atomic structure and radiation

what did john Dalton believe?

Matter was made of atoms that were too small to be seen by the human eye.

what did JJ Thompson do?

Discovered electrons

~Plum pudding model (negative electrons were spread throughout the sphere of positive charge )

what experiment did Rutherford do?

Alpha particle scattering

what did this alpha scattering experiment show?

They expected most alpha particles to pass straight through with only a little being slightly deflected.

However more than expected were deflected at large angles and some fully back

Most still passed through showing atom is mainly empty space

The deflecting back shows that most of the mass is concentrated in the centre of the atom

The nucleus must also be positively charged as it reflected the positive alpha particles

What did Niels Bohr do?

used theoretical calculations with experimental data to prove that electrons orbited the nucleus at fixed distances in fixed energy levels

what is the radius of an atom?

1 x 10^-10 metres

how can electrons move with the atom?

if they gain energy through absorbing EM radiation, they can move to a higher energy level away from the nucleus

if they lose energy through releasing EM radiation, they move to a lower energy level

what are isotopes?

Different forms of the same element, which have the same number of protons but a different number of neutrons

how many stable isotopes do elements normally have?

1 - 2

What do unstable isotopes do?

Decay into other elements giving out radiation in an attempt to become stable.

what do radioactive substances emit from their nucleus?

alpha, beta or gamma ionising radiation

what else can radioactive substances release?

neutrons

what is ionising radiation?

Radiation that knocks electrons off atoms, creating positive ions

what is ionising power?

How easily a radiation source can knock electrons off atoms

what is an alpha particle?

Two neutrons and two protons, the same as a helium nucleus

what are the properties of an alpha particle?

very low penetration- stopped in a few cm in air or paper

very ionising due to large size

what are beta particles?

a fast moving electron released by the nucleus. charge of -1

what are the properties of beta particles?

moderately ionising

moderately penetrating- have a range in air of few m and absorbed by a sheet of aluminium

For every beta particle emitted, a neutron turns into a proton

what are gamma rays?

wave of EM released by nucleus

highly penetrating absorbed by thick sheets of lead or m of concrete. Travel long distance in air

very low ionising as they tend to pass through rather than collide with atoms

what are nuclear equations a way of showing?

radioactive decay by using element symbols

what is the main rule of nuclear equations?

The total mass and atomic numbers must be equal on both sides

what does alpha decay do to a nucleus?

Decreases the charge and mass ( atomic number by 2 and mass number by 4)

written as He

what does beta decay do to a nucleus?

increases charge ( increases atomic number by 1 hence charge by 1)

mass doesn't change as it loses a neutron but gains a proton

written as e

what does gamma decay do to a nucleus?

they dont change mass or charge of a nucleus as it is a way of getting rid of excess energy from a nucleus

why cant you predict when a nucleus will decay?

radioactive decay is completely random so you don't know when or which one will decay

what can radiation be measured with?

- Geiger-Muller tube and counter

- records the count-rate - number of radiation counts reaching it per second

what can you calculate then in radiation?

half life

what is half life?

the time taken for the number of radioactive nuclei in an isotope to halve

what is activity?

The rate at which a source decays, measured in Bq

what happens each time a nucleus decays?

the activity decreases

why do we use the idea of a half life?

to calculate the rate at which it decays as the activity never reaches zero

why are short half life isotopes very damgerous?

they release high radiation at the start but will quickly release very little

why is a high half life isotope not dangerous?

most of the nuclei don't decay for a long time, releases small amounts of radiation at a time

What is background radiation?

The low-level radiation that's around us all the time

what are the different sources of background radiation?

radioactivity of naturally occurring isotopes

radiation from space called cosmic rays such as from the sun

radiation due to human activity such as fallouts form a nuclear explosion or nuclear waste

what is the radiation dose?

the risk of harm to body tissues due to exposure of radiation

measured in sieverts (sv)

what is irradiation?

exposure to radiation

what does irradiation not do?

does not make one radioactive

how do you reduce effects of irradiation?

-Keeping sources in lead-lined boxes

-Standing behind barriers

-Being in a different room

-Using remote-controlled arms

what is contamination?

When unwanted radioactive atoms get onto or into an object

why can contamination be harmful?

the radioactive atoms can decay release radiation causing harm

what can be used to reduce contamination?

using gloves or tongs when handling sources

protective suits

what is the seriousness of contamination and irradiation dependent on?

the radiation type of the source

what is most dangerous outside the body?

beta and gamma as it is able to penetrate through the skin reach organs

what is most dangerous inside body?

alpha as it is highly ionising meaning it can knock electrons out of atoms in organs causing mutations hence why contamination is more of a worry with alpha. Beta and gamma are less dangerous as some may pass straight through organs or be absorbed by a wider area

what are the risks of radiation?

- radiation can enter living cells and ionise atoms with them leading to tissue damage

- lower dosage tends to cause minor damage which can cause mutations inside cells which divide uncontrollably

- higher dosage can kill cells entirely causing radiation sickness

how can radioactive sources be used as medical tracers?

radioactive isotopes can be injected in body and be tracked around the body using an external detector

usually gamma as it is highly penetrating and hence be detected outside the body easier without causing much harm as low ionisation

also often have short half lives to prevent the substance decaying and releasing higher amounts of radiation for a long time inside the body

how can radioactive sources be used to treat cancer?

high doses of radiation can kill cells and can hence kill cancer cells if pointed carefully with a right amount of dosage at the cell without killing other cells

what is nuclear fission?

A type of nuclear reaction that is used to release energy from large and unstable atoms by splitting them into smaller atoms

what is the process of nuclear fission?

- spontaneous fission rarely happens. the nucleus has to absorb a neutron before it splits

- when the atom splits, it forms 2 lighter elements that are roughly the same size and that have some energy in kinetic stores

-2 or 3 neutrons are also released when an atom splits. If any of the neutrons are moving slow enough to be absorbed by another nucleus, it will cause more fission to occur called a

chain reaction

- the energy not transferred to kinetic energy stores of products are carried away by gamma rays

- the gamma rays that carry the energy and the kinetic energy in the free neutrons or other decay products can be used to heat water to make steam for turbines and generators

-the amount of energy released by fission in a nuclear reactor is controlled by changing how quickly chain reactions can occur. this is done by control rods which are lowered and raised inside a nuclear reactor to absorb neutrons, slow down the chain reaction and control energy released

-uncontrolled chain reactions can release lots of energy as an explosion

What is nuclear fusion?

two light nuclei collide at high speed and join to create a larger heavier nucleus

Why doesn't the heavier nucleus produced by fusion not have as much mass as the two separate light nuclei did?

because some of the mass of the lighter nuclei was converted to energy and is then released as radiation

does fusion release more energy than fission?

yes

what are fusion reactors hard and expensive to build?

the temperatures and pressures fusion requires is really high