Topic 5- Atomic Physics

Question 1

Describe the structure of a nucleus

Answer 1

The nucleus is positively charged and is
made of protons (positive) and neutrons
(neutral).

Question 2

What are the relative charges of protons,

electrons and neutrons?

Answer 2

Protons: +1
Electrons: -1
Neutrons: 0

Question 3

What are the relative masses of protons,

electrons and neutrons?

Answer 3

Protons: 1
Electrons: 0 (0.0005)
Neutrons: 1

Question 4

Describe Rutherford’s experiment

Answer 4

● Alpha particles (charge +2) were fired at a thin
sheet of gold foil
● Most particles went straight through
● Some particles were deflected by small angles
(< 90º)
● A few particles were deflected by large angles
(> 90º)

Question 5

What are the conclusions of Rutherford’s

experiment?

Answer 5

● Most of the atom is made of empty space: most of the alpha particles passed straight through the gold foil

●The nucleus has a positive charge (protons) as some of the positive alpha particles deflected.

● The nucleus is dense and concentrated and located in a very small volume within the atom. Few alpha particles deflected at a small angle from their original and very few deflected at a large angle

Question 6

Describe Rutherford’s atomic model

Answer 6

There is a positive nucleus at the centre of an atom, with negative electrons existing in a “cloud”/region around the nucleus.

(1913)

Question 7

What is an isotope?

Answer 7

Atoms that are the same element with the same number of protons. However they have a different number of neutrons so therefore different masses.

Question 8

Describe the key features of the plum pudding model. Include the following details: a) where is the negative charge; b) where is the positive charge

Answer 8

atom is a sphere of positive charge with electrons (negative charge) dotted inside like raisins in a pudding

Question 9

What do all atoms of the same element

share?

Answer 9

The same number of protons (atomic

number).

Question 10

What is nuclear fission?

Answer 10

heavy unstable nucleus (eg. uranium or plutonium) splits into two smaller nuclei and releases energy

2/3 neutrons are also released in the process
- the energy from the neutrons powers a nuclear reactor

Question 11

What is nuclear fusion?

Answer 11

• two small nuclei fuse to form a
  different element (heavier nucleus)
  eg. hydrogen nuclei can fuse to produce a helium nucleus
• releases a lot of energy
• requires HIGH temperatures and pressures (how energy is released in the stars)

Question 12

What does Z represent?

Answer 12

The atomic (proton) number; the number
of protons in the nucleus of an atom.

Question 13

What does A represent?

Answer 13

The nucleon number; the numbers of
protons and neutrons in the nucleus of
an atom.
This is also known as the atom’s mass.

Question 14

describe the structure of an atom

Answer 14

• central nucleus made of protons and neutrons
• electrons orbit at high speed around the nucleus
• protons +, electrons (-), neutrons 0 (normally electron n = proton n.; so total charge is 0)
• protons and neutrons -> nucleons
• electrons held in orbit by force of attraction between opposite charges
• protons and neutrons bound tightly together by strong nuclear force

Question 15

what is the difference between Rutherford’s and Thomson’s model (

Answer 15

in Thomson’s model, the positive and negative charges are spread throughout the atom

Question 16

how does the Rutherford model of the atom explain why

a) most alpha particles go straight through the foil
b) some alpha particles are deflected at large angles

Answer 16

a) nucleus is extremely small

b) repelled by highly concentrated charge of nucleus

Question 17

why do the results of Rutherford’s experiment suggest the nucleus has a positive charge

Answer 17

alpha particles are positively charged and repelled by like charges

Question 18

define nuclide

Answer 18

a nucleus with a specific combination of protons and neutrons
- characterised by the mass number (A) and the atomic number (Z)

each different type of atom, lithium 7 for example, is called a nuclide

Question 19

why does radioactive decay cause a rise in temperature

Answer 19

emitted particles transfer energy to surrounding atoms when they collide with them

Question 20

what is meant by

a) fission
b) chain reaction

Answer 20

a) splitting of heavy unstable nucleus into 2 lighter nuclei

b) when emitted particles (neutrons) trigger further fission and so on

Question 21

give an example of

a) a controlled chain reaction
b) an uncontrolled chain reaction

Answer 21

a) energy release in nuclear reactor

b) explosion in nuclear bomb

Question 22

where does plutonium-239 come from and why is it so dangerous

Answer 22

formed in reactor when U-238 is bombarded by neutrons

its toxic and dust can get into the lungs

Question 23

from this data how can you tell energy is being released by the reaction (uranium-235 absorbs a neutron creating a nucleus which splits to form barium-141, krypton-92 and three neutrons)

               mass/kg neutron                            1.673x10^-27 uranium-235 nucleus      390.250x10^-27 barium-141 nucleus          233.964x10^-27 krypton-92 nucleus         152.628x10^-27

Answer 23

total mass of products is slightly less than total mass of U-235 nucleus and neutron

Question 24

how do you maintain a chain reaction in a nuclear reactor and how do you control the rate of reaction

Answer 24

• a moderator is needed (eg. graphite) to slow down the neutrons or too many get absorbed by uranium-238
• control the rate of reaction by raising/lowering control rods which absorb neutrons (contain boron/cadmium)

Question 25

what is background radiation

Answer 25

a low level of nuclear radiation always present because of radioactive materials in the environment (natural/man made)

Question 26

what are sources of background radiation

Answer 26

natural:
soil, rocks, air, building materials, food, drink, cosmic rays

man-made:
nuclear power and weapons testing, medical

Question 27

name 3 ways to detect radiation

Answer 27

Photographic film
GM tube
Cloud Chamber

Question 28

describe how photographic film is used to detect radiation

Answer 28

• more radiation absorbed by film -> darker it gets (film initially white)
• worn as badges by people who work with radiation to check how much exposure they have had

Question 29

describe how a GM tube is used to detect radiation

Answer 29

• ionising effect of radiation is used in the tube to detect radiation
• each time it absorbs radiation, it transmits an electrical pulse to the machine which makes a clicking sound
• greater frequency of clicks -> more radiation present
• *
• GM tube is a hollow cylinder filled with a gas at low pressure
• thin window made of mica at one end
• central electrode inside the GM tube
• high voltage supply is connected across the casing of the tube and the central electrode

Question 30

describe how a cloud chamber is used to detect radiation

Answer 30

• small container full of water vapour

- alpha particles create short, broad tracks while beta particles produce long wispy tracks

Question 31

discuss the random nature of radioactive emission

Answer 31

radioactive decay is spontaneous and random meaning:

• you cannot predict when a particular nucleus will disintegrate
• you cannot predict which direction a particle will be emitted
• process is unaffected by pressure, temperature, chemical change

some types of nucleus are more unstable and decay faster