Physics GCSE 4.3 Particle Model of Matter and 4.4 Atomic Structure

Solids

Strong forces of attraction

Holds the particles close together in fixed position creating a regular lattice structure

Definite shape and structure

Can vibrate

When heated - they vibrate more and when the temperature hits that object’s melting point - the particles break free of their bonds - solid melts into a liquid

Liquids

Have weak forces of attraction between particles therefore are free to move

They are still compact therefore have a definite volume

Their shape can change therefore it can flow to fit a container

When heated, the particles gain more energy therefore move around faster. The bonds between particles weaken until the temperature hits the substance’s boiling point and the bonds break - turning the liquid to a gas

When liquid is cooled down, the particles won’t have enough energy to overcome the attracting between the molecules and even more bonds will form, keeping the particles in place to form a solid - Freezing

Gases

Forces of Attraction between different particles are very weak - free to move around by themselves

Don’t keep a definite shape or volume - Always fill a container

Gases are constantly moving with random motion (straight lines in any direction) - get deflected by solid walls and other particles

When heated, the particles get more energy and expand (in a expandable container like a balloon) or if the container is fixed, the pressure will increase

If gas is cooled down, the particles won’t have enough energy to overcome forces and bonds will start to form again between particles, condensing the gas into liquid

States of matter in a closed system

Changes in state won’t change the mass - still the same number of particles

Density of the substance will change - Solid (highest density), Gas (lowest density)

Density

Measure of how much mass a substance has, per unit of volume

Density (kg/m³)= mass (kg) /volume (m³)

How to find Density of Solids?

Mass: Place the solid a measuring scale to measure the mass

Volume:

• Regular Shape (cube) : Measuring length, width and height and multiplying together

• Irregular Shape: Eureka Can filled with water with an empty measuring cylinder under the outlet. The irregular shape can be placed in the eureka can and the volume of water exactly equal to that of the solid will flow out of the can into the measuring cylinder

Use equation to find Density

How to find Density of Liquids?

Mass: Empty measuring cylinder on balance and zero the balance. Then add liquid to measuring cylinder and record the mass of the volume of liquid

Volume: measured using measuring cylinder

Use equation to find density

The larger the volume, the more accurate the density: minimises the effects of uncertainty

Can also take multiple measurements to calculate a mean

Latent Heat

If a change of state happens:

The energy needed for a substance to change state is called latent heat. When a change of state occurs, the energy supplied changes the energy stored (internal energy) but not the temperature.

Specific Latent Heat (of vaporisation and fusion)

The specific latent heat of a substance is the amount of energy required to change the state of one kilogram of the substance with no change in temperature.

Cooling: Amount of energy released by change in state

Specific Latent heat of vaporisation: Energy change when a substance changes between a liquid and Gas

Specific Latent heat of Fusion: Energy change when a substance changes between a solid and liquid

Specific Latent Heat equation

E = ml

Energy (J) required or released = mass (kg) x Specific Latent Heat (J/kg)

Factors affecting Gas Pressure

• Temperature

• Concentration

• Volume

Factors affecting Gas pressure - Temperature

• When gas is heated up, the particles have more kinetic energy and therefore move around faster

• As they are moving around faster, there are more collisions with the walls of the container and each collision involves more force

• Therefore pressure increases with temperature

Factors affecting Gas pressure - Concentration

• Increasing the number of particles in a container but keeping the volume the same increases the concentration

• This increases the amount of collisions with the container wall and other particles

• Therefore pressure increases with concentration

Factors affecting Gas pressure - Volume

• Making the container smaller but keeping the number of particles the same increases the number of particles per unit of volume (higher concentration)

• More collisions

• As volume decreases, pressure increases

Flexible containers (balloon)

Changes in temperature and concentration changes the volume of the container rather than the pressure

Increase in the number and force of collisions will cause the balloon to expand

Pressure and Volume

Pressure and volume are inversely related

Pressure (Pascals) x Volume (m³)= constant value

pV = Constant OR p₁V₁ = p₂V₂

John Dalton’s discoveries

• Substances were made of atoms that were like tiny hard spheres 

• Each chemical element had its own atoms that differed in mass 

• Believed that they could not be divided or split

• In chemical reactions, atoms rearranged themselves and combined with other atoms in new ways

JJ Thomson’s Discoveries

• Discovered electron - tiny, negatively charged particle that was found to have a mass about 2000 times smaller than the lightest atom 

• Plum Pudding Model - Negatively charged electrons must be embedded in a cloud of positive charge

• Atoms don’t carry a charge so charges in atom must balance out 

Ernest Rutherford Discoveries

• Fired positively charged alpha particles at a thin sheet of gold 

• Some of the particles deflected to the side and some came back the way they were fired

• Rutherford Suggested Nuclear Model - Compact Nucleus with all the dense positively charged protons and negative charge existed as a cloud around the nucleus  

Niels Bohr Discoveries

• Noticed that light given out when atoms were heated only had specific amounts of energy

• Bohr proposed that electrons are orbiting nucleus in fixed energy levels or shells at set distances which prevent the atom from collapsing 

Energy is given out when electrons fall to a lower energy level

James Chadwick’s Discoveries

• Second subatomic particle proposed to explain missing mass in atoms 

Neutrons must have no charge but have the same mass as a proton

Isotopes

Atoms with the same number of protons but different numbers of neutrons

Electrons in Atoms

• Arranged in a series of energy levels “shells”

• Increasing energy level

• Can jump to a higher energy level if they gain enough energy (from electromagnetic radiation “excited”

• Electron will fall back and re-emit the energy as EM radiation

What is ionisation?

An electron can absorb so much energy that they are able to leave the atom

It then will have a positive charge - positive ion

Ionising radiation - able to knock electrons off atoms (ionise them)

What is radioactive decay?

Unstable atomic nuclei that give out radiation as it changes to become more stable

What is activity (Radioactive)?

Activity is the rate at which a source of unstable nuclei decays
Activity is measured in becquerel (Bq)

What is the count rate?

Count-rate is the number of decays recorded each second by a detector (eg: Geiger-Muller Tube)

What are the 4 types of nuclear radiation?

Beta particles, Alpha particle, Gamma rays and neutrons

What is alpha radiation?

Alpha particles are made up of 2 protons and 2 neutrons (same as Helium - “He”)

Doesn’t have any electrons - overall charge of 2+

Strongly ionising due to large size

What is the penetration of alpha particles?

They are relatively large - can’t penetrate very far into other materials

Only travel a few centimetres in air

Can be absorbed by a single sheet of paper

What is beta radiation/particles?

Is a high speed electron ejected from the nucleus as a neutron turns into a proton (stays in nucleus)

What is the penetration of Beta Particles?

Moderately ionising as they penetrate moderately far into materials

Penetrate through several metres of air

Needs 5mm of aluminium to stop

What are gammy rays/radiation?

High frequency Waves of electromagnetic radiation - often emitted after alpha or Beta radiation as excess

Electromagnetic radiation from the nucleus

Very weakly ionising

What is the penetration of Gamma rays?

They don’t have any mass or charge so they pass through materials

Penetrate really far through air and takes thick sheets of lead to stop it

What is neutron radiation?

If a nucleus contains too many neutrons - neutrons are released to make it more stable

Nuclear equations of alpha radiation

4/2 He - represents alpha particles

An unstable nucleus undergoing alpha decay emits 2 protons and 2 neutrons

-4 from mass number and -2 from atomic number

-4/-2

Becomes different element

new element + 4/2 He

Nuclear equation of beta radiation

Loses neutron and gains proton

Mass number stays the same

Atomic number increases by 1

element + 0/-1 e

Nuclear equation of gamma radiation

Gamma radiation has no mass or energy

So doesn’t change anything

element + a gamma ray (Y)

Nuclear equations for neutron emission

-1 mass number

same atomic number

element + 1/0n

What is the half life of a radioactive atoms?

Time taken to activity to half

The half life is the time it takes for half of the radioactive atoms to decay - or the activity to reduce by half

Time/Half life - amount of half-lives

What is radioactive contamination?

Is the unwanted presence of materials containing radioactive atoms on other materials

Issue of contamination

The radioactive material on the other material is likely to decay and irradiate the material

Increased risk of irradiation

What is irradiation?

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

What determines how harmful radiation is?

Types of radiation, where you’re exposed to it, the amount of radiation

What is the most dangerous types of radiation and why?

Ionising radiation as it can enter living cells and ionise DNA which can cause mutations or even cause cells to uncontrollably divide, developing cancer (ultraviolet as well)

What are the Location of radiation source dangers?

Alpha - stops being harmful outside the body - can’t penetrate skin

Beta and Gamma - can penetrate skin so are the most dangerous outside the body

Alpha - most dangerous on the body or inside it as it can make it into your cells

What are the dangers of dosage of radiation?

Depends on how far away from the source you are, how long you’re exposed to it, how radioactive that substance is

How to minimise irradiation and Avoid contamination

Wearing protective gear - gloves and overalls

Handling radioactive items with tools - tongs

Keeping the item in a lead lined box

What are some sources of Background radiation?

Natural sources such as rocks and cosmic rays from space
Man-made sources such as the fallout from nuclear weapons testing and nuclear accidents

May be affected by occupation or location

Uses of radiation in medicine:

Radiotherapy to treat cancer
Medical tracers

What is radiotherapy?

Radiation can be used to kill off cancer cells

Externally: Gamma rays can target cancer cells from lots of different angles - highest dose

Internally: Radioactive source inside cancer cells or next to it. Uses Beta radiation which is more damaging but can’t pass far through the body

Side effects: can damage normal cells which can cause patients to feel sick

What are medical tracers?

Radioactive isotopes can be injected or swallowed by a patient

Can track movement of isotopes around the body by tracking radiation they emit

Can check if organs are working properly by measuring how much they absorb

Gamma radiation is used as it is less harmful and use isotopes with as short a half life as possible so they only emit radiation for a short period before they stop being harmful

Benefits and risk of using radiation in medicine

Medical tracers can help us diagnose diseases - worth the risk - low dose used to minimise risk

What is nuclear fission?

The splitting of a large and unstable nucleus - eg: uranium or plutonium

2 types of nuclear fission

Happens spontaneously - Fission is unforced and happens by itself - rare

Absorbing a neutron - can split a nucleus

Process of Nuclear fission

Slow moving neutron fired at unstable nuclei- making it even more unstable

This releases 2 daughter nuclei and more neutrons as well as load of energy (gamma radiation)

All of the fission products have kinetic energy

The neutrons may go onto start a chain reaction and repeat the process again

What happens in a nuclear reactor?

Energy from the fission heats up water which creates steam which is used to drive turbines and are connected to electricity generators

How is a chain reaction controlled in Nuclear reactors?

Controlled in a nuclear reactor to control the energy released.

Control rods are lowered into the reactor to absorb neutrons - slow down the reaction

Pros of Nuclear energy

Uranium or Plutonium fuel is relatively cheap - produce large and steady amounts of energy

Clean energy source - doesn’t produce greenhouse gases

Cons of Nuclear Energy

Power plants are very expensive to build

Nuclear waste is expensive to get rid of - buried under the ground

Risk of major disasters

What is nuclear fusion?

2 lighter nuclei (hydrogen) join or fuse to form a heavier nuclei (helium) - releases energy

Some of the mass from H released as energy - He is lightly lighter

Process that fuels stars

How all elements heavier than hydrogen were made

Pros and Cons of Nuclear Fusion

Pros: Produces NO radioactive waste and we can easily make the hydrogen that’s needed as fuel

Cons: only happens at really high temperatures and pressures - can’t currently do it on earth