Topic 4 physics

Atomic structure

• Most of the mass of an atom is concentrated in the nucleus

• The nucleus is made up of protons and neutrons - overall positive charge

• Electrons orbit the nucleus at different distances - energy levels or shells

• Atoms have no overall charge because the positively charged nucleus and the negatively charged electrons cancel eachother out - number of protons = number of electrons

• The radius of an atoms is 1 × 10^-10 metres

• The radius of a nucleus is less than 1/10 000 of the radius of an atom

How do electrons move between energy levels?

1) The further an energy level is from the nucleus, the more energy an electron in that energy level has

2) Electrons can move between energy levels by absorbing or releasing electromagnetic radiation

3) If an electron in an outer energy level absorbs electromagnetic radiation, it can leave the atom

4) If an atom loses one or more of its outer electrons, it turns into a positively charged ion

Mass number and atomic number

• Mass number is the number of protons and neutrons

• Atomic number is the number of protons (and electrons)

What are isotopes?

Isotopes are atoms of the same element that have the same number of protons but different number of neutrons

The development of the model of the atom

• Before the discovery of the electron, atoms were though to be tiny spheres that could not be divided (John Dalton)

• Plum pudding model - the discovery of the electron led to the 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 (JJ Thompson)

• Nuclear model - Rutherford fired positively charged alpha particles at a thin sheet of gold. From the plum pudding model, the particles were expected to pass straight through the sheet. Most of the particles did go through, but some were deflected more than expected and a small number were deflected backwards

→ Most of the alpha particles went straight through the foil. Therefore the atoms were mainly empty space

→ Some of the alpha particles were deflected. Therefore the centre of the atom must have a positive charge that repelled the alpha particles

→ Sometimes the alpha particles bounced straight back. Therefore the mass of the atom must be concentrated in the centre - called the nucleus

So Neils Bohr adapted the model by suggesting electrons orbit the nucleus at specific distances. Bohr’s work agreed with the results of experiments by other scientists. ‘Orbits’ are called shells or energy levels

• Later experiments led to the idea that the positive charge in the nucleus in due to positive particles in the nucleus called protons

• James Chadwick discovered that the nucleus contains neutrons - these have no charge and are neutral

What is radioactive decay?

• Some isotopes have an unstable nucleus

• To become stable, the nucleus gives out radiation. This is called radioactive decay

• Radioactive decay is a totally random process

What is activity?

• The activity is the rate at which a source of unstable nuclei decay (give out radiation)

• Activity is measured in Becquerel (Bq)

• 1 Bq = 1 decay per second

How can you measure activity of a radioactive source?

• By using a Geiger-Muller tube

• The count-rate is the number of decays recorded each second by the detector

What are the 4 ways radiation may be emitted by?

• An alpha particle (a) - 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 (Y) - high frequency electromagnetic wave (radiation) from the nucleus

• A neutron (n) - can be released when atoms decay to rebalance their atomic and mass numbers

What is ionising radiation and ionising power?

• Ionising radiation is radiation that knocks electrons off atoms, creating positive ions

• Ionising power is when radiation collides with atoms which causes the atoms to lose electrons and form ions

Properties of Alpha radiation

Ionising power:

• Strong

Range in air:

• 5 cm before collide with air particles and stop

Stopped by:

• Sheet of paper

Properties of Beta radiation

Ionising power:

• Moderate

Range in air:

• 15 cm before collide with air particles and stop

Stopped by:

• Sheet of aluminium

Properties of Gamma radiation

Ionising power:

• Weak

Range in air:

• Several metres

Stopped by:

• Thick sheets of lead or concrete

Nuclear equations

Alpha particle:

4,2

Beta particle:

0, -1

Gamma particle:

0, 0

What is half-life?

• The half-life of a radioactive isotope is the time it takes for the number of unstable nuclei of the isotope in a sample to halve

• 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 its initial level

What is irradiation?

• Irradiation is exposing an object to nuclear radiation (alpha, beta, gamma or neutrons)

• Used for STERILIZATION (kill bacteria) of medical equipment

How could a needle be sterilized?

1) Place object in a plastic wrapper to stop bacteria entering

2) Place needle near a radioactive isotope that emits gamma radiation

3) Workers should go inside a lead sheild to protect themselves

4) Gamma radiation kills any bacteria present - THE OBJECT DOES NOT BECOME RADIOACTIVE → object only comes in contact with the radiation not the radioactive isotope

What is the risk of ionising radiation?

• Ionising radiation can increase the risk of cancer

• People who work with radioactive isotopes must take precautions

How can Alpha, Beta and Gamma radiation be stopped?

• Alpha radiation has a low penetrating power so can be stopped by wearing gloves

• Beta and gamma radiation are more penetrating so can be reduced using a lead apron or shielding can be used (e.g lead walls and lead-glass screen)

What is a radiation monitor?

Measures how much radiation has been received

What is radioactive contamination?

• When unwanted radioactive isotopes end up on other materials

• This is hazardous as the radioactive atoms decay and emit ionising radiation - you have the radioactive source on you

Irradiation and contamination - Alpha sources

Irradiation:

• Least dangerous - alpha sources cannot penetrate the skin and is stopped by a small air gap

Contamination:

• Most dangerous - strongly ionising but easily stopped by dead cells on the surface. Alpha emittes can be dangerous if inhaled or swallowed

Irradiation and contamination - Beta sources

Irradiation:

• Dangerous - can penetrate the body and damage delicate organs

Contamination:

• Less dangerous - can penitrate the skin into the body but its aborbed over a wider area

Irradiation and contamination - Gamma sources

Irradiation:

• Least dangerous - can penetrate the body and damage delicate organs

Contamination:

• Most dangerous - mostly pass straight out of the body

Research about radiation

It is important that research about radiation is published. The data is peer-reviewed and can become accepted, leading to improvement in how we protect ourselves