P4

Question 1

The radius of an atom

Answer 1

Atoms have a radius of about 1x10-10 metres

Question 2

The basic structure of an atom

Answer 2

The basic structure of an atom is a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons.

Question 3

Radius of the nucleus

Answer 3

Radius of the nucleus is 1x10-14 m - less than 1/10,000 of the radius of an atom

Question 4

Where is most of the mass in an atom?

Answer 4

Most of the mass of an atom is concentrated in the nucleus.

Question 5

How does electromagnetic radiation affect an electron?

Answer 5

Absorption of electromagnetic radiation causes electrons to move further away from the nucleus - a higher energy level.
Emission of electromagnetic radiation causes electrons to move closer to the nuceus - a lower energy level.

Question 6

Why do atoms have no overall charge?

Answer 6

The number of electrons is equal to the number of protons in the nucleus.

Question 7

What do all atoms of an element have?

Answer 7

All atoms of a particular element have the same number of protons.

Question 8

What is an element’s atomic number?

Answer 8

The number of protons in an atom of an element is called its atomic number.

Question 9

What is an element’s mass number?

Answer 9

The total number of protons and neutrons in an atom is called its mass number.

Question 10

How do atoms from positive ions?

Answer 10

Atoms turn into positive ions if they lose one or more outer electron(s).

Question 11

Define isotope

Answer 11

Isotopes of an element are atoms with the same number of protons (atomic number and charge on the nucleus) but a different number of neutrons (different mass number).

Question 12

Define unstable isotope

Answer 12

Unstable isotopes are atoms that have an unstable nuclei. These are radioactive isotopes which have an excess or absence of neutrons.

Question 13

What is radioactive decay?

Answer 13

When unstable isotopes decay into other elements and give out radiation to try to become more stable (balance the number of protons and neutrons in their nucleus).

Question 14

What were atoms thought to be before the discovery of the electron?

Answer 14

Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.

Question 15

When were electrons discovered and who discovered it?

Answer 15

1897 - J. J Thomson

Question 16

What did the discovery of electrons lead to?

Answer 16

The discovery of electrons led to the plum pudding model of the atom. The plum pudding model suggested that the atom has an internal structure and is a ball of positive charge with negative electrons embedded in it.

Question 17

When did the alpha scattering experiment take place and who carried it out?

Answer 17

1909 - Rutherford

Question 18

How was the alpha scattering carried out?

Answer 18

Rutherford used a piece of gold-foil - can hammer gold into very thin foil which is just a few atoms thick.
Fired alpha particles (positively charged particles) at the gold-foil.

Question 19

What 3 things were discovered from the alpha scattering experiment?

Answer 19

• Most of the alpha particles passed straight through the gold-foil without changing direction - atoms are mainly empty space.
• Sometimes an alpha particle was deflected (changed direction) - the centre of an atom must have a positive charge and that repelled the alpha particles.
• Sometimes an alpha particle bounced straight back - the mass of the atom is concentrated in the centre (the nucleus).

Question 20

What model replaced the plum pudding model after the alpha scattering experiment?

Answer 20

The nuclear model

Question 21

Who discovered that electrons orbit the nucleus at specific distances and updated the nuclear model?

Answer 21

Niels Bohr

Question 22

How were protons discovered?

Answer 22

Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.

Question 23

When were neutrons discovered and who discovered them?

Answer 23

1932 - James Chadwick (about 20 years after the nucleus became an accepted scientific idea)

Question 24

Define radioactive decay

Answer 24

When the nucleus gives out radiation as it changes to become more stable.

Question 25

Can we predict when radioactive decay will happen?

Answer 25

No, it is a random process.

Question 26

Define activity

Answer 26

Activity is the rate at which a source of unstable nuclei decay.

Question 27

How is activity measured?

Answer 27

Activity is measured in becquerel (Bq).

1 Bq = 1 decay per second.

Question 28

Define count rate

Answer 28

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

Question 29

Name a detector used to measure the activity of a radioactive source

Answer 29

Geiger-Muller tube

Question 30

4 types of nuclear radiation

Answer 30

• An alpha particle (α)
• A beta particle (β)
• A gamma ray (γ)
• A neutron (n).

Question 31

What is an alpha particle?

Answer 31

Consists of 2 neutrons and 2 protons, it is the same as a helium nucleus.

Question 32

What is a beta particle?

Answer 32

A high speed electron ejected from the nucleus and is formed when a neutron changes into a proton and an electron.

Question 33

What is a gamma ray?

Answer 33

Electromagnetic radiation from the nucleus.

Question 34

Penetration through materials of alpha radiation

Answer 34

Stopped by a single sheet of paper.

Question 35

Penetration through materials of beta radiation

Answer 35

Stopped by a few millimetres of aluminium.

Question 36

Penetration through materials of gamma radiation

Answer 36

Stopped by several centimetres of lead.

Question 37

Range in air of alpha radiation

Answer 37

Large so can travel around 5cm in air before colliding with air particles and stopping.

Question 38

Range in air of beta radiation

Answer 38

Travels around 15cm in air before stopping.

Question 39

Range in air of gamma radiation

Answer 39

Travels several metres in air before stopping.

Question 40

Define ionising radiation

Answer 40

Ionising radiation is radiation that knocks electrons off atoms, creating positive ions. The ionising power of a radiation source is how easily it can do this.

Question 41

Ionising power of alpha radiation

Answer 41

Large so very strongly ionising.

Question 42

Ionising power of beta radiation

Answer 42

Quite strongly ionising.

Question 43

Ionising power of gamma radiation

Answer 43

Weakly ionising.

Question 44

What happens when radiation collides with atoms?

Answer 44

When radiation collides with atoms, that causes the atoms to lose electrons and form ions.

Question 45

Function of nuclear equations

Answer 45

Nuclear equations are used to represent radioactive decay.

Question 46

Change in mass during alpha decay

Answer 46

The mass number deceases by 4.

Question 47

Change in charge during alpha decay

Answer 47

The charge of the nucleus decreases by 2.

Question 48

Change in mass during beta decay

Answer 48

The mass does not change - protons and neutrons have the same mass.

Question 49

Change in charge during beta decay

Answer 49

The charge of the nucleus increases by 1.

Question 50

Why does the emission of gamma ray not change the mass or the charge if the nucleus?

Answer 50

They are only a way of getting rid of excess energy from a nucleus.

Question 51

Define half-life

Answer 51

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

The 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 initial level.

Question 52

Why is ionising radiation dangerous?

Answer 52

Ionising radiation can enter the living cells and ionise atoms within them. This can damage the cells (increasing the risk of cancer) or kill them off completely.