Topic 4 - Atomic Structure and Radioactivity Flashcards
Give an approximation for the radius of an atom.
1x10−10 metres
Where is most of the mass of an atom concentrated?
In the nucleus.
Approximately what proportion of the total radius of an atom is the radius of the nucleus?
1/10000
Describe the arrangement of protons, neutrons and electrons in an atom.
- the nucleus contains protons and neutrons
- the is a positive charge
- the nucleus is surrounded by electrons in energy levels
energy levels
Electrons lie at different distances from the nucleus (different energy levels).
What type of charge does the nucleus of an atom
have? Why?
-The nucleus contains protons and neutrons
=Positive charge
Protons have a positive charge
Neutrons have no charge
explain how energy levels affect energy
energy levels which are further away from the nucleus are at a higher energy
Give two ways that an atom’s electron arrangement can be changed.
Absorbing electromagnetic radiation
Emitting electromagnetic radiation
Explain how an atom’s electron arrangement
changes when it absorbs EM radiation.
atoms can absorb electromagnetic radiation this excites the electrons and cause if to move from a lower energy level to a higher energy level
Explain how an atom’s electron arrangement
changes when it emits EM radiation.
when atoms can emit electromagnetic radiation. it loses the energy and returns back to its original energy level
How does the ratio of electrons to protons in an atom result in the atom having no overall charge?
●The number of protons is equal to the number of electrons
● Protons and electrons have equal and
opposite charges, so charge cancels
What do all forms of the same element have in common?
They all have the same number of protons.
What is the name given to the number of protons in an atom?
Atomic Number
What is an atom’s mass number?
The total number of protons and neutrons in the atom.
What is an isotope of an atom?
isotopes are atoms of the same element that have a different number of neutrons but the same number of protons
How do atoms turn into positive ions?
- atoms can lose electrons from their outer shell/energy level
- Electrons are negatively charged
- so the charge of the atom is positive
What may lead to a scientific model being changed or replaced?
The discovery of new experimental
evidence which doesn’t agree with the existing theory.
How did the plum-pudding model describe the atom?
an atom is a ball of positive charge with negative electrons embedded in it
Prior to the discovery of the electron, what was believed about the atom?
atoms cannot be divided
1800 john dalton
everything was made of tiny spheres of atoms that could not be divided
1897-jj Thomson
discovery of electrons
suggested the plum pudding model
Which experiment led to the plum-pudding model being discarded?
Rutherford’s alpha scattering experiment.
rutherford discovered that
most of the atom was empty space
describe the alpha scattering particle
a beam of alpha particles was fired at a thin gold foil
MOST- alpha particles went straight through
SOME- alpha particles slightly deflected
FEW-alpha particles repelled
why did they use gold?
because they were able to hammer the gold out into thin foil
State the conclusions of the Alpha-Scattering experiment.
most alpha particles went straight through therefore the atoms are mainly empty space
some alpha particles were deflected therefore the centre of an atom must have a positive charge
some alpha particles repelled so that Most of the mass of the atom is
concentrated at the centre in the nucleus
the plum pudding model was replaced for the …
nuclear model
alpha scattering particles took place as well
Neil Bohr finalised the theory by
produced the final model of the atom that electrons orbit around the nucleus at a specific distance
What did James Chadwick’s experiments on the atom prove?
discovered that the nucleus also contains neutrons these have no overall charge they are neutral
so the updated nuclear model concludes that
electrons orbit around the nucleus at a specific point
the positive charge is due to the tiny positive particles called protons
the nucleus also contains neutrons
so atoms have no overall charge
Why do unstable nuclei give out radiation?
● Unstable nuclei undergo decay to become more stable
● As they release radiation their stability increases
define radioactive decay
some isotopes have an unstable nucleus and to become stable it gives out radiation
why can’t scientists predict when a nucleus will decay
radioactive decay is a totally random process
Define the activity of an unstable nucleus.
activity is the rate at which a source of unstable nuclei decay
What is the unit of radioactive activity?
Becquerel (Bq)
1Bq=1 decay per second
Give an example of a detector that may be used to measure count-rate.
Give an example of a detector that may be used to measure count-rate.
What is count-rate?
The number of decays per
second
State four types of nuclear radiation.
- Alpha particles
- Beta particles
- Gamma rays
- Neutrons
What does an alpha particle consist of?
Two protons and two neutrons
an alpha particle is the same as an …
It is the same as a helium nucleus
What is the range of an alpha particle through the air?
A few centimetres
beta particle
beta radiation is an electron ejected from the nucleus this happens because the neutron splits into protons and electrons
where is a beta particle formed?
inside the nucleus when a neutron splits into a proton and an electron
gamma particle
gamma rays are electromagnetic radiation that emits from the nucleus
properties of alpha particles
- alpha particles can travel few centimetres in air
- weakly penetrating
- stopped by paper
- highly ionising
What will stop beta radiation from passing through a point?
● A thin sheet of aluminium
● Several metres of air
Describe the nature of radioactive decay.
● Random
● It is impossible to predict which nuclei will decay
properties of beta particles
- can travel further they reach around 15 cm
- medium penetration
- stopped by thin sheet of aluminium
- medium ionising
properties of gamma particles
- travels very far in air before stopping
- highly penetration
- low ionising
- stopped by lead
What will stop beta radiation from
passing through a point?
● A thin sheet of aluminium
● Several metres of air
What will stop gamma radiation from passing through a point?
● Several centimetres of lead
● A few metres of concrete
Which type of radiation is most ionising?
Alpha radiation.
Which type of radiation is the least ionising?
Gamma radiation.
State any changes to mass or charge
that occur due to the emission of a gamma-ray.
Both mass and charge remain
unchanged.
Nuclear Equations alpha particles :
4(take away 4)
He
2 (take away 2 )
Nuclear Equations beta particles
0 (stays the same
e
-1 (add one)
rules for alpha decay
the atomic number - number at he bottom decrease by 2
- the mass number decrease by 4
the symbol for alpha decay
-4
α
2
rules for beta decay
the atomic number increases by one but the mass number does not change
Gamma Decay:
Does not cause the mass or charge to change.
this is because in the nucleus the high frequency electronic magnetic wave means that no particles are emitted
Define the half-life of a radioactive isotope.
● The time it takes for the number of
unstable nuclei in a substance to halve
● The time it takes for the count rate
from a sample to fall to half its initial level
So if 80 atoms fall to 20 over 10mins, the half-life?
o 80/2 = 40
o 40/2 = 20 – so two half lives in 10mins
So half-life is 5mins
What is irradiation?
The process of exposing a material to nuclear radiation
The material does not become radioactive
Medical items are irradiated
to kill bacteria
what can ionising radiation do
increase the risk of cancer
what are the precaution of ionising radiation
- shielding
- lead apron
- radiation monitor
- leave the room
What is radioactive contamination?
The presence of unwanted radioactive
nuclei on other materials.
Why is it important for the results of studies on the
effects of radiation to be published and shared with
other scientists?
To allow the findings to be
independently checked
This is known as peer review
J.J. Thomson discovered this sub-atomic particle in 1897.
electrons
James Chadwick discovered this sub-atomic particle in 1932.
neutrons
Ernest Rutherford discovered this sub-atomic particle in 1909.
proton
what is an ion
ions are charged particles that form when an atom either loses or gains electrons , ions are either positive or negatively charged
negative ions
are created when an atom gains electrons
positive ions
are created when an atom loses electrons
What subatomic particle has to be lost or gained for an atom to become charged?
Electron
Detecting Radioactivity
There are three primary ways of detecting radioactivity.
Cloud chamber
Photographic film
Geiger counter (Geiger-Muller tube)
How does photographic film detect radiation?
a bright spot appears where ionising radiation hits the film
How does the cloud chamber detect radiation?
A cloud chamber is a container full of supersaturated alcohol vapour.
how does the Geiger-Muller tube (Geiger counter) detect radiation
Ionising radiation enters a tube full of low-pressure gases.
It ionises the atoms in the gas, knocking electrons out of the atoms.
The gas can now conduct electricity and completes an electric circuit. Current flows between electrodes.
The current produces a clicking noise.
The count-rate is the number of decays recorded each second.
Practical Applications of Radiation-gamma
Sterilisation
We can use gamma radiation to sterilise medical equipment in hospitals. Sterilisation kills bacteria and viruses.
Practical Applications of Radiation-alpha radiation
Smoke detectors
A radioactive material fires alpha particles at a smoke detector.
If there is smoke between the radioactive material and the smoke detector, then fewer alpha particles will reach the detector.
This will set off the smoke alarm.
How is blood flow in the body imaged using radiation?
Gamma-emitting isotopes are injected into the body.
We can trace the gamma-emitting particles as they travel in the blood around the body.
We can use the detected signal to image the organs in the body.
Adjusting for background radiation
substance radiation=total radiation−background radiation then divide 2
If the background count rate is 85, and the initial count rate is 1065, what is the count rate at the half-life?
Substance initial count rate = 980.
So half-life is the time when the corrected count rate is 490, or the uncorrected count rate is 575.
further away it is from the nucleus
the higher the energy level
the closer it is to the nucleus
the lower the energy level
atomic number
smaller number
number of protons
number at the bottom
atomic mass
larger number
number of p and n
