P4 Flashcards
Developing the atom: 1
John Dalton: 1804. Agreed with Democritus that matter made up of tiny spheres that can’t be broke up. Each element made of diff type atom.
JJ Thomson: near 100 years later. Discovered electrons that can be removed from atoms. Dalton incorrect. T suggested atoms were spheres of + charge with tiny - electrons stuck in like fruit in plum pudding.
Developing the atom: 2
1909: scientists in Rutherford’s lab fired a beam of alpha particles at gold foil-alpha scattering experiment.
They expected particles to pass through sheet, instead more were deflected than were expected. So plum pudding model incorrect. Scientists realised most of mass of atom must be concentrated at centre in tiny nucleus. This must have positive charge as it repelled positive alpha particles. As near all particles passed straight though, thought most of atom was empty space.
Developing the atom: 3
Niels Bohr: said electrons orbiting the nucleus do so at certain distances: energy levels. His theoretical calculations agreed with experimental data.
James Chadwick: 1932. Proved the existence of the neutron, which explained imbalance between atomic and mass numbers.
Current model of atom
Tiny nucleus contains + protons (+1 rc) & neutral neutrons(0 relative charge)
Radius 10,000 smaller than atom radius
Rest of atom: empty space & negative electrons (-1 rc): whizz round outside.
No. protons=no. electrons. No overall charge.
Facts about atoms
All atoms of each element have a set no. of protons.
No. of protons=atomic no.
Mass no. of atom (mass of nucleus) = no. of protons + no. of neutrons.
On periodic table: mass number above atomic number.
Isotopes
Atoms with the same no. of protons, but a different no. of neutrons.
All elements have diff isotopes. Usually 1 or 2 stable ones.
Radioactive decay
An element’s unstable isotopes tend to decay into other elements & give our radiation as they try to become more stable.
They try to balance no. of protons & neutrons in their nucleus or get rid of any excess energy.
Radioactive substances
Spit out 1+ types of ionising radiation from their nucleus.
also release neutrons when they decay to rebalance no. of protons+neutrons.
Ionising radiation knocks electrons off atoms, making +ions. Ionising power = how easy it is to do this.
Give out radiation from nuclei of their atoms.
Alpha radiation
When an alpha particle is emitted from nucleus.
An a-particle is 2 neutrons and 2 protons (like a helium nucleus).
They don’t penetrate v far into materials & are stopped quickly.
Can only travel a few cm in air & are absorbed by a sheet of paper.
Strongly ionising.
Beta radiation
Fast moving electron released by the nucleus.
Have virtually no mass.
Charge of -1.
Moderately ionising. Penetrate moderately far into materials before colliding, have a range in air of a few m.
Absorbed by a sheet or aluminium.
For every beta particle emitted, a neutron in nucleus turns into a proton.
Gamma radiation
Waves of electromagnetic radiation released by the nucleus.
Penetrate far into materials and travel long distance through air.
So they’re weakly ionising as they tend to pass through rather than collide with atoms. Eventually they hit&do damage.
Can be absorbed by thick sheets of lead/metres of concrete.
Nuclear equations
Show radioactive decay by using element symbols
Atom before decay - atom after decay+radiation emitted.
Total mass&atomic no equal both sides
Alpha: +4 on top +2 below.
Beta: 0 on top -1 below.
Gamma rays are a way of getting rid of excess energy from nucleus. So no charge to the atomic mass/no of atom.
Radioactive decay
Entirely random.
Can’t predict which nucleus in sample will decay next, or when any will decay.
Radiation can be measure with a Geiger Muller tube and counter, which records count rate - no. of radiation counts reaching it per second.
Half Life
Time it takes for the amount of radiation emitted by source to halve
Can be used to make predictions abt radioactive sourced, even though decays are random.
Can be used to find rate at which source decays- its ACTIVITY.
Activity is measured in Bq (1 decay/s).
Half life 2
The time taken for no of radioactive nuclei in an isotope to halve.
Older sources emit less radiation.
Half life can also be described as the time taken for the activity(+count rate) to fall to half its initial value).
The half life is found from GRAPH by finding time interval on bottom axis, corresponding to a halving of activity on vertical axis.
