Atomic structure and radioactivity

What are the basic features of a atoms structure?

• The atom size is 1×10^-10cm

• The nucleus is less than 1/10000 of the radius of the atom

• The nuclues contains protons (positive) and neutrons (neutral) meaning the nuclues has a positive charge

• Surrounded by electrons (negative) in energy levels

• Atoms have a overall negative charge

What are happens to the energy levels further from the nucleus?

They have higher energy then shells closer to the nuclues

How do electrons change energy levels if they lose or gain energy?

• If the atom absorbs electromagnetic radiation an electron can move from a lower energy level to a higher energy level

• The atom can now emit electromagnetic radation and the electron returns back to the lower energy level

• The electromagnetic radation by transfering energy to the or taking it back

What does the atomic number show?

The number of protons and electrons

What does the mass number show?

Protons and neutrons

How do we find the number of neutrons?

Subtract the atomic number and mass number

What are isotopes?

Isotopes of atoms of the same element that have different numbers of neutrons

What are ions?

Atoms can lose electrons from their outer shells which are shown with postive or negative ions

Loose electron = +1 charge

Gain electron = -1 charge

What did ancient greeks belive atoms where?

Tiny spheres which cannot be divided that make up everything

What was discovered in 1897 by scientist?

Atoms contain tiny negative particles called electrons

This showed atoms must have an enternal structure

What did the plum pudding model suggest?

An atom is a ball of positive charge with negative electrons embedded in it

Describe the alpha-scattering experiment

• Take a piece of gold foil

• Then fire tiny alpha particles (positive charge) at the gold foil

Why is gold foil used in the alpha-scattering experiment?

Gold can be hammered thin

What happened in the alpha-scattering model? What does the mean?

• Some particles passed straight through

  • Told us atoms are mainly empty space

• Some particles changed direction as they passed through the foil

  • The centre of an atom must have a positive charge meaning they must have reflected (both are postitive)

• Some particles bounced off the foil

  • The mass of the atom must be concentrated in the centre which is now called the nucleus

Describe the nuclear model before Niel Bohr

What did Niels Bohr suggest?

• Electrons orbit around the nucleus at specific distances

• These orbits are now called energy levels/shells

Describe the updated nuclear model after Neils Bohrs discovery

What did James chadwick discover?

Neutrons

What do some isotopes have?

A unstable nucleus

What is ionising power?

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

What is radioactive decay?

When the unstable nucleus in a isotope gives out radiation to become stable

Radioactive decay is a totally random process and is unpredictable

What is the activity? What is this measured in?

The rate at which the nucleus decays

This is measured in Becquerel (Bq)

1 Bq = 1 decay per second

What can we use to measure the rate of decay? What is the count rate?

Geiger-muller tube

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

What is the difference between count rate and activity?

Activity is the rate at which unstable nuclei decay, whereas count rate is the rate at which radioactive emissions are detected

What are the four types of radation given out while the nucleus decays?

• A alpha particle

• Beta particles

• Gamma rays

• A neutron

Describe alpha particles

• Consist of two protons and neutrons

• This means a alpha particle is the same as a helium nucleus

What are the properties of alpha particles?

• Large and can travel around 5cm in air before they collide with other air particles and stop

• Alpha particles can be stopped with a sing sheet of paper and are easy to stop → penetrating power

• Very strongly ionising

Describe beta particles

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

• A beta particle is formed inside the nucleus when a neutron changes into a proton and an electron

What are the properties of beta particles?

• Beta particles can reach around 15cm in the air before stopping

• Beta particles can be stopped by a few millimetres of aluminium → penetrating power

• Quite strongly ionising

Describe gamma rays

• Not particles

• A type of electromagnetic radiation

What are the properties of gamma rays?

• Gamma radiation can travel several metres in air before stopping

• Gamma rays can be stopped by several centimetres of lead → penetrating power

• Weakly ionisng

Describe a neutron

• Neutral charge

What does a nuclear equation show us?

Shows us whats produced when radioactive nucleus decay

What is the rule of alpha decay?

During alpha decay, the atomic number decreases by 2

During alpha decay, the atomic number decreases by 4

Example of alpha decay

• Has two protons and neutrons which is the same as a helium atom

• Alpha decay is represent by the helium symbol ⁴₂He

^226₈₈Ra → Rn + ⁴₂He

(radium) (radon) (alpha)

Atomic number = 88 -2 = 86

Mass number = 226 - 4 = 222

What is the rule of beta decay?

The atomic number increases by 1 but the mass number does not change

Example of beta decay

• In beta decay, a neutron changes to a proton and an electron

• The electron is ejected from the nucleus and becomes a beta particle (⁰_-1e)

²⁹₁₃Al → _?Si + ⁰_-1e

Atomic number = 13 + 1 = 14

Mass number = stays the same (29)

What is the rule of gamma decay?

Both the atomic number and the mass number are not changed at all

What are the two definitions of half-life?

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

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 inital level

What is a long and short half-life?

A long half-life means it takes a while for the nuclues to decay in half whilst a short long-life means it occurs quickly

How can we work out the half-life from a graph?

Half the number of undecayed nuclei and find the point of the graph where this number lines up with the time and find that number

A radioactive isotope has a half-life of 15 days and an initial count rate of 200 counts per second. Determine the count rate after 45 days

15 goes into 45 3 times

So half 200 3 times

200 counts

100 counts

50 counts

What is the risk of ionising radation?

Can lead to a risk of cancer

What is irradiation?

Exposing an object to nuclear radation eg sterelisation

How are objects sterilised?

• The object to be sterilised is wrapped in plastic to stop bacteria from entering

• Place the object under a radioactive isotope in a lead shield to protect workers

• The gamma radation irradiates the object

• This kills any bacteria present

What is the key fact about irradation?

Doesnt become radioactive as the objecy only comes in contact with the radation and not the actual isotope

How can precautions be taken to avoid the affects of ionising radiation?

• Shielding eg lead apron to stop beta and gamma radiation or lead walls and glass

• Monitoring

What is radioactive contamination?

When unwanted radioactive isotopes end up on other materials

Why is contamination hazardous?

The radioactive atoms decay and emit ionising radiation

How dangerous is it to be contaminated by alpha radiation?

Strongly ionising but easily stopped by dead cells on the skin surface

Can be dangerous if swallowed or inhaled

How dangerous is it to be contaminated by beta radiation?

Quite ionising and can penetrate skin into the body

How dangerous is it to be contaminated by gamma radiation?

Weakly ionising and can penetrate the body but likely to pass straight through

What is peer view?

Findings scientist have made checked by other scientist