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Scales in space and time
Scales in space and time

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2.12 Protons and neutrons

The following slide show considers the size of subatomic particles, protons and neutrons, and their role in determining the identity of an element. You will consider the size of protons and neutrons relative to an atom, and how to use isotopes of carbon to date samples for a specific period.

While working through the slides, record the following information to use in the questions that follow:

  • the diameter of a neutron
  • the half-life of carbon-14 (14C).
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Subatomic particles: protons and neutrons
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Question 1

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Question 2

In another part of this topic you learn that the diameter of a carbon atom is about 1.4 × 10−10 m. How many times larger is a carbon atom than a neutron? Report your answer in full, not using scientific notation, and leaving no spaces between digits.

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Question 3

Radioactive 14C (read aloud as ‘14-C’) decays to 14N in a predictable way. The half-life of 14C is the number of years it takes half the amount of 14C remaining in a sample to decay.

Table 9 shows the percentage of 14C remaining against years since plant death for a sample. Using the value of the half-life you noted from the slide show, select the correct values for the list below to replace W, X, Y and Z.

Table 9 Decay of 14C in dead plant tissue.
Years since plant death % 14C remaining
0 100
5730 50
11 460 25
W 12.5
X 6.3
28 650 Y
34 380 Z
40 110 0.8
45 840 0.4
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Question 4

There are equations describing the curve of the decay graph that was shown in the slide show, and these allow an organic sample to be dated accurately. However, you can use Table 9 to estimate the age of sample.

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Question 5

Different radioactive isotopes have different half-lives. The time frame varies substantially across the elements. For uranium-232, the half-life is just 68.9 years. Use the diameter of a uranium atom (312 pm or picometres), together with the half-life of 232U to add the ‘Half-life of a uranium-232’ bar to the size–time explorer [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . (Note that 1 pm = 1 × 10–12 m).

Summary

In this section you have learned that atoms consist of subatomic particles including protons and neutrons. The number of protons determines the element. The number of neutrons in atoms can vary, which results in different isotopes of the same element (e.g. 12C, 13C and 14C). Neutrons are lost from atoms at predictable rates. This phenomenon has various practical scientific applications, including radiocarbon dating.

Next: Look at the smallest scale now: quarks, which you can do with the ‘Next >’ button. Alternatively, you can return to the size–time explorer and choose a level for yourself.