Atomic and Nuclear Physics

Inside the atom

Everything is made of atoms, and inside every atom is a tiny, dense centre called the nucleus, holding protons and neutrons, with electrons moving around it. Nearly all the mass of an atom sits in that nucleus, even though it is a minute part of the atom. Some nuclei are unstable and give out radiation as they change, which is what we mean by radioactivity. This module covers the parts of the atom, what isotopes are, the three types of nuclear radiation, the idea of half-life and how to calculate with it, and where radiation is useful and where it is dangerous.

Atomic words

Learn these terms before the ideas are described. They name the parts of the atom and of radioactivity.

Match each particle to its property

Alpha against gamma

The three types of nuclear radiation differ in how far they travel and what stops them. Comparing the extremes makes the pattern clear.

Where is the mass?

Where is nearly all the mass of an atom found?

• In the nucleus
• In the electrons
• Spread evenly through the whole atom
• In the empty space around the atom

Understanding half-life

Radioactive decay is random for any one nucleus, but across a large sample it follows a steady pattern measured by the half-life. The half-life is the time it takes for half of the radioactive nuclei to decay, and it is the same length of time whatever the starting amount. So after one half-life the activity has dropped to a half, after two it is a quarter, after three an eighth, and so on. To calculate, count how many half-lives have passed, then halve the amount that many times. Count the half-lives, then halve for each: that is the whole method, and it works for any radioactive sample.

Pick the true radiation facts

Select every true statement about the atom and radiation.

• The nucleus holds protons and neutrons
• Alpha radiation is stopped by paper
• Gamma radiation is the most penetrating
• Most of the mass of an atom is in the electrons
• Every nucleus is stable and never decays

Order a half-life calculation

Put the steps for working out how much of a sample remains after several half-lives in order.

• Note the starting amount or count rate
• Find the length of one half-life
• Count how many half-lives have passed
• Halve the amount once for each half-life
• State the amount that is left

Complete the atomic facts

The tiny dense centre of an atom is the _____. A positive particle inside it is a _____. Atoms of the same element with different numbers of neutrons are _____. The time for half the nuclei to decay is the _____.

One sample, three half-lives

Suppose a radioactive sample has a count rate of eighty when you first measure it, and its half-life is one hour. After one hour, one half-life has passed, so the count rate falls to half, which is forty. After a second hour, that is two half-lives, so it halves again to twenty. After a third hour, three half-lives, it halves once more to ten. Notice that the same length of time, one hour, always halves the count, no matter how much is there to begin with. Counting the half-lives and halving each time is all the calculation needs, and it lets you predict how a sample fades away.

Tap the types of radiation

Tap the TWO things that are types of nuclear radiation.

• alpha radiation
• gamma radiation
• a sound wave
• a magnet

Work out the count rate

A radioactive sample has a count rate of 80. After two half-lives, what is the count rate? Halve it once for each half-life.

Apply the nuclear physics

Read each situation and choose the best answer.

• A source is completely stopped by a single sheet of paper. Which type of radiation is it most likely to be?
• Two atoms are the same element but have different numbers of neutrons. What are they?
• A sample has a count rate of 40 and a half-life of one day. What will the count rate be after one day?

Build a point about isotopes

Choose the word for each gap to complete one clear point about atoms and radiation.

Explain atomic and nuclear physics

A friend thinks an atom is just a solid ball and that all radiation is the same. Using what you have learned, explain the structure of the atom and how nuclear radiation behaves.

• Explain the parts of an atom and where the mass is
• Explain what an isotope is
• Explain the three types of radiation and their penetrating power
• Explain what half-life means and how to use it