Atomic structure

Estimation for radius of an atom

1x10^-10 metres

What are the three subatomic constituents of an atom?

1. Proton

2. Neutron

3. Electron

Relative Mass of proton

1

Relative mass of neutron

1

Relative mass of electron

0

Relative charge of proton

+1

Relative charge of neutron

0

Relative charge of electron

-1

Where is most of the mass of an atom concentrated

nucleus

What is the proportion of total radius of atom is the radius of nucleus

1/10000

Describe arrangement of protons, neutrons and electrons in an atom

● The protons and neutrons are found in the atom's nucleus

● The electrons are found in discrete energy levels around the nucleus

What type of charge does the nucleus have and why

● Positive charge

● The nucleus contains protons and neutrons

● Protons have a positive charge

● Neutrons have no charge

Isotope

An atom with the same number of protons and a different number of neutrons from other atoms of the same element.

Give two ways that an atoms electron arrangement can be changed

1. Absorbing electromagnetic radiation

2. Emitting electromagnetic radiation

Explain how an atom's electron arrangement changes when it absorbs EM radiation.

● Electrons move further away from the nucleus

● They move to a higher energy level

Explain how an atom's electron arrangement changes when it emits EM radiation.

● Electrons move closer to the nucleus

● They move to a lower energy level

What does the ratio of electrons to protons in an atom result in atom having no overall change

The number of protons is equal to number of electrons

Protons and electrons have equal and opposite charges so charge cancels

What do all forms of the same element have in common

They all have the same number of protons.

What is the name given to the number of protons in an atom

atomic number

What reinforces a scientific theory

When experimental results agree with the hypothesised theoretical calculations and theories.

1800 dalton theory

Everything made of tiny spheres and can't be divided

1897 Thompson theory

Plum pudding model- overall charge of atom is neutral so negative electrons were disperse through positive sea of charge to cancel out charges

1911 Rutherford theory

Nuclear model-gold foil experiment

Most alpha particles went through-most of atom is empty space

Some alpha particles deflected-nucleus must be charged

Few alpha particles bounced back so nucleus contained most of mass

1913 Bohr theory

Protons and neutrons in nucleus

Electrons on shells

What did Chadwick's experiments on the atom prove

Existence of neutrons

Why do unstable nuclei give out radiation

● Unstable nuclei undergo decay to become more stable

● As they release radiation their stability increases

What is the name of the process in which an unstable nucleus gives out radiation to become more stable?

radioactive decay

Define the activity of an unstable nucleus

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

What is the unit of radioactive activity

Becquerel (Bq)

What is count rate

The number of radioactive decays per second for a radioactive source.

Give an example of a detector that may be used to measure count rate

Geiger-Muller tube

State four types of nuclear radiation

1. Alpha particles

2. Beta particles

3. Gamma rays

4. Neutrons

Alpha particle

Highly ionising

Weakly penetrating

5cm of air

Gamma radiation

Low ionising

Highly penetrating

Few cm of lead

Beta minus

Medium ionising

Medium penetrating

Sheet of paper

What are the constituents of an alpha particle

● Two protons and two neutrons

● It is the same as a helium nucleus

What is the range of an alpha particle through air

A few centimetres (normally in the range of 2-10cm).

What will stop beta radiation from passing through a point

● A thin sheet of aluminium

● Several metres of air

What will stop gamma radiation from passing through a point?

● Several centimetres of lead

● A few metres of concrete

What type of radiation is most ionising?

alpha radiation

What type of radiation is least ionising

gamma radiation

What type of radiation is most penetrating

gamma radiation

Alpha decay

Mass and charge if nucleus decrease

Beta decay decay

Mass doesn't change but charge of nucleus increases

State any changes to mass or charge that occur due to the emission of a gamma ray.

Both mass and charge remain unchanged.

Describe nature of radioactive decay

● Random

● Which nuclei decays and when is determined only by chance

● It is impossible to predict which nuclei will decay and when

Define the half-life of a radioactive isotope.

● The time it takes for the number of unstable nuclei in a substance to halve

● The time it takes for the count rate from a sample to fall to half its initial level

If 80 atoms falls to 20 over 10mins,what is the half-life?

80/2

40/2=20- so two half lives in 10 mins so half life I 5mins

Short half life

Less of a risk

Long half life

- remains weakly radioactive for a long period of time

Net decline

initial number - number after X half lives / initial number

What is radioactive contamination

The presence of unwanted radioactive nuclei on other materials.

What is irradiation

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

Why is it important for the results of studies on the effects of radiation to be published and shared with other scientists

● To allow the findings to be independently checked

● This is known as peer review

Give 4 sources of background radiation

1. Rocks

2. Cosmic rays from space

3. Nuclear weapon testing

4. Nuclear accidents

How should background radiation be dealt with in calculations?

The background count should be subtracted from any readings before calculations (half life etc.) are attempted.

What unit is used to measure radiation dosage

Sieverts (Sv)

How many millisieverts (mSv) make up 1 sievert (Sv)?

1000 millisieverts (mSv) = 1 sievert (Sv)

Why might the radiation dosage that different people experience differ

● Some occupations involve working with radiation

● Background radiation differs with location due to things such as the locality of nuclear power stations or radiation related testing

What factors determine how dangerous a particular radioactive isotope is

half-life of isotope

Why are isotopes with long half-lives particularly harmful?

● They remain radioactive for much longer periods of time

● They must be stored in specific ways to avoid humans and the environment from being exposed to radiation for too long

State two uses of nuclear radiation in the field of medicine

1. Examining of internal organs

2. Controlling and destroying unwanted

tissue

How is radiation used in sterilisation?

Gamma emitters are used to kill bacteria/parasites on equipment.

Explain the process of radiotherapy

● Gamma emitters direct gamma rays at

the cancerous cells

● The cancerous cells absorb the

radiation and are killed

How are medical tracers chosen?

They should have a short half life and decay into

a stable isotope which can be excreted.

They should only release gamma radiation since

it is weakly ionising and can easily pass through

body tissue without damaging it.

Technetium as a medical tracer

-decays into safe isotope to be excreted by body

Its injected/swallowed with enough time for body to detect before it decays away to minimise harm

Gamma emitter so can pass through body tissue without being absorbed

What is nuclear fission

The splitting of large, unstable nuclei to form smaller more stable nuclei (+the emission of spare neutrons).

Give an example of a fissionable isotope

Uranium-235

What usually needs to happen to induce fission

● The unstable nuclei must absorb a neutron

● Spontaneous fission (where no neutron absorption occurs) is rare

What else is emitted during fission reaction

Two or three neutrons

Gamma rays

Energy

What form of energy do all fission products have

kinetic energy

What takes place during a chain reaction in a nuclear reactor

● An unstable nucleus absorbs a neutron

● The nucleus undergoes fission and releases 2 or 3 further neutrons

● These induce more fission, which results in a chain reaction

What is the consequence of an uncontrolled chain reaction

● The rate of fission events becomes to high and results in the production of too much energy

● This can lead to a nuclear explosion

What are three main components of the core of a nuclear reactor

1. Fuel rods

2. Control rods

3. Moderator

What is the role of the moderator in a nuclear reactor

To slow down the neutrons so they are travelling at speeds which allow them to be absorbed by fissile nuclei and cause fission.

How is the chain reaction in a fission reactor kept under control

● Control rods are positioned in between the fuel rods

● The rate of fission is controlled by moving these rods up and down

● The lower the rods are inserted, the slower the rate of fission

What erm is used to describe nuclei in which fission can be induced through the absorption of slow neutrons

Fissile nuclei

What is nuclear fusion?

The joining of two light nuclei to produce a heavier nuclei and release energy

Name two isotopes of hydrogen which are commonly used in nuclear fusion.

deuterium and tritium

What releases more energy, fusion or fission?

fusion

Explain the difficulty of generating energy through nuclear fusion.

Fusion requires very high temperatures which in itself requires large quantities of energy and also requires casing which can withstand them.

Explain why nuclear fusion is currently not a viable way to produce energy on Earth.

With current equipment/techniques, the energy required is greater than the energy produced, resulting in a net energy loss.