Radioactivity

Question 1

Describe the basic structure of an atom and give the relative masses and charges of each

Answer 1

Small (10000 x smaller than the whole atom) nucleus made of protons (mass 1, charge +1) and neutrons (mass 1, charge 0) orbited by electrons (mass negligibe, charge -1)

Question 2

How did modern theories about atomic structure (the nuclear model) come about?

Answer 2

Rutherford and Marsden fired alpha particles at gold foil and observed that MOST passed through but a few were deflected. This showed that atoms are mostly empty space. It proved that the ‘plum pudding’ model was inaccurate. It showed that all positive charge is in the nucleus

Question 3

Define a radioactive substance

Answer 3

An unstable material which gives out radiation from the nucleus in the form of alpha particles, beta particles or gamma rays all the time, no matter what conditions they are under

Question 4

What are the origins of background radiation?

Answer 4

Rocks (such as uranium ore), cosmic rays from space, fallout from nuclear weapons and nuclear accidents

Question 5

Define an alpha particle, a beta particle and gamma radiation

Answer 5

Alpha particles are two neutrons and two protons, the same as a helium nucleus

Beta particles are electrons from the nucleus

Gamma radiation is electromagnetic gamma waves of very high frequencies

Question 6

What happens to the nucleus when an ALPHA particle is ejected? Give the nuclear equation, using uranium-238 (92 protons) as an example

Answer 6

Nucleus loses 2 protons and 2 neutrons therefore atomic number goes down by two, mass number goes down by 4

Equation: ²³⁸₉₂U – ⁴₂ a → ²³⁴₉₀ Th + (ENERGY)

Question 7

What happens to the nucleus when an BETA particle is ejected? Give the nuclear equation, using carbon-14 (6 protons) as an example

Answer 7

Nucleus loses one electron therefore atomic number goes down by one, mass number stays the same

Equation: ¹⁴₆C – ⁰_-1ß → ¹⁴₇N + (ENERGY)

Question 8

Compare properties of alpha beta and gamma radiations in terms of relative ionising power, their penetration through materials and their range in air

Answer 8

Alpha: causes very strong ionisation, can be stopped by a piece of paper, few centimetres in air

Beta: weak ionisation, can be stopped by a few mm of aluminium, few metres in air

Gamma: very weak ionisation, can be reduced by several metres of lead and concrete, infinite range in air

Question 9

Define half-life

Answer 9

The half-life of a radioactive isotope is the average time it takes for the number of nucliei of the isotope in a sample to halve, OR the activity to halve, OR the mass of the isotope remaining to halve

Question 10

How are alpha, beta and gamma radiation each used?

Answer 10

Alpha: in paper mills, to alert a computer if the thickness of the paper is not ideal (it cannot pass through if too thick, and its energy is increased if too thin). This needs long half-lifes

Beta: the same, except in foil manufacture (alpha particles would not be able to pass through foil)

Gamma: in radioactive tracers for medical purposes because gamma rays can pass through the body without difficulty, allowing us to detect them. They are also the least damaging of the three types. They need half-lifes long enough that they can be monitored but not too long

Question 11

How does radioactive dating work?

Answer 11

An isotope in the sample with a similar half life to the age of the sample is needed

Question 12

What makes radioactive particles and waves dangerous?

Answer 12

They knock electrons out of atoms and the atoms become charged (ionised). In a living cell, this ionisation can damage or kill the cell. It damages DNA and also turns water into highly reactive H and OH radicals

Question 13

Describe what happens when each type of radiation enters a magnetic field?

Answer 13

Alpha: radiation is deflected by the magnetic field in one direction. They are difficult to deflect because they have a large mass.

Beta: radiation is deflected in the opposite direction to alpha radiation. They have a smaller mass than alpha particles.

Gamma: rays pass through undeflected because they have no mass.

Question 14

Explain what happens when each type of radiation enters an electric field

Answer 14

Alpha particles are attracted to the -ve plate as they are +vely charged (opposites attract). They have a charge twice that of beta particles but a much greater mass so deflection is less

Beta particles are attracted to the +ve plate as they are -vely charged. They deflect more than alpha particles because their mass is so much smaller

Gamma rays do not deflect because they are electromagnetic and so have no charge