the third energy level to the second then an exited electron falling from the second energy level to the first
Less energy will be released. So the frequency of the generated radiation decreases as you get further from the nucleus
What do changes in the nucleus itself lead to produce
The production High energy, high frequency gamma rays
What can happen if an outer electron absorbs radiation with enough energy
It can move so far that it leaves the atom. It then becomes a free electron and the atom is said to have been ionised. The atom is now a positive ion. It’s positive as there are now more protons then electrons. An atom can lose more then. One electron. The more electrons it loses, the greater it’s positive charge
What is ionising radiation
Any radiation that can knock electrons off of atoms.how likely it is that each type of radiation will ionise an atom varies
What’s an isotope
A different form of the same element. Atoms with the same number of protons but with a different number of neutrons (a different mass number)
What is the number of protons in an atom called
It’s atomic number air proton number
What is the mass (nucleon) number of a an atom
The proton number + the number of neutrons in the nucleus
Finish this m
All elements have different isotopes but
But there’s only one or two stable ones
What happens to unstable isotopes
They tend to decay into other elements and give out radiation as they try to become more stable.
What’s the process of when unstable isotopes decay into other elements and give out radiation as they try to become more stable called
Radioactive decay
What do radioactive substances spit out
One or more types of ionising radiation when they decay: alpha, beta, gamma. They can also emit neutrons
What’s an alpha particle
A helium nucleus
What’s alpha radiation
When an alpha particle is emitted from the nucleus (2 neutrons and 2 protons)
What are the properties of alpha particles
Don’t penetrate very far into materials and are stopped quickly - they can only travel a few cm in air and are absorbed by a thin sheet of paper
They are strongly ionising due to their size
What’s a beta minus particle
A fast moving electron released by the nucleus. Beta minus particles have virtually no mass and a relative charge of negative one (-1)
What’s a beta plus particle
A fast moving positron. The positron is an anti particle of the electron. This means that it has exactly the same mass as the electron, but a positive charge (+1)
How ionising are the beta particles
Moderately ionising
How are beta minus particles absorbed
Beta minus particles have a range in air of a few meters and are absorbed by a sheet of Aluminium (around 5mm thick)
Explain positrons
Positrons have a smaller range, because when they hit an electron the two destroy each other and produce gamma rays - this is called annihilation and it’s used in PET scanners
What are gamma rays
EM waves with a short wavelength released by the nucleus
What happens after a nucleus has decayed
It often undergoes nuclear rearrangement and releases some energy
How do gamma rays penetrate
Penetrate Far into materials without being stopped and will travel a long distance through air
How ionising are gamma regs and why
Weakly ionising because they tend to pass through rather then collide with atoms . Eventually they hit something and do damage
How are gamma rays absorbed
By thick sheets of less
Or by meters of concrete
What are nuclear equations a way of showing
Radioactive decay by using elements symbols
What form is radioactive decay written in
Atom before decay—>atom after decay + radiation emitted
What’s the golden rule to remember about nuclear equations
The total mass and atomic mass must be equal on both sides
What happens when a nucleus emits an alpha particle
The nucleus loses 2 protons and 2 neutrons
So the mass number decreases by 4
And atomic number decreases by 2
What happens in beta minus decay
A neutron changes into a proton and electron so:
The mass number does not change - as it’s lost a neutron but gained a proton
The atomic number increases by 1 - because it has one more proton
What happens in beta plus decay (positron emission)
In beta plus decay, a proton changes into a neutron and a positron, so:
The mass number doesn’t change - as it’s lost a proton but gained a neutron
The atomic number decreases by 1 - because it has one less proton
What happens when a nucleus emits a neutron (neutron emission)
The mass number decreases by 1 - as it has lost a neutron
The atomic number stays the same
What are gamma rays a way of getting rid of
Excess energy from an atom - the nucleus goes from an exited state to a more stable state by emitting a gamma ray
What happens the the mass and atomic numbers after a gamma ray has been emitted
The mass and atomic numbers stay the same
What do radioactive sources contain that give out radiation from the nuclei of their atoms
Radioactive isotopes
How is ,how quickly unstable nuclei unstable Nuclei decay measured
Using activity and half-life
What does it mean by ‘the process of radioactivity entirely random’
This means if you you have 1000 unstable nuclei you can’t say when anyone of them are going to decay, or which one will decay next
If there is lots of nuclei what can you predict
How many will have decayed in a given time based on the half-life of the source.
What is the rate of which a source decays at called
It’s activity
How is activity measured
In becquerels(Bq). 1 Bq is 1 decay per second
What’s another way activity can be measured
Using a Geiger-muller tube, which clicks each time it detects radiation.
The tube is attached to a counter, which displays the number of clicks per second (the count rate)
What’s another way to detect radiation
Using a photographic film. The more radiation the films exposed to, the darker it becomes (just like when you expose it to light)
