P4 - Atomic Structure Flashcards
who came up with the plum pudding model?
JJ thomson
describe the plum pudding model of the atom
a sphere of positive charge with negative electrons embedded in it.
what was the “alpha scattering experiment”?
Rutherford fired a beam of alpha particles at a thin sheet of gold foil
what did rutherford expect to happen in the alpha scattering experiment?
From the plum pudding model, he expected that the alpha particles would pass straight through or be very slightly deflected
what was the result of the alpha scattering experiment, and why did it contradict the plum pudding model?
Most of the alpha particles did pass straight through, but some were deflected more than expected and a few were deflected back the way they had come. This could not be explained by the plum pudding model, as there was no concentrated centre of mass that would deflect only some of the particles back
what was concluded from the alpha scattering experiment?
Because a few alpha particles were deflected back, they realised that most of the mass of the atom must be concentrated at the centre in a tiny nucleus. this nucleus must also have a positive charge, since it repelled the positive alpha particles. they also realised that because nearly all the alpha particles passed straight through, most of an atom is just empty space. this was the first nuclear model of the atom.
what changes did Niels Bohr make to the nuclear model of the atom?
he said that the electrons orbiting the nucleus do so at certain distances called energy levels (he came up with the idea of shells around the nucleus). His theoretical calculations agreed with experimental data.
who proved the existence of the neutron, and when? What did this explain?
James Chadwick in 1932. This explained the imbalance between the atomic and mass numbers
what is the radius of an atom?
about 1 × 10 to the power of -10 m
if electrons gain energy by absorbing EM radiation, do they move to a lower or higher energy level?
higher energy level, further from the nucleus
if electrons release EM radiation, do they move to a lower or higher energy level?
lower energy level, closer to the nucleus
what are isotopes?
different forms of the same element - they have the same number of protons but a different number of neutrons
what is radioactive decay? Why does it happen?
unstable elements decaying into other more stable elements, and giving out radiation as they do so. It happens because all elements have different isotopes, but there are usually only one or two stable ones. The other unstable isotopes tend to decay into other elements and give out radiation as the try to become more stable (they try to balance the number of protons and neutrons in their nucleus to get rid of any excess energy.
what type of radiation are helium nuclei?
alpha particles
what are alpha particles made up of?
two protons and two neutrons (like a helium nucleus)
what is alpha radiation’s range in air?
a few cm - less than ten
what is alpha radiation stopped by?
a sheet of paper
how ionising are alpha particles?
very strongly
what are beta particles?
high speed electrons released by the nucleus. Beta particles have virtually no mass and a charge of -1
how ionising is beta radiation?
moderately
what is beta radiation’s range in air?
a few metres
what is beta radiation stopped by?
a sheet of aluminium (around 5mm)
what are gamma rays?
Electromagnetic waves with a short wavelength
what is gamma radiation’s range in air?
very far - kilometres
what is gamma radiation stopped by?
a thick sheet of lead or metres of concrete
when an atom emits an alpha particle, what does the mass number decrease by?
4
when an atom emits an alpha particle, what does the atomic number decrease by?
2
what is an alpha particle written as in atomic equations?
4
He (a helium nucleus)
2
when an atom emits a beta particle, what does the atomic number change by?
+1
when an atom emits a beta particle, what happens to the mass number?
it stays the same
what is a beta particle written as in atomic equations?
0
_ e (an electron)
-1
do gamma rays change the charge or the mass of the nucleus?
no
radioactive decay is random. true or false?
true
what is the half-life of a radioactive substance?
the time it takes for the number of radioactive nuclei in an isotope/sample to halve (it can also be described as the time taken for the activity (and so count rate) to fall to half of its initial value)
what is “activity” in radioactive samples?
the rate at which a source decays - the amount of decays per second
what is activity measured in?
Becquerels, Bq (where 1 Bq = 1 decay per second)
what happens to the radioactivity of a source over time?
it decreases - older sources emit less radiation
why do we use half-life as a measurement?
the activity never reaches zero, but half-life allows us to measure how quickly the activity drops off.
why is radiation dangerous?
ionising radiation can enter living cells and ionise atoms within them. This can damage the cells (which can cause things like cancer) or kill them off completely.
what is irradiation?
exposure to radiation (objects near a radioactive source are irradiated by it)
what is contamination? (in the context of radioactivity)
radioactive particles getting onto or into objects
what are some precautions to take in order to avoid contamination?
gloves and tongues should be used when handling sources, and some industrial workers wear protective suits to stop them breathing in particles
approximately, how big is the radius of an atom?
1 x 10^-10 metres
who was the first person to come up with the idea of an atom? What did their model look like?
a Greek man called Democritus first came up with the idea of an atom in the 5th century BC. He thought that all matter, whatever it was, was made up of identical lumps called “atomos”. This view wasn’t widely challenged until the 1800s
what changes did John Dalton make to the model of the atom?
In 1804 John Dalton agreed with Democritus that matter was made up of tiny spheres (“atoms”) that couldn’t be broken up, but he thought that each element was made up of a different type of “atom”
what lead Thomson to creating the plum pudding model of the atom?
He discovered particles called electrons that could be removed from atoms.
describe the nuclear model of the atom that resulted from the alpha particle scattering experiment
it was a positively charged nucleus surrounded by a cloud of negative electrons
describe the current model of the atom
The nucleus is tiny but it makes up most of the mass of the atom. It contains protons and neutrons, giving it an overall positive charge. The radius of the nucleus is about 10,000 times smaller than the radius of the atom.
The rest of the atom is mostly empty space. Negative electrons orbit the outside of the nucleus, travelling at extremely high speeds. They give the atom its overall size - the radius of an atom is about 1 x 10^-10 m.
The number of protons = the number of electrons, as protons and electrons have an equal but opposite charge and atoms have no overall charge.
Electrons in energy levels can move within (or sometimes leave) the atom. If they gain energy by absorbing EM radiation they move to a higher energy level, further from the nucleus. If they release EM radiation, they move to a lower energy level that is closer to the nucleus.
what is ionising radiation?
radiation that knocks electrons off atoms, creating positive ions. The ionising power of a radiation source is how easily it can do this.
give an example of how alpha radiation is used, and explain how it works
alpha radiation is used in smoke detectors - it ionises air particles, causing a current to flow. If there is smoke in the air, it binds to the ions - meaning the current stops and the alarm sounds
what is the symbol for beta radiation?
β
what is the symbol for alpha radiation?
⍺
what symbol is used to represent gamma radiation?
γ
what happens in the nucleus when a beta particle is emitted?
a neutron in the nucleus turns into a proton
what are beta emitters used for?
they are used to test the thickness of sheets of metal, as the particals are not immediately absorbed by the material like alpha radiation would be and do not penetrate as far as gamma rays.
how ionising is gamma radiation? why?
gamma radiation penetrates far into materials without being stopped and will travel a long distance through air. This means that they are weakly ionising because they tend to pass through rather than collide with atoms. Eventually they hit something and do damage
what are nuclear equations used for?
to show radioactive decay by using element symbols
how are nuclear equations written?
in the form:
atom before decay -> atom after decay + radiation emitted
what rule do you have to remember when writing nuclear equations?
the total mass and atomic numbers must be equal on both sides
why are gamma rays released?
they are released as a way of getting rid of excess energy from a nucleus
how can radiation be measured?
with a Geiger-Muller tube and counter, which records the count-rate (the number of radiation counts reaching it per second)
what can half-life be used for?
it can be used to make predictions about radioactive sources, even though their decays are random. Half life can be used to find the rate at which a source decays (its activity)
what happens each time a radioactive nucleus decays to become a stable nucleus?
the activity as a whole will decrease (older sources emit less radiation)
give 2 common ways of reducing the effects of irradiation
keeping sources in lead-lined boxes and standing behind barriers when using sources. In some industries, the source may be in a different room and remote-controlled arms are used to handle it
why is contamination dangerous?
the contaminating atoms might then decay, releasing radiation which could cause you harm. Contamination is especially dangerous because radioactive particles could get inside your body.
what does the seriousness of irradiation and contamination depend on?
the source (different amounts of harm can be caused depending on the radiation type.
which types of radiation are the most dangerous outside the body? Why?
outside the body, beta and gamma sources are the most dangerous. This is because beta and gamma can penetrate the body and get to the delicate organs. Alpha is less dangerous because it can’t penetrate the skin and is easily blocked by a small air gap - high levels of irradiation from all sources are dangerous, but especially from ones that emit beta and gamma
which type of radiation is most dangerous inside the body? Why is this?
inside the body, alpha sources are the most dangerous, because they do all their damage in a very localised area. Beta sources are less damaging inside the body, as radiation is absorbed over a wider area and some passes out of the body altogether. Gamma sources are the least dangerous inside the body, as they mostly pass straight out, and the have the lowest ionising power.
when working with alpha sources, is contamination or irradiation more dangerous?
contamination, rather than irradiation, is the major concern when working with alpha sources
