Atomic structure

Question 1

What is the radius of an atom?

Answer 1

1 × 10^-10 metres.

Question 2

What is the radius of a nucleus?

Answer 2

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

Question 3

How do electrons gain energy and what happens if they do?

Answer 3

• by absorbing EM radiation
• they move to a higher energy level, further from the nucleus

Question 4

How do electrons lose energy?

Answer 4

• release EM radiation
• they move to a lower energy level that is closer to the nucleus

Question 5

What is the atomic number?

Answer 5

The number of protons

Question 6

What is the mass number?

Answer 6

The total number of protons and neutrons

Question 7

What are isotopes?

Answer 7

A different form of the same element with the same number of protons but different number of neutrons

Question 8

Before the discovery of the electron, what were atoms thought to be?

Answer 8

Democritus - tiny spheres that could not be divided

Question 9

Who discovered electrons?

Answer 9

J J Thomson

Question 10

Who made the plum pudding model?

Answer 10

J J Thomson

Question 11

What was the plum pudding model?

Answer 11

Suggested that an atom was a ball of positive charge with negative electrons embedded in it

Question 12

What was the Rutherford alpha particle scattering experiment?

Answer 12

fired a beam of alpha particles at thin gold foil

Question 13

What happened in the alpha particle scattering experiment?

Answer 13

• most of the alpha particles did go straight through
• some deflected more than they had expected
• a small amount of particles came straight back at them

Question 14

What were the conclusions of the Rutherford scattering experiment?

Answer 14

1. most of the atom must be empty space because most of the alpha particles passes straight through the foil
2. The nucleus must have a large positive charge as some positively-charged alpha particles were repelled and deflected by a big angle
3. The nucleus must be concentrated in a small area
   at the centre as very few alpha particles were deflected back

Question 15

How did Bohr adapt the nuclear model?

Answer 15

He suggested that the electrons orbit the nucleus at specific distances

Question 16

Who discovered neutrons?

Answer 16

James Chadwick

Question 17

Why was Bohr’s model accepted?

Answer 17

Bohr’s theoretical calculations were found to agree with experimental data so the model was accepted

Question 18

What is radioactive decay?

Answer 18

When an unstable nuclei gives out radiation randomly to become more stable

Question 19

What is activity and what is it measured in?

Answer 19

The rate at which a source of unstable nuclei decays

Bequerels (Bq)

Question 20

What is the count rate?

Answer 20

the number of decays recorded each second by a detector (Geiger Muller tube)

Question 21

What is an alpha particle?

Answer 21

Consists of 2 neutrons and 2 protons

So decreases the atomic number by 2

decreases the mass number by 4

Question 22

What is a beta particle?

Answer 22

a fast moving electron released from the nucleus as a neutron turns into a proton

increases the atomic number by 1

Question 23

What are alpha particles used in and how?

Answer 23

Smoke detectors - ionises air particles, causing a current to flow

If there is smoke in the air, the smoke binds to the ions, reducing the number available to carry a current

The current falls and the alarm sounds

Question 24

What is the penetration through materials, range in air and ionising power of alpha particles?

Answer 24

low penetration
range in air >5cm
high ionising power

Question 25

What are beta particles used for and how?

Answer 25

used to test the thickness of thin sheets of metal, as the particles are not immediately absorbed by the material like alpha radiation would be, and do not penetrate as far as gamma rays

Question 26

What is the penetration through materials, range in air and ionising power of beta particles?

Answer 26

penetration is more than alpha but less than gamma ionising is more gamma but less than alpha range in air is about 1m absorbed by aluminium (5mm)

Question 27

What are gamma rays used for?

Answer 27

detecting cracks or blockages in underground pipes or flaws in thick sheets of metal

Question 28

What is the penetration through materials, range in air and ionising power of gamma rays?

Answer 28

high penetration low ionising range in air (>1km) absorbed by lead

Question 29

What is the half-life?

Answer 29

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 from a sample containing the isotope to half

Question 30

What is irradiation?

Answer 30

The process of exposing an object to nuclear radiation. The irradiated object does not become radioactive

Question 31

What is radioactive contamination?

Answer 31

the prescence of unwanted radioactive atoms on or inside an object

Question 32

Why is contamination hazardous?

Answer 32

the contaminating atoms might then decay, releasing radiation which could cause harm

Question 33

How do you prevent irradiation?

Answer 33

- Keep sources in lead lined boxes - standing behind barriers or being in a different room

Question 34

How do you prevent the risk of contamination?

Answer 34

- gloves - tongs

Question 35

What is radiation dose?

Answer 35

A measure of the risk of harm to your body due to exposure to radiation The higher the radiation dose, the more at risk you are of developing cancer

Question 36

What is radiation dose measure in?

Answer 36

siverts (Sv) 1000millisieverts (mSv) = 1 (Sv)

Question 37

Where does background radiation come from?

Answer 37

- natural sources such as rocks and cosmic rays from space - man-made sources such as nuclear weapons testing and nuclear accidents

Question 38

What affects the amount of radiation you're exposed to?

Answer 38

Location: certain underground rocks such as granite and some other rocks which release radioactive radon gas which could be in pipes, high altitudes as more cosmic rays Occupation: Nuclear industry workers, radiographers, underground

Question 39

How do low doses of ionising radiation damage cells?

Answer 39

ionisation inside the cell leads to damage Damaged cell divides uncontrollably, forming a tumour

Question 40

How do high doses of ionising radiation damage cells?

Answer 40

Ionisation causes lots of damage to the cells, causing them to die

Question 41

What does the extent of the harmful effects of radiation depend on?

Answer 41

- how much exposure you have to the radiation - the energy and penetration of the radiation

Question 42

What are medical uses of radiation?

Answer 42

Medical tracers Radiotherapy

Question 43

What is nuclear fission?

Answer 43

- unstable nucleus absorbs a neutron - the nucleus splits into two smaller nuclei and emits two or three neutrons plus gamma rays, energy is released by the fission reaction - 1/3 is absorbed by another nucleus causing a chain reaction

Question 44

How is nuclear fission controlled?

Answer 44

Control rods are lowered and raised, absorbing the neutrons (cadmium/boron), less neutrons so less fission reactions and less energy produced Moderator - graphite or water which slows the neutrons down so they are more likely to be absorbed and reducing the rate of reaction Coolant - rapidly carry energy away from the core

Question 45

What is nuclear fusion?

Answer 45

Joining of two light nuclei at a high speed to form a larger, heavier nucleus Hydrogen nuclei can fuse to produce a helium nucleus

Question 46

Why does the heavier nucleus produced by fusion not have as much mass as the two separate light nuclei?

Answer 46

Some of the mass of the lighter nuclei is converted into energy which is then released as radiation

Question 47

Why can't we do nuclear fusion?

Answer 47

High temperature and high pressures are needed so fusion reactors are really hard and expensive to build.