P4 Flashcards
What did Democritus discover in the 5th Century BC?
He thought that all matter, whatever it was, was made up of identical lumps called “atomost”
What did Jonh Dalton discover in 1804?
That matter was made up of tiny spheres that couldn’t be broken up, but reckoned that each element was made up of a different type of “atom”
Nearly 100 years later after Dalton, what did Thompson discover?
Particles called electrons could be removed from atoms. He suggested that atoms where spheres of positive charge with tiny negative electrons stuck in them (plum pudding model)
In 1909, what did Rutherford’s alpha scattering experiment determine?
- Because of Thompson’s model, they expected the particles to pass straight through or slightly deflected, but some particles were deflected more than expected and even back the way they came
- 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
- It also meant that the nucleus must have a positive charge
- Also that most of the atom is empty space (the first nuclear model of the atom)
- Basically a positively charged nucleus surrounded by a cloud of negative electrons
What did Bohr discover about the atom?
- Electrons orbiting the nucleus do so at certain distances called energy levels
What did Chadwick discover 20 years after Bohr?
- The idea of a nucleus was accepted
- Proved the existence of the neutron which explained the imbalance between the atomic and mass numbers
What are isotopes?
Elements that have the same amount of protons (same atomic number), but a different number of neutrons
What usually happens to unstable isotopes (Radioactive decay)?
- They tend to decay into other elements and give out radiation as they try to become more stable
- They try to balance the number of protons and neutrons in their nucleus to get rid of any excess energy
What are the 3 different types of ionising radiation from the nucleus?
- Alpha
- Beta
- Gamma
What is ionising radiation?
- Radiation that knocks off atoms, creating positive ions
- The ionising power of a radiation source is how easily it can do this
Alpha Particles (radiation)
- Alpha radiation is when an alpha particle is emitted from the nucleus
- It is 2 neutrons and 2 protons (like a helium nucleus)
- They don’t penetrate that far into materials and are stopped quickly - they can only travel a few cm in air and are absorbed by a sheet of paper
- Because of their size they are strongly ionising
Beta particles (Radiation)
- A beta particle is simply a fast moving electron released by the nucleus. They have virtually no mass, and a charge of -1
- They are moderately ionising, and can penetrate moderately far into materials before colliding and have a range in air of a few metres
- They can be absorbed by a sheet of aluminium
- For every beta particles emitted, a neutron in the nucleus has turned into a proton
Gamma rays / Electromagnetic radiation
- Gamma rays are waves of electromagnetic radiation released by the nucleus
- They penetrate far into materials without being stopped and will travel long distance through air
- This means they are weakly ionising because they tend to pass through rather than collide with atoms. eventually they hit something and do damage
- They can be absorbed by thick sheets of lead or metres of concrete
Nuclear equations
- A way of showing radioactive decay by using element symbols
- They are written in the form: atom decay -> atom after decay + radiation emitted
What must be conserved in nuclear equations (what must be equal on both sides)?
The total mass and atomic numbers
