P7 Radioactivity Flashcards
What was first believed by Dalton about atom?
- Matter made out of tiny spheres (“atoms”)
- Each element made up of a different type of “atom”
What was Thomson’s discovery?
- Particles called electrons that could be removed from atoms (Dalton incorrect)
- Suggested atoms where spheres of positive charge w/ tiny negative electrons stuck in them (plum pudding model)
Describe the alpha particle scattering experiment
1) Beam of alpha particles fired at thin gold foil
2) Because of plum pudding, they expected the particles to pass straight through the gold sheet, or only be slightly deflected
3) Although most of the particles did go straight through the sheet, some were deflected more than expected, & a few were deflected back the way they had come - something the plum pudding model couldn’t explain
4) Because a few alpha particles were deflected back, the scientists realised that most of the mass of the atom must be concentrated at the centre in a tiny nucleus
5) This nucleus must also have a positive charge, since it repelled the positive alpha particles
6) They also realised that because nearly all the alpha particles passed straight through, most of the atom is just empty space
What is the first nuclear model of the atom?
- Rutherford’s nuclear model
How did the nuclear model develop into the current model of the atom?
1) Nuclear model that resulted from the alpha particle scattering experiment was a positively charged nucleus surrounded by a cloud of negative electrons
2) Niels Bohr said that electrons orbiting the nucleus do so at certain distances called energy levels. His theoretical calculations agreed with experimental data
3) Evidence from further experiments changed the model to have a nucleus made up of a group of particles (protons) which all had the same positive charge that added up to the overall charge of the nucleus
4) About 20 years after the idea of a nucleus was accepted, in 1932, James Chadwick proved the existence of the neutron, which explained the imbalance between the atomic & mass numbers
Describe the current model of the atom
- Tiny nucleus (radius 1/10000 of atom), makes of most of atom mass
- Contains protons & neutrons -> gives overall positive charge
- Rest of atom is empty space
- Electrons quickly move around outside of nucleus; gives overall atom size (1 x 10^-10m)
Describe the energy levels of an atom
- Electrons in energy levels can move within (or sometimes leave) the atom
- If one or more outer electrons leaves the atom, the atom becomes a positively charged ion
What happens if an electron gains energy by absorbing EM radiation?
- Moves to higher energy level, further from nucleus
What happens if an atom releases EM radiation?
- They move to a lower energy level that is closer to the nucleus
What is atomic number?
Number of protons
What is mass number?
Protons + neutrons
What are isotopes?
- Atoms w/ same number of protons but different number of neutrons
What is radioactive decay?
- Unstable isotopes decay into other elements & give out radiation as they try to become more stable (they try to balance number of protons & neutrons in nucleus or get rid of excess energy)
How do radioactive substances release neutrons?
- When they decay to rebalance the number of protons & neutrons
What is ionising radiation?
- A radiation that knocks electrons off atoms, creating positive ions
