Atomic Structure

Page 1

Model of the Atom

In 1804 John Dalton agreed with Democritus that matter was made up of tiny spheres (atoms) that couldn't be broken up

Said each element was made up of different type of atom

Nearly 100 years later, JJ Thomson discovered electrons that could be removed from atoms

Dalton's theory wasn't quite

Suggested atoms were spheres of positive charge with tiny negative electrons stuck in them

However, in 1909, scientists in Rutherford's lab tried firing a beam of alpha particles at thin gold foil - gold foil experiment

Expected particles to pass straight through gold sheet, or only be slightly deflected

Most particles did go straight through the sheet, but some were deflected more than expected and a few were deflected back

Plum Pudding Model couldn't explain this

As a few alpha particles were deflected back, scientists realised most mass of the atom must be concentrated at the centre in a tiny nucleus

Nucleus must also have positive charge, since it repelled positive charged alpha particles

Also realised that because nearly all alpha particles passed through, most an atom is just empty space

First nuclear model of the atom

Nuclear model that came from gold foil experiment was a positively charged nucleus surrounded by a cloud of negative electrons

Niels Bohr said electrons orbiting nucleus do so at certain distances called energy levels

Theoretical calculations agreed with experimental data

Evidence from further experiments changed model to have a nucleus made up of a group of particles

Which all had the same positive charge that added up to overall charge of the nucleus

About 20 years after idea of nucleus was accepted, in 1932, James Chadwick proves existence of neutron

Explained imbalance between atomic and mass numbers

Current Model of the Atom

Nucleus is tiny

Makes up most of the mass

Contains protons (positively charged - relative charge of +1) and neutrons (neutral charge - relative charge of 0)

Radius is about 10,000 times smaller than radius of atom

Radius of atom is mostly empty space

Negative electrons (relative charge of -1)

Whizz around outside of nucleus fast

Give atom overall size - radius of atom is about 1 x 10^-10 m

Number of protons = number of electrons

As protons and electrons have an equal an opposite charge

Electrons in energy levels can move within atom

If they gain energy by absorbing EM radiation they move to a higher energy level - further from nucleus

If they release EM radiation, they move to a lower energy level that is closer to the nucleus

If one or more outer electrons leave the atom, atom becomes positively charge ion

No saying that it won't change

Just like for the plum pudding, new experiments sometimes mean they might need to change or completely get rid of current models

Page 2

Isotopes and Nuclear Radiation

All atoms of elements have a set number of protons

Number of protons in an atom = atomic number

Mass number is number of protons + number of neutrons

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

All elements have different isotopes

But they're usually only one or two stable ones

Other unstable isotopes tend to decay into other elements and give out radiation as they try to become more stable

Try to balance number of protons and neutrons in nucleus or get rid of any excess energy - radioactive decay

Radioactive substances produce one or more types of ionising radiation from their nucleus

Can also release neutrons when they decay to rebalance number of protons and neutrons

Ionising radiation knocks electrons off atoms, creating positive ions

Ionising power of radiation source is how easily it can do this

Alpha Particles are Helium Nuclei

When an alpha particle is emitted from the nucleus

An alpha particles is two neutrons and two protons (like a helium nucleus)

Don't penetrate very far into materials and are stopped quickly

Can only travel a few cm in air

Absorbed by a sheet of paper

Strongly ionising - because of size

Beta Particles are high-speed electrons

Simply a fast-moving electron released by nucleus

Have virtually no mass and charge of -1

Moderately ionising

Penetrate moderately far into materials before colliding and have a range in air of few metres

Absorbed by a sheet of aluminium (around 5mm)

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

Gamma Rays are EM waves with a short wavelength

Waves of electromagnetic radiation released by nucleus

Penetrate far into materials without being sopped

Will travel a long distance through air

Means they are weakly ionising

Tend to pass through rather than collide with atoms

Eventually hit something and do damage

Can be absorbed by thick sheets of lead or metres of concrete

Page 3

Nuclear Equations

Nuclear equations are a way of showing radioactive decay by using element symbols

Written in form: atom before decay -> atom after decay + radiation emitted

One rule to remember:

Total mass and atomic numbers must be equal on both sides

Alpha decay decreases charge and mass of nucleus

Alpha particles are made up of two protons and two neutrons

When an atom emits an alpha particle, its atomic number reduces by 2 and mass number reduces by 4

A proton is positively charged and a neutron is neutral - charge of nucleus decreases

In nuclear equations, an alpha particle can be written as helium nucleus

Beta decay increase charge of nucleus

When beta decay occurs, a neutron in nucleus turns into a proton and releases a fast-moving electron (beta particle)

Number of protons in nucleus has increased by 1

Increases positive charge of nucleus

Because nucleus has lost a neutron and gained a proton, mass doesn't change

Gamma rays don't change charge or mass of nucleus

Gamma rays are a way of getting rid of excess energy from a nucleus

Means there is no change to atomic mass or atomic number

Page 4

Half-Life

Radioactive substances give out radiation from nuclei of atoms

Can be measured with Geiger-Muller tube and counter

Records count-rate - number of radiation counts reaching it per second

Radioactive decay is random

Can't be exactly predicted which nucleus will decay next

Or when any one of them will decay

Can find out the time taken for amount of radiation to halve

Known as half-life

Can be used to make predictions about radioactive sources - even though they're random

Can be used to find rate at which a source decays - its activity

Measured in becquerels, Bq (where 1 Bq is 1 decay per second)

Radioactivity of a source decreases over time

Each time a nucleus decays to become stable, the activity will decrease

Older sources emit less radiation

For some isotopes it takes just a few hours before nearly all unstable nuclei have decayed, whilst others last for million of years

Problem with trying to measure this is that activity never reaches zero

Half-life is time taken for the number of radioactive nuclei in an isotope to halve

Half-life can also be described as the time taken for activity (count rate) to fall to half of its initial value

Page 5

Irradiation and Contamination

Ionising radiation can enter living cells and ionise atoms with them

Can damage cells (can cause cancer) or kill them completely

Important to know precautions to take when working with any sources

Objects near a radioactive source are irradiated by it

Simply means they're exposed to it

Irradiating something doesn't make it radioactive

Keeping sources in lead-lined boxes and standing behind barriers when using sources are common ways of reducing effects of irradiation

In some industries, source may be in a different room and remote-controlled arms are used to handle it

If unwanted radioactive atoms get onto or into an object, the object is contaminated

Contaminating atoms might then decay, releasing radiation which could cause harm

Contamination is especially dangerous because radioactive particles could get inside body

Gloves and tongs should be used when handling sources, to avoid particles getting stuck into skin or under nails

Some industrial workers wear protective suits to stop them breathing in particles

Contamination or irradiation can cause different amounts of harm - based on type

Outside the body, beta and gamma sources are the most dangerous

Can penetrate the body and get into organs

Alpha is less dangerous - can't penetrate skin and is easily blocked by a small air gap

High levels of irradiation from all sources are dangerous

Especially ones that emit beta and gamma

Inside the body, alpha sources are the most dangerous

They do all their damage in a very localised area

Contamination is the major concern when working with alpha sources

Beta sources are less damaging inside the body, as radiation is absorbed over a wider area

And some passes out of the body altogether

Gamma sources are least dangerous inside the body, as they mostly pass straight out

Have lowest ionsing power

More we understand how different types of radiation affect bodies, the better we can protect ourselves when using them

Why it's so important that research is published

Data is peer-reviewed and can quickly become accepted, leading to many improvements in use of radiation sources