PHYSICS UNIT 9 - ATOMIC STRUCTURE

Isotopes carbon

Nuclear Decay Equations for Alpha/Beta Decay decay

Half-life graph

Hazards of Radiation haz

Models of the Atom

Bohr 1912

Key features: electrons contained in shells (orbits) or would be attracted to nucleus & cause collapse

Modern Atomic Structure atom

Rutherford 1911

2) Nuclear Model ruth

Thomson 1897

Key features: -ve charged electrons embedded within a +vely charged "soup"

1) Plum Pudding Model - Spheres of +ve charge with tiny -ve electrons stuck in them like "plum pudding" plum

1804 Dalton

Agreed with D but thought each element made of a different type of "atom"

Greek called Democritus in 5th Century BC - all matter made up of identical lumps called "atomos" that couldn't be broken up

Key features : atom has a small +ve nucleus with -ve electrons orbiting around the nucleus

Experiment - fired alpha particles at thin gold foil. Plum pudding predicted they would travel straight through but some reflected back. Showed must be a small +ve nucleus & most of atom empty space.

Molecule bigger as made up of atoms, atom significantly bigger than nucleus which is like a full stop if the atom were magnified to size of school hall

Almost all the mass of the atom is in the nucleus

Atom = +vely charged nucleus surrounded by -vely charged electrons

Nucleus = protons & neutrons

Atoms of same element can differ in nuclear mass by having different numbers of neutrons

Same number of protons (atomic number) so same charge on nucleus

Different number of neutrons (mass number)

= Different forms of same element

Unstable isotopes tend to decay into other elements & give out radiation as try to become more stable = radioactive decay e.g.. alpha, beta & gamma radiation

Ionising Radiation rad

Beta e

Gamma gamma

Alpha alpha

Don't penetrate very far & stopped quickly, only travel few cm in air & absorbed by sheet of paper

Strongly Ionise atoms when collide into them because of their size

Alpha particle (2 neutrons & 2 protons - like helium) is emitted from nucleus

Use - smoke detectors - contains small amount of radioactive material & receiver counts Alpha made, if smoke present it blocks Alpha particles to receiver & alarm goes off

Moderately ionising & penetrates moderately, range in air of few metres & absorbed by sheet of aluminium (5mm thick)

For every beta particle emitted a neutron in nucleus has turned into a proton

Fast moving electron released by nucleus, virtually no mass & a charge of -1

Use - test thickness of sheets of metal as slight variations in thickness affect amount of radiation passing through

Penetrate far into materials & travel long distance through air

Weakly ionising as pass through rather than collide, eventually hit and do damage

Waves of electromagnetic radiation released by nucleus

Absorbed by thick sheets of lead or metres of concrete

Uses - killing cancer cells & sterilising equipment

Half-life used to find rate at which source decays, its ACTIVITY (measured in becquerels, Bq) 1Bq = 1 decay per second

Each time a radioactive nucleus decays to become stable the activity as a whole decreases - older sources emit less radiation

Radioactive decay is entirely random so can only find out time it takes for amount of radiation emitted by a source to halve = half-life

Isotopes can take just few hours or millions of years

Radiation can be measured with a Geiger-Muller tube & counter which records count rate = number of radiation counts reaching it per second

Half-life = time taken for activity (count rate) to halve

Find half-life by dividing total Bq by 2

Irradiation

Contamination

We're always being irradiated by background radiation e.g.. natural sources = cosmic rays from sun, rocks & soil, artificial = X-rays & nuclear power stations

Irradiating something does not make it radioactive - 0bject is no longer radioactive when source removed e.g.. cancer treatment

= Exposed to radiation

Keep sources in lead-lined boxes, standing behind barriers or different room, using remote controlled arms when working with radioactive sources

Contaminating atoms may then decay releasing radiation causing you harm & radioactive particles could get inside body

Gloves & tongs should be used to avoid particles getting stuck to skin or under nails. Industrial workers may use protective suits.

= Radioactive particles get onto or into an object e.g.. touching radioactive source without gloves

Inside body alpha most dangerous because most ionising & damage localised area

Outside body beta & gamma are the most dangerous as can penetrate body

Alpha

Beta

Gamma rays don't change charge or mass of nucleus so don't do equations

Atomic number reduces by 2 & mass by 4

Decay decreases charge & mass of nucleus

Written as Helium nucleus he

Number of protons in nucleus increased by 1 & increases atomic number, mass stays the same

Beta particle be

Neutron in nucleus turns into proton & releases fast-moving electron

Atom before decay --------> atom(s) after decay & radiation emitted (total mass & atomic numbers must be equal on both sides) equa

Object is still radioactive as source is within e.g.. injected tracer treating cancer

Experimented with cathode ray tubes which showed all atoms contain tiny -vely charged subatomic particles or "electrons"

Experiment - electricity passed through a gas & made to glow & the light analysed using a spectroscope . Bright lines seen in the spectroscope

3) Shells bo