Nuclear physics.

Question 1

What are the features of the ‘plum pudding model’ of the atom?

Answer 1

The ‘plum pudding’ model of the atom was proposed by JJ Thomson, who had also discovered the electron. … According to this model, the atom is a sphere of positive charge, and negatively charged electrons are embedded in it to balance the total positive charge. The electrons are like plums in a pudding.#

Question 2

What are the features of the ‘nuclear model’ of the atom?

Answer 2

Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in energy levels around the nucleus.
The table shows the properties of these three sub-atomic particles.

SEE BOOK FOR DIAGRAM.

Question 3

What were the observations in the alpha scattering experiment?

Answer 3

Some of the alpha particles emerged from the foil at different angles, and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.

Question 4

What are the relative masses of protons, neutrons and electrons?

Answer 4

Particle Relative mass Relative charge
proton 1 +1
neutron 1 0
electron 0.0005 −1

Question 5

What are the relative charges of protons, neutrons and electrons?

Answer 5

Particle Relative mass Relative charge
proton 1 +1
neutron 1 0
electron 0.0005 −1

Question 6

Why are atoms neutral?

Answer 6

When an atom is electrically neutral, it means that the overall charge of the atom is zero. Atoms are made up of positively charged particles called protons and negatively charged particles called electrons as well as non-charged particles called neutrons. These cancel each other out,

Question 7

What is ionisation?

Answer 7

Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons to form ions, often in conjunction with other chemical changes.

Question 8

What does the atomic number tell us about the make up of an atom?

Answer 8

The atomic number uniquely identifies a chemical element. In an atom of neutral charge, atomic number is equal to the number of electrons. The atomic number is closely related to the mass number, which is the number of protons and neutrons in the nucleus of an atom.

Question 9

What does the mass number tell us about the make up of an atom?

Answer 9

The mass number of an atom is the total number of protons and neutrons it contains. The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger.

Question 10

What are isotopes?

Answer 10

Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.

Question 11

What is radioactivity?

Answer 11

The emission of ionising radiation or particles caused by the spontaneous disintegration of atomic nuclei.

Question 12

What are the origins of background radiation?

Answer 12

Natural sources of background radiation include: Cosmic rays – radiation that reaches the Earth from space. Rocks and soil – some rocks are radioactive and give off radioactive radon gas. Living things – plants absorb radioactive materials from the soil and these pass up the food chain.

Question 13

What is an alpha particle?

Answer 13

A helium nucleus emitted by some radioactive substances, originally regarded as a ray.

Question 14

What is a beta particle?

Answer 14

A fast-moving electron emitted by radioactive decay of substances (originally regarded as rays).

Question 15

What is gamma radiation?

Answer 15

Gamma waves have a very high frequency. Gamma radiation cannot be seen or felt. It mostly passes through skin and soft tissue, but some of it is absorbed by cells.

Question 16

What is ionising power?

Answer 16

The ionizing power of ionising radiation measures how many ions are formed in a given area when the radiation passes through it. Alpha particles have a double charge and are very massive compared to beta and gamma they are therefore very strongly ionizing, interacting with many electrons in the orbitals of atoms before they lose their energy

Question 17

What does penetrative power mean?

Answer 17

The penetrating power of nuclear radiation depends upon the ionising power of the radiation. The penetrative power is how powerful a ray is, this varies between gamma, alpha and beta particles.

Question 18

Which types of radiation are affected by electric and magnetic fields?

Answer 18

Electric fields
Alpha particles are positively charged, beta particles are negatively charged and gamma radiation is electrically neutral. This means that alpha radiation and beta radiation can be deflected by electric fields, but gamma radiation is not deflected.

Magnetic fields
Because they consist of charged particles, alpha radiation and beta radiation can also be deflected by magnetic fields. Just as with electric fields, gamma radiation is not deflected by magnetic fields.
Check your understanding of this by trying the animation.

Question 19

What are the uses of nuclear radiation?

Answer 19

Tracers
Radioisotopes are used as tracers in industry and hospitals. They’re used to find out what is happening inside objects without the need to break into the object.
In industry they can be used to:
find leaks or blockages in underground pipes
find the route of underground pipes
track the dispersal of waste

Smoke alarms
One type of smoke detector uses Americium-241, an alpha source, to detect smoke.

Power stations
Nothing is burned or exploded in a nuclear power plant. Rather, the uranium fuel generates heat through a process called fission. Nuclear power plants are fueled by uranium, which emits radioactive substances. Most of these substances are trapped in uranium fuel pellets or in sealed metal fuel rods.

Question 20

What are the dangers associated with nuclear radiation?

Answer 20

When radiation collides with molecules in living cells it can damage them. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. The cell then goes out of control, divides rapidly and causes serious health problems.

Alpha, beta and gamma radiation
The degree to which each different type of radiation is most dangerous to the body depends on whether the source is outside or inside the body.
If the radioactive source is inside the body, perhaps after being swallowed or breathed in:

Alpha radiation is the most dangerous because it is easily absorbed by cells.

Beta and gamma radiation are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it
If the radioactive source is outside the body:

Alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body

Beta and gamma radiation are the most dangerous sources because they can penetrate the skin and damage the cells inside
Notice that these effects are opposites and make sure you get them the right way around.

Question 21

What is half-life?

Answer 21

The time taken for the radioactivity of a specified isotope to fall to half its original value.
“iodine-131 has a half-life of 8.1 days”

Question 22

How can half-life be found using a graph of count rate against time?

Answer 22

The graph shows the decay curve for a radioactive substance. The count rate drops from 80 to 40 counts a minute in two days, so the half-life is two days.

Question 23

What is nuclear fission?

Answer 23

A nuclear reaction in which a heavy nucleus splits spontaneously or on impact with another particle, with the release of energy.

Question 24

What elements are used as fuels in nuclear power stations?

Answer 24

Uranium and Plutonium

Question 25

How do chain reactions occur?

Answer 25

The fission of uranium can set up a chain reaction that will keep on releasing energy as long as there are uranium nuclei present. If this chain reaction is allowed to get out of control, energy is released very quickly and the result is a nuclear bomb. Nuclear power stations are designed to keep chain reactions under control.

Question 26

What are the functions of control rods in nuclear power stations?

Answer 26

Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning.

Question 27

What is nuclear fusion?

Answer 27

A nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy.

Question 28

Why are nuclear fusion power stations so difficult to build?

Answer 28

Extremely high temperatures are difficult to achieve, and it is difficult to safely manage the extremely hot gases produced. However, the oceans could provide almost limitless amounts of the hydrogen isotopes needed for nuclear fusion, so scientists believe that it is worth trying to design and build a fusion power station. This is so expensive that several countries are working together on the project.

Question 29

How do stars and planets form?

Answer 29

A cloud collapses to form a star and disk. Planets form from this disk. According to our current understanding, a star and its planets form out of a collapsing cloud of dust and gas within a larger cloud called a nebula.

Question 30

What is the lifecycle of a star about the same size as the sun?

Answer 30

SEE DIAGRAM ON BITESIZE OR IN BOOK.

Question 31

What is the lifecycle of a star with much more mass than the sun?

Answer 31

SEE DIAGRAM ON BITESIZE OR IN BOOK

Question 32

Why is a star stable during its ‘main sequence’ stage?

Answer 32

During its ‘main sequence’ period of its life cycle, a star is stable because the forces in it are balanced. The outward pressure from the expanding hot gases is balanced by the force of the star’s gravity. Our Sun is at this stable phase in its life.

Question 33

How are elements heavier than hydrogen formed?

Answer 33

During most of a star’s lifetime, hydrogen nuclei fuse together to form helium nuclei. As the star runs out of hydrogen, other fusion reactions take place forming the nuclei of other elements. Heavier elements than hydrogen and helium (up to iron) are formed. Elements heavier than iron are formed in supernovas.