Atomic structure and Radioactivity (Paper 1)

What is the size of the radius of an atom?

1×10^-10 m

What is the size of the radius of the nucleus?

Less than 1/10 000 of the radius of an atom

Where is most of the mass of an atom concentrated ?

In the nucleus

What does the nucleus contain?

Protons (+) and neutrons (neutral) meaning that the nucleus is overall positive

What is the nucleus surrounded by?

Electrons (-) in energy levels.

How does the distance from the nucleus affect the energy of the energy level?

Energy levels which are further from the nucleus are at a higher energy than those which are closer to the nucleus.

How does the position of the electron change if the atom absorbs electromagnetic radiation?

It can move from a lower energy level to a higher energy level.

How does the position of an electron change if the atom emits electromagnetic radiation?

It moves from a higher energy level to a lower energy level.

What is the atomic number?

The number of protons in the atom.

Why do atoms have no overall charge?

Because the number of electrons is equal to the number of protons, so the positive and negative charges cancel each other out

What is the mass number?

The number of protons and neutrons in an atom?

How do we calculate the number of neutrons?

Atomic mass-atomic number

What are isotopes?

Atoms of the same element that have different numbers of neutrons.

If an atom loses 1 or more electrons what does it become?

A positive ion

If an atom gains 1 or more electron what does it become?

A negative ion

Development of atomic structure

1. Before the discovery of the electron, atoms were thought to be tiny spheres that couldn’t be divided.

2. 1897- Electrons were discovered by JJ Thompson which led to his plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.

3. Earnest Rutherford carried out the alpha particle scattering experiment which led to the conclusion that the mass of an atom was concentrated at the centre in a positive nucleus. This nuclear model replaced the plum pudding model

4. Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. This was accepted as his suggestion agreed with the results of other scientists. These orbits are called energy levels.

5. Scientists discovered that the positive charge in the nucleus is due to positive particles called protons.

6. Around 20 years after the nuclear model was first proposed, James Chadwick discovered that the nucleus also contains neutral neutrons.

Describe the alpha scattering experiment

1. The scientist took a piece of gold foil. The reason that they used gold is because in can be hammered down into very thin foil, only a few atoms thick .

2. They then fired positively charged alpha particles at the gold foil.

3. They found that most of the alpha particles passed straight through the gold foil without changing direction, some of the alpha particles were deflected and some bounced straight back off the gold foil.

4. The fact that most passed straight through meant that atoms are mainly empty space, so the plum pudding model had to be wrong.

5. Because some of the alpha particles were deflected, this meant the centre of the atom must have a positive charge that repelled the alpha particles.

6. Because some alpha particles bounced straight back, this meant that the mass of the atom must be concentrated in the centre. This central part of the atom is now called the nucleus

Compare the plum pudding model and the nuclear model

• They both have electrons

• The plum pudding model has no nucleus whereas the nuclear model has a positive nucleus.

• The plum pudding model has electrons embedded in a positively charged ‘pudding’ while the nuclear model has electrons around the edge of the atom.

• The plum pudding model assumes the atom is a solid sphere, while the nuclear model shows that the atom is mostly empty space.

What do some isotopes have?

An unstable nucleus

What is radioactive decay?

When the unstable nucleus gives out radiation in order to become stable

What type of process is radioactive decay?

Random-unpredictable

What is activity?

• The rate at which a source of unstable nuclei decay

• Measured in becquerel (Bq)

• 1 Bq= 1 decay per second

What is used to measure the activity of a radioactive source?

A Geiger-Muller tube

What is the count rate?

The number of decays recorded each second by a detector

What is an alpha particle?

A positive particle that consists of 2 protons and 2 neutrons (same nucleus as a helium nucleus)

What is a beta particle?

An electron which is ejected from the nucleus at very high speed

How do beta particles form if the nucleus doesn’t contain electrons.

When inside the nucleus, a neutron changes into a proton and an electron.

The electron is ejected from the nucleus, forming a beta particle.

What are gamma rays?

A type of electromagnetic radiation from the nucleus

What are the 4 ways nuclear radiation may be emitted?

• Alpha particle (α)

• Beta particle (β)

• Gamma waves (γ)

• Neutron (n)

How far can alpha particles travel?

• (They’re large)

• They can travel around 5 cm in air before they collide with air particles and stop.

How far can beta particles travel?

• They can travel around 15cm in air before stopping

How far can gamma radiation travel?

• They can travel several metres in the air before stopping.

What can stop alpha particles?

A single sheet of paper

(least penetrating)

What are beta particles stopped by?

A few millimetres of aluminium

What is gamma radiation stopped by?

• Several centimetres of lead

• (most penetrating)

What does ionising power mean?

When radiation collides with atoms, that can cause atoms to lose electrons and form ions.

How ionising are alpha particles?

Very strongly ionising- this means they can produce a lot of ions when they collide with a material as they cause the atoms to lose their electrons.

How ionising are beta particles?

Quite strongly ionising but not as ionising as alpha particles

How ionising is gamma radiation?

Weakly ionising

Beta particle

• 0

• e

• -1

What happens during alpha decay to the element?

• The mass number decreases by 4

• The atomic number decreases by 2

What happens during beta decay to the element?

• The mass number stays the same

• The atomic number increases by 1

What happens during gamma decay to the element?

• The mass number and atomic number aren’t changed

What is meant by 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 to halve.

OR

The half-life is also the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half it’s initial level.

Why do scientists use half-life?

Because radioactive decay is random

What is the big risk linked to ionising radiation?

It can increase the risk of cancer in humans so people who work with radioactive isotopes have got to take precautions.

What is irradiation?

Exposing an object to nuclear radiation

Why do irradiated objects not become radioactive?

Because the object only comes in contact with the radiation but not the radioactive isotope itself.

Precautions that people take if they work with radioactive isotopes

• Shielding

  • Because alpha radiation isn’t very penetrating, gloves are used to successfully protect against alpha radiation.

  • Because beta and gamma radiation are more penetrating, aprons are used to protect e.g. lead aprons.

  • However, for high levels of radiation, this might not be enough so lead walls and lead-glass screens are used.

• Monitoring- With a radiation monitor we can measure how much radiation has been received. If the person has received too much radiation, we can stop them from working with radioactive isotopes.

What’s meant by radioactive contamination?

When unwanted radioactive isotopes end up on other materials making them radioactive- they decay and emit nuclear radiation.

This is hazardous as you now have the radioactive source on you or in you and you could get a large dose of radiation.

The level of hazard depends on the type of radiation emitted.

How hazardous the radioactive contamination is based off the type of radiation.

• Alpha radiation: Strongly ionising but easily stopped by dead cells on the skin surface as they’re not very penetrating. However, alpha emitters can be very dangerous if inhaled or swallowed as the alpha particles can then crash into living cells and damage their DNA.

• Beta radiation: Quite ionising and can penetrate into the body where they can damage cells.

• Gamma radiation: Weakly ionising. They can penetrate body but are likely to pass straight through, making them less dangerous then alpha or beta radiation

Background radiation

• Radioactive rock e.g granite

• Cosmic rays

• Nuclear weapon testing

• Nuclear accidents

What are cosmic rays

Very high energy particles which travel through space and crash into Earth’s atmosphere.

What can affect your exposure to background radiation?

Your location and occupation

Exploring internal organs using a radioactive tracer. Remember thyroid gland absorbs iodine and uses it to make hormones

• To check the functioning of the thyroid gland, patient can drink radioactive iodine. (tracer)

• Radioactive iodine emits gamma radiation which passes out of the body and can be detected.

• If the scan shows that the thyroid gland has absorbed too much or too little iodine then the doctor can use thid to diagnose the patients condition.

Exploring internal organs- considerations

• The tracer must emit radiation that can pass out of the body (gamma or beta but not alpha as it’s weakly penetrating.)

• Tracer must not be strongly ionising to minimise damage to body tissue (gamma as beta is second most ionising).

• Tracer must not decay into another radioactive isotope.

• Tracer must have a short half- life so it is not present in the body for a long period.

What’s radiotherapy?

Destroying certain cancers using ionising radiation

Radiotherapy considerations

• Healthy tissue may also be damaged as the radiation passes through the body.

• When radiation source is placed inside body, radiation is targeted precisely to tumour so less risk of damaging healthy tissue.

What can ionising do do body cells?

Mutate DNS→ cancer

but if the cells receive a large enough dose then the cells can be killed off completely which is good for treating cancers.

Ways of radiotherapy

External and internal

Nuclear Fission - Chain Reaction

• Large and unstable nuclei split e.g. uranium

• Usually happens when nucleus absorbs neutron, but fission can also happen spontaneously.

1. Neutron is absorbed, triggering the nucleus to undergo fission.

2. Two smaller daughter nuclei are formed (around equal sizes).

3. 2 or 3 neutrons + gamma radiation is emitted.

4. Energy is also released.

5. Neutrons can now be absorbed by more unstable and large nuclei, triggering fission again and repeat- chain reaction.

Uses of fission when controlled- Controlled chain reaction

Used to release energy in a nuclear reactor.

Fission when uncontrolled- uncontrolled chain reaction

Example of this is the explosion in a nuclear weapon.

Nuclear fusion

• Two light nuclei e.g. hydrogen join to form a heavier nucleus.

• Some of the mass of the nuclei can be converted into energy.

• This energy is released as radiation.