Topic 3 - Radioactivity and Atomic Structure Flashcards
What were the first ideas about atoms
- John Dalton thought that atoms were tiny balls of matter that couldn’t be broken up
Describe the plum pudding model
A ball of positive charge with negatively charged electrons embedded in it
What was the gold foil experiment
- Positively charged alpha particles were fired at a thin gold foil
- Circular detector screen surrounding gold foil detects alpha particles deflected
What results were Rutherford expecting?
- most of the alpha particles would pass straight through or some would be very slightly deflected ( < 1°)
Results of alpha particle experiment
- most alpha particles went straight through as expected
- however some were reflected more than expected
- a few alpha particles were reflected straight back at them
What did this explain?
- most of the atom was empty space because most alpha particles passed through
- nucleus must have large positive charge as some particles reflected/deflected by big angle
- nucleus must be small as very few particles were deflected/ reflected
2 differences between nuclear model and plum pudding
- nuclear model has positive charge concentrated in small nucleus but P.P has it spread out
- nuclear model is mostly empty space but P.P is a ‘ solid mass ‘
3 developments to nuclear model
- Niels Bohr discovered that electrons orbit nucleus at certain distances in shells : this stopped atom collapsing
- nucleus could be split into a group of atoms with positive charge(protons)
- James Chadwick proved existence of neutral atoms in nucleus( neutrons)
what happens when an atom absorbs/emits EM radiation?
- when an atom absorbs EM radiation electrons move to a higher energy level : further
- when an atoms emits EM radiation electrons move to a lower energy level : closer
ion
atom that has lost or gained electron(s)
isotopes
different form of same element with same no. of protons but different no. of neutrons
Radioactive decay
when unstable nuclei emit radiation to try and become stable
random nature of radioactive decay
- you don’t know which nuclei is going to decay
- and when they are going to decay
alpha particle
2 protons, 2 neutrons ( helium nucleus )
properties of alpha particles
- low penetration
- high ionising power
- travels few cm in air
- absorbed by a tissue/ skin cells
beta particle
electron ejected from unstable nucleus
properties of beta particles
- medium penetration
- medium ionising power
-travels few metres in air - absorbed by a sheet of aluminium
