4. Atoms & Radioactivity Flashcards
Examples of nucleons?
Protons and neutrons
What is the majority of the mass of an atom?
The nucleus, as the mass of an electron is almost zero compared to the mass of protons and neutrons
What was Democritus’ theory about the atom?
400BC - all matter is made of small invisible units called atoms
What was Dalton’s theory about the atom?
1803 - agreed with Democritus and said atoms of the same element are identical and indestructible
What was Thomson’s theory about the atom?
1904 - Plum Pudding Model
What was Rutherford’s theory about the atom?
1911 - Nuclear Model; that most of the mass of the atom was concentrated in the nucleus which carries a positive charge
What was Bohr’s theory about the atom?
1913 - Bohr’s model, electrons orbit nucleus in certain energy levels
What was Chadwick’s theory about the atom?
1932 - discovered the neutron
Which scientists were involved with the history of the atom?
- Democritus
- Dalton
- Thomson
- Rutherford
- Bohr
- Chadwick
Which was the first sub-atomic particles to be discovered?
The electron, by Thomson
What was the Plum Pudding Model?
A sphere of positive charge (dough) with electrons dotted inside (plums)
What did the discovery of the electron show about atoms?
That atoms were not the smallest thing, as the electron was smaller than the atom
What did the Bohr model show?
- That electrons can only orbit the nucleus in certain energy levels
- Electromagnetic energy is emitted from the atom if an electron jumps from an excited energy level to one that’s closer to the nucleus
- If an electron absorbs energy it can jump to a higher energy state from the nucleus
What did later experiments suggest about the atom?
- the positive charge of any nucleus could be subdivided into a whole number of smaller individual sub-atomic particles called protons
- 1934 - Chadwick used alpha scattering experiment to provide evidence for existence of the neutron
Who was the first to call the nucleus of a hydrogen atom a proton?
Rutherford
In the scattering experiment, how could have the alpha particles been supplied and been moving in the right direction?
From radioactive matter. Surrounded by lead shielding apart from a small opening facing the way they wanted the beam
Why was the chamber evacuated where the scattering experiment took place?
So the alpha particles didn’t change direction and so no other particles would affect the results
Why should alpha particles be repelled by positive atomic material?
They both have positive charges and like charges repel
What happens when isotopes have unstable nuclei?
They break down or disintegrate
What happens when isotopes break down or disintegrate?
They give out radiation said to be radioactive
What is the process when isotopes with unstable nuclei disintegrate?
Radioactive decay
What may nuclear radiation be?
- an alpha particle
- a beta particle
- a gamma ray
- a neutron
What does an alpha particle consist of?
2 neutrons and 2 protons - the same as a helium nucleus
What is a beta particle?
When a high speed electron is ejected from the nucleus as a neutron changes into a proton
What is a gamma ray?
High energy electromagnetic radiation from the nucleus
What are nuclear equations used to represent?
Radioactive decay
What happens to the mass and charge of the nucleus during alpha decay?
Both the mass and charge of the nucleus decease
What happens to the mass and charge of the nucleus during beta decay?
The mass of the nucleus doesn’t change however the charge of the nucleus increases
What happens to the mass and charge of the nucleus during gamma decay?
The emission of a gamma ray doesn’t cause the mass or charge of the nucleus to change
What is an ion?
An atom which has lost or gained an electron
What happens when radioactive radiation strikes atoms in a substance?
They can knock an electron from one atom to another
Properties of alpha particles?
- Relatively big, heavy and slow moving
- Large relative charge (+2)
- Strongly ionising - bash into lots of atoms and knock electrons off them
Properties of beta particles?
- Quite small and move quite fast
* Moderately ionising because they have a smaller relative charge (-1)
What is the relative charge of an alpha particle?
+2
What is the relative charge of a beta particle?
-1
Properties of gamma radiation?
Weakly ionising as gamma is not charged
What is the range of alpha in air?
A few cm
What is the range of beta in air?
A few metres
What is the range of gamma in air?
Infinite
What can alpha radiation be stopped (absorbed) by?
Paper, card, skin
What can beta radiation be stopped (absorbed) by?
Sheet of aluminium (5mm)
What can gamma radiation be stopped (absorbed) by?
Thick sheet of lead or metres of concrete
Which type of radiation is most ionising?
Alpha
Which type of radiation is least ionising?
Gamma
What is the safest type of radiation to have inside the body?
Gamma as it’s the least ionising and escapes easily
What is the most dangerous type of radiation to have inside the body?
Alpha as it’s most ionising and cannot escape
What is the safest type of radiation to have outside the body?
Alpha as it’s least penetrating
What is the most dangerous type of radiation to have outside the body?
Gamma as it’s most penetrating
What is alpha used in?
Smoke detectors
How do smoke detectors work?
Alpha ionises air particles causing a current to flow. If there is smoke in the air, the smoke binds to the ions reducing the number available to carry a current. Current falls and alarm sounds
Why is alpha used in smoke detectors?
- Ionising
- Charged
- Cannot penetrate though a plastic shield
- Can be absorbed by smoke
- Shortest range
Why aren’t beta or gamma used in smoke detectors?
They are less ionising and have a larger range
Are smoke detectors safe given they contain alpha particles?
Yes as alpha has a short range and not penetrating, provided it is not pulled apart
What is beta used in?
Beta emitters test the thickness of thin sheets of metal
Why is beta used to test thickness of metal?
Particles are not immediately absorbed by the material like alpha, and do not penetrate as far as gamma
What is gamma used for?
- Tracers in medicine
- Leak detection in pipes
- Radiotherapy
- Food irradiation
What did Rutherford expect the alpha particles to do in the scattering experiment? Why?
He expected them to go through the thin sheet of atoms easily because they were moving at a very high speed
What was the material of the sheet of atoms in the scattering experiment?
Gold
What was Rutherford expecting to observe in the scattering experiment? Why?
For slight changes in the direction of the alpha particles due to them repelling from the thinly spread positive material of the atoms
Why should alpha particles be repelled by positive atomic material?
They’re both positively charged
Why did Rutherford not expect the electrons to have any noticeable effect on the movement of the alpha particles?
Alpha particles are much larger and have momentum and so need force for them to deflect their direction
What was the set-up that allowed Geiger and Marsden to observe what happened to the alpha particles in the scattering experiment?
A fluorescent screen that when hit by alpha gives off spots of light - looked at using a microscope
During the scattering experiment, which path of the alpha particles agreed with the plum pudding model?
The path which was slightly deflected
During the scattering experiment, which path of the alpha particles disagrees most strongly with the prediction of the plum pudding model?
The path which was reflected over 90 degrees -because alpha have a positive charge and repelled the positive protons in the nucleus of the gold atoms. The nucleus was so dense that it caused the alpha particles to bounce back
During the scattering experiment, only 1 in 10,000 alpha particles were deflected over 90 degrees. What does this suggest about the nucleus of an atom?
It is very small so have very few collisions with alpha particles
What does ionising mean?
When atoms are turned into ions by losing or gaining electrons
What is irradiation?
When the radiation stops as soon as the source of radiation has been removed
Example of irradiation?
When a light is turned on the room is filled with electromagnetic radiation (visible light). When the light is turns off the electromagnetic radiation is gone
What is contamination?
When the source of ionising radiation is transferred
Example of contamination?
When radioactive isotopes in solids, liquids or gases are introduced into the environment
Why is a patient more likely to get cancer through contamination than through irradiation?
In contamination the source of radiation is transferred and will stay in the body for longer. It is inside the body so is closer and therefore more intense.
