4.4- Atomic Structure and Radiation

What was the first model of the atom

Hard Spheres which can't be broken down any further
John Dalton

Second Model of the atom

Plum Pudding Model- Positive sphere with electrons dotted around and embedded within- electrons are the plums
JJ Thompson

Experiment which caused developments

Alpha Particle Scattering experiment- Alpha particles were fired at gold foil (they have a positive charge). It was expected that the majority of the alpha particles would've been reflected , if the plum pudding model was true
However, the majority passed through, proving atoms were mostly empty space. A few were deflected and a few were reflected proving the positive charge was concentrated in the centre in the nucleus, and that the mass was concentrated in the centre of the atom

Third model of the atom

Nuclear Model- Positive nucleus surrounded by electrons orbiting
Rutherford

Fourth model of the atom

Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.

What was later discovered about the nucleus

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.

Final model of the atom

The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.

Radius of an atom in m

Radius of a nucleus in m

Charge and mass of a proton

Charge = +1
Mass = 1

Charge and mass of a neutron

Charge = 0
Mass = 1

Charge and mass of an electron

Charge= -1
Mass= 0

Describe the basic structure of an atom

• The basic structure of an atom is a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons.

• The radius of a nucleus is less than 1/10 000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus.

• The electrons are arranged at different distances from the nucleus (different energy levels).

• The electron arrangements may change with the absorption of electromagnetic radiation (move further from the nucleus; a higher energy level) orby the emission of electromagnetic radiation (move closer to the nucleus; a lower energy level).

Describe how electrons can move between energy levels

• Electrons can absorb electromagnetic radiation, gain a higher energy level, overcome the electrostatic forces of attraction between the nucleus and itself to an extent, and move to a higher energy level

• Eventually, the electron is pulled back by the forces of attraction, so releases electromagnetic radiation to reduce its energy levels and move back to a lower energy level

Isotope

Atoms of the same element with different numbers of neutrons

Mass number

The sum of total neutrons and protons in an atom
The mass of an atom relative to 1/12 of a carbon-12 atom

What happens if a nucleus is unstable

• Some atomic nuclei are unstable.

• The nucleus gives out radiation as it changes to become more stable.

• This is a random process called radioactive decay.

What is the activity

• Activity is the rate at which a source of unstable nuclei decays .

• Activity is measured in becquerel (Bq)

What is count rate

Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube).

What are the 4 types of nuclear radiation

• an alpha particle (α) – this consists of two neutrons and two protons, it is the same as a helium nucleus

• a beta particle (β) – a high speed electron ejected from the nucleus as a neutron turns into a proton

• a gamma ray (γ) – electromagnetic radiation from the nucleus

• a neutron (n).

Describe the features of alpha (α) radiation

• 2 protons and 2 neutrons (a helium nucleus)

• Minimum absorption material- sheet of paper

• Range in air- 2-3cm

• Ionising power- Strong

• Penetration - weak

• Used in smoke detectors

Describe the features of beta (β) radiation

• Electron

• Created when a neutron emits an electron to become a proton

• Minimum absorption material- sheet of aluminium

• Range in air- 2-3m

• Ionising power- Medium

• Penetration - Medium

• Used in material thickness testing

Describe the features of gamma (γ) radiation

• Electromagnetic radiation from the nucleus

• Minimum absorption material- Thick sheets of lead

• Range in air- about 90m

• Ionising power- Weak

• Penetration power- Strong

• Used in medical tracers

Rules for alpha decay equations

• Mass number decreases by 4

• Atomic number decreases by 2

Rules for beta decay equations

- Mass number remains the same
- Atomic number increases by 1

Nuclear decay equation for Radon (Rn)-219 decaying alpha particles

Nuclear decay equation for Carbon (C)-14 decaying beta particles

What is Radioactive Contamination

• Radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials.

• The hazard from contamination is due to the decay of the contaminating atoms.

• The type of radiation emitted affects the level of hazard.

What is Irradiation

• Irradiation is the process of exposing an object to nuclear radiation.

• The irradiated object does not become radioactive.

What are the precautions against radiation

Keep sources in lead lined boxes
Stand behind barriers or be in a different room to the source
Handle sources with remote controlled arms
Protective equipment such as lead aprons
Contamination showers

What is Half life

The half-life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level.

What is the random nature of radioactive decay

• It cannot be predicted when a nucleus will decay

• It cannot be predicted which nucleus will decay next

What is the half-life of a sample where the activity drops from 1,200 Bq down to 300 Bq in 10 days?

1200/300=4
4= 2 half lives (2^2=4)
2 half lives=10 days
1 half life = 5 days

Risk of radiation to humans

Radiation can enter cells and ionise atoms within it, causing DNA to mutate, which can cause cancer, or causing the cell to die

Which is most and least dangerous inside the body

Alpha most dangerous
Gamma least dangerous

Which is most and least dangerous outside the body

Gamma most dangerous
Alpha least dangerous

What does a short half life mean

Isotope will decay faster- emits high amounts of radiation to start off with but becomes safer quickly

What does a long half life mean

Isotope will decay slower- emits low amounts of radiation over a longer period of time, nearby areas are exposed for longer

What is background radiation and where does it come from

Background radiation is around us all of the time. It comes from:

• natural sources such as rocks and cosmic rays from space

• man-made sources such as the fallout from nuclear weapons testing, medical imaging (eg x rays) and nuclear accidents like Chernobyl.

The level of background radiation and radiation dose may be affected by occupation and/or location.

What is radiation dose, and what is it measured in

The risk of harm to the body tissues due to exposure to radiation
Measured in Sieverts (Sv)

What does dose depend on

- Job
- Living location

What is the maximum dosage per year for workers

20mSv

What can employers do to prevent too much dosage for their workers

Schedule workers shifts so that no worker receives too much at a time
Use badges that change colour based on how much radiation has been absorbed

What type of radiation should be used for detecting cancers, and what should the half life be

Gamma radiation- Easier to detect outside the body, and the least dangerous inside the body
Short Half life- The inside of the body is exposed to radiation for a shorter amount of time- less harm caused

What type of radiation should be used for a smoke detector

Alpha- Smoke will stop the alpha particles, so the geiger counter will notice the difference when smoke is present
Long half life- It won't have to be replaced very often

What is nuclear radiation used for in medicine

Nuclear radiations are used in medicine for the:

• exploration of internal organs

• control or destruction of unwanted tissue.

What is nuclear fission

Nuclear fission is the splitting of a large and unstable nucleus (eg uranium or plutonium).

What are the steps involved in fission

• Spontaneous fission is rare.

• Usually , for fission to occur the unstable nucleus must first absorb a neutron.

• The nucleus undergoing fission splits into two smaller nuclei, roughly equal in size, and emits two or three neutrons plus gamma rays.

• Energy is released by the fission reaction.

• All of the fission products have kinetic energy.

• The neutrons may go on to start a chain reaction.

Why must the chain reaction be controlled in fission

• The neutrons may go on to start a chain reaction.

• The chain reaction is controlled in a nuclear reactor to control the energy released.

• The explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction.

What is nuclear fusion

• Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.

• In this process some of the mass may be converted into the energy of radiation.