The size and structure of the atom, mass number and atomic number, isotopes, and how the alpha scattering experiment overturned the plum pudding model.
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1.What is the approximate radius of an atom?
About 1 x 10 to the power minus 10 metres.
2.How does the radius of the nucleus compare with the atom?
It is less than one ten thousandth of the radius of the atom, roughly 1 x 10 to the power minus 14 metres.
3.Where is almost all the mass of an atom?
In the nucleus.
4.What is the atomic number?
The number of protons in the nucleus.
5.What is the mass number?
The total number of protons and neutrons in the nucleus.
6.How do you find the number of neutrons?
Subtract the atomic number from the mass number.
7.What are the relative charges of the three particles?
Proton +1, neutron 0, electron -1.
8.Why is an atom neutral overall?
It has equal numbers of protons and electrons, so the charges cancel.
9.What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons.
10.Do isotopes have the same atomic number?
Yes. They differ only in mass number.
11.What happens if an atom loses an electron?
It becomes a positive ion.
12.How can an atom become ionised?
By absorbing enough energy, for example from radiation, to remove an outer electron.
13.What happens when an electron absorbs electromagnetic radiation?
It can move to a higher energy level, further from the nucleus.
14.What happens when an electron falls back to a lower energy level?
It emits electromagnetic radiation.
15.What was the plum pudding model?
The idea that the atom is a ball of positive charge with negative electrons embedded within it.
16.Describe the alpha scattering experiment.
Alpha particles were fired at very thin gold foil and the directions they travelled afterwards were observed.
17.What were the three observations?
Most passed straight through, some were deflected, and a very small number bounced back.
18.What did most passing straight through show?
That the atom is mostly empty space.
19.What did the small number bouncing back show?
That the mass and positive charge are concentrated in a very small central nucleus.
20.What model replaced the plum pudding model?
The nuclear model.
21.What did Niels Bohr contribute?
That electrons orbit the nucleus at specific distances in fixed energy levels, which matched experimental observations.
22.What did James Chadwick discover?
The neutron, about twenty years after the nuclear model was accepted.
23.Why do scientific models change?
New experimental evidence appears that the existing model cannot explain, so it is modified or replaced.
24.An atom has mass number 14 and atomic number 6. State its composition.
6 protons, 6 electrons and 8 neutrons.
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