The model of the atom has changed over time. Thomson, Rutherford and Bohr each added to it.
(a)(i) Which one of Thomson, Rutherford and Bohr found the electron?
Our own questions on this topic, marked straight away, so you see what to revise. Free now.
Show model answer for 18.1Hide model answer for 18.1
Thomson. He found that atoms contain tiny negative particles, the electrons.
(a)(ii) Which one of the three developed the plum pudding model?
See how you would score on Atomic StructureShow model answer for 18.2Hide model answer for 18.2
Thomson. His model was a ball of positive charge with electrons spread through it, like plums in a pudding.
(a)(iii) Which one of the three said electrons can only travel in set orbits, called shells?
See how you would score on Atomic StructureShow model answer for 18.3Hide model answer for 18.3
Bohr. He suggested electrons move in fixed shells around the nucleus, not at any distance.
(b) In the gold foil experiment, most alpha particles went straight through and some were deflected. Say what each result shows about the atom.
See how you would score on Atomic StructureShow model answer for 18.4Hide model answer for 18.4
Because nearly all the alpha particles went straight on, the atom must be mostly gaps, with a very small nucleus.
Some alpha particles being deflected shows that the atom has a positively charged centre, the nucleus, which repels the positively charged alpha particles.
How the marks are earned
- One mark: the straight-through particles show the atom is largely empty space (a tiny nucleus is also accepted).
- One mark: the deflections show that part of the atom is positively charged. Saying positive charge is enough.
- One mark: naming a nucleus. Saying it is small, dense or at the centre is good physics but is not needed.
- If none of these marks is earned, one mark is allowed for any correct conclusion given for the wrong observation.
Why it scores
If the atom were a solid ball, as in the plum pudding model, the particles would not pass through so easily. This earns the first mark.
Alpha particles are positive, and like charges repel. Only a concentrated positive charge could turn a fast alpha particle aside. This earns the other two marks: positive charge, and nucleus.
Revise Atomic Structure(c) A molecule is 20 atoms in a straight line, each with radius 1.0 × 10-10 m. Give its length in standard form.
See how you would score on Atomic StructureShow model answer for 18.5Hide model answer for 18.5
Each atom is a sphere, and in a line the atoms touch across their full width, so each uses its diameter:. 2 × 1.0 × 10-10 = 2.0 × 10-10 m. For 20 atoms:. 20 × 2.0 × 10-10 = 4.0 × 10-9 m
How the marks are earned
- One mark: each atom takes up its diameter, twice the radius, so the length is 2 × 1.0 × 10-10 × 20.
- One mark: the answer 4.0 × 10-9 m.
- Using the radius instead of the diameter, giving 2.0 × 10-9 m, earns 1 mark. A correct value not in standard form, such as 0.000000004 or 40 × 10-10, earns 1 mark.
- The correct answer, 4.0 × 10-9 m, on the answer line earns full marks.
Why it scores
The common slip is using the radius, which gives half the true length and only 1 mark. Standard form needs one digit before the decimal point, here 4.0, times a power of ten.
Revise Atomic Structure