Why might scientists replace an atomic model? Tick one box.
Our own questions on this topic, marked straight away, so you see what to revise. Free now.
Show model answer for 07.1Hide model answer for 07.1
A model is replaced when experiments produce results it fails to explain.
How the marks are earned
- Tick the statement saying a model changes when experiments give results the old model fails to explain.
Early researchers were both irradiated and contaminated by radioactive materials. Describe how irradiation differs from radioactive contamination.
See how you would score on Uses & Hazards of RadiationShow model answer for 07.2Hide model answer for 07.2
Irradiation is exposure to radiation, for example from a source outside the body, and no radioactive material stays on the person. Contamination is when radioactive atoms get onto or inside a person or object, so they keep emitting radiation there.
How the marks are earned
- One mark: an irradiated person has been exposed to radiation. Saying that nothing radioactive is left behind is extra detail, not required.
- One mark: a contaminated person or object carries unwanted radioactive material on the surface or inside.
Why it scores
Each term needs its own definition. The key difference is whether radioactive material is present on or in the person.
Revise Uses & Hazards of RadiationPolonium-210 decays to lead. Find each ?: 210Po (atomic number ?) → ?Pb (atomic number 82) + 4He (atomic number 2).
See how you would score on Radioactive DecayShow model answer for 07.3Hide model answer for 07.3
The helium nucleus given out is an alpha particle. The product has atomic number 82, as given.. An alpha particle removes 2 protons, so for the atomic numbers to balance across the arrow, the starting element has this atomic number:. 82 + 2 = 84. The alpha particle also removes 4 from the mass number, so for the mass numbers to balance, the product has this mass number:. 210 − 4 = 206
How the marks are earned
- One mark: the atomic number of the starting element, 84. Work it out from the given 82 plus the 2 protons an alpha particle takes away.
- One mark: the mass number of the product, 206. Work it out as 210 − 4, the mass number an alpha particle takes away.
Why it scores
Atomic numbers and mass numbers must each balance across the arrow. Checking that each total balances gives both missing numbers without guessing.
Revise Radioactive DecayThe half-life of polonium-210 is 138 days. Calculate how long 256 000 atoms of it take to fall to 16 000.
See how you would score on Half-LifeShow model answer for 07.4Hide model answer for 07.4
Halving 256 000 four times gives 128 000, then 64 000, then 32 000, then 16 000. So 4 half-lives are needed.
Time = 4 × 138. = 552 days
How the marks are earned
- One mark: find how many half-lives are needed by repeated halving. The number is 4.
- One mark: multiply the number of half-lives by the length of one half-life, 138 days.
- One mark: the final time. The unit, days, is not needed for the mark here, but always write it.
Why it scores
Halving step by step is the safest method and makes the number of half-lives clear.
Multiplying the count by the half-life gives the time, and the answer must be in the same unit as the half-life.
Revise Half-LifeHalf the atoms in a sample are polonium-209, half-life 125 years. The rest are polonium-210, half-life 138 days. Compare the two activities and explain.
See how you would score on Half-LifeShow model answer for 07.5Hide model answer for 07.5
Polonium-209 has the lower activity of the two isotopes, because its half-life is much longer. Each polonium-209 nucleus is less likely to decay in a given second, so with equal numbers of atoms of each isotope, polonium-209 gives fewer decays per second than polonium-210.
How the marks are earned
- One mark: name polonium-209 as the isotope with the lower activity, compared with polonium-210.
- One mark: give its much longer half-life as the reason. This mark counts only if the comparison mark is scored.
Why it scores
The first sentence uses the word activity to name which isotope has the lower value, and gives the reason, its much longer half-life. Describing only the rate of decay is not enough.
Revise Half-Life