Active reading exercise: Isotopes


In 1932 James Chadwick, a British scientist, discovered the neutron. His discovery explains how isotopes are formed. Isotopes have the same number of protons, but different numbers of neutrons.

We can also say that the atomic number is the same, but the mass numbers are different. Isotopes are the same element because their atomic number does not change.

We refer to an isotope by adding its mass number to the element’s name. The isotope in the diagram below is called carbon-12.


Carbon isotopes

Carbon can occur as three different isotopes. They are called carbon-12, carbon-13 and carbon-14.

Carbon dating is used to identify the age of very old objects, for example the remains of extinct animals such as mammoths. The amount of the carbon isotope carbon-14 in an object is examined to determine its age.

Things to do

  1. Fill in the missing words and numbers to describe the similarities and differences between isotopes of the same element.                                                                                 a) Isotopes are atoms with the same            number but different                   number. b) They have the same number of protons and electrons but different numbers of                      .
  2. Name the scientist who discovered the neutron.
  3. Why are isotopes the same element?
  4. How do we refer to isotopes?
  5. Name the three isotopes of carbon.
  6. Why is the isotope carbon-14 useful?
  7. Using your knowledge about isotopes, fill in the gaps in the table a, b and c.
Isotope name Atomic number Mass number
 Tin-116  50 a
b c 118

Remember that the atomic number is the same for isotopes of the same element.

Active reading exercise: The Atom

A bit of history

In 1805 the English Chemist John Dalton published his atomic theory that said:

  • Everything is made up from tiny particles called atoms
  • Atoms are tiny hard spheres (= balls) that cannot be broken down into smaller parts
  • Atoms in one element are all identical

This helped to explain many properties of materials. However, later experiments showed that atoms contained even smaller particles. In 1897 another British scientist, JJ Thomson, discovered the electron. The nucleus which makes up the middle of an atom was discovered by Ernest Rutherford in 1913.

The Structure of the atom

Today we know that atoms are made from three subatomic particles: proton, neutron and electron.

Protons and neutrons are found in the centre of the atom which is called the nucleus. Both have a mass of 1. Protons have a positive (+) charge and neutrons are neutral (= no charge).

Electrons have a negative (-) charge and have almost no mass at all. They are found on the electron shells on the outside of the atom, circling the nucleus.

What you need to remember

  • Atoms are made from protons, neutrons and electrons called subatomic particles
  • Protons: found in nucleus, positive (+) charge and a mass of 1
  • Neutrons: found in nucleus, neutral (no charge) and a mass of 1
  • Electrons: found on electron shells, negative (-) charge and almost no mass


Things to do

  1. Name the scientist who first introduced atomic theory.
  2. Name the scientists who discovered the electron and the nucleus.
  3. State the names of the three subatomic particles as well as their masses and charges.
  4. State where in the atom protons and neutrons are found.
  5. State where in the atom electrons are found.
  6. Copy and label the image of the atom. Words: electron, proton, neutron, shellLithium atom
  7. Describe in your own words what an atom looks like. Include information about the charges and masses of the subatomic particles.

Reading Exercise: The History of the Atomic Model

Democritus’ atoms

Around 440 BC, Greek philosopher Democritus was the first to suggest the existence of atoms, tiny particles that make up all matter. The word atom comes from the Greek ”atomos” meaning indivisible.

However, most of his colleagues, especially Aristotle, did not agree with Democritus. Instead, they thought that matter was made up of the four ”elements” fire, water, wind and earth.

Dalton’s spheres

It took over 2000 years until another scientist would challenge Aristotle’s ”element” theory.

In 1803, John Dalton, an English teacher from Manchester, carried out experiments proving that all matter is made up of tiny particles. He chose to use Democritus’ name and called them atoms.

In Dalton’s model atoms were tiny, hard spheres that vary in size and mass, but cannot be split into smaller pieces.

Thomson’s plum pudding

It took a much shorter time to reach the next step in the discovery of the atomic model.

J.J. Thomson, another English scientist, discovered the electron in 1897 and developed the plum pudding model of the atom for which he received the Nobel prize in 1906.

The plum pudding model said that the tiny negative electrons were distributed in a positive mass inside the atom. The electrons were like negative raising in a positive plum pudding dough.

Rutherford’s gold foil experiment

The next experiment to develop the atomic model even further were carried out by one of Thomson’s former students, Ernest Rutherford, who was originally from New Zealand.

In 1909, Rutherford and his team conducted one of the most important experiments in the history of science. They used a gold foil which was bombarded with alpha particles (= Helium nuclei which have 2 protons and 2 neutrons).

If Thomson’s plum pudding model were true, you would expect all alpha particles to punch holes through the positive ”dough” and pass straight through the foil.

The results looked somewhat different. Most alpha particles did pass straight through the gold foil. However, some particles did not pass through and bounced back. This suggested that there were parts in the foil where mass was very concentrated, while the rest seemed to be empty space.

As a consequence, Rutherford introduced the modern planetary model of the atom where the electrons circulate around a nucleus. The nucleus is small, but contains most of the atom’s mass and is where the alpha particles bounced back in his experiment. The major part of the atom is empty space where the alpha particles could pass through.

By the way, Ernest Rutherford received a Nobel prize as well. However, it was for discovering the concept of half-life for radioactive substances rather than his work on the atomic model.


  1. Draw a timeline including the four stages of the atomic model’s development.
  2. Describe what the word ”atom” means.
  3. Describe what atoms were like in Dalton’s atomic model.
  4. State what Thomson discovered.
  5. Describe what atoms were like in Thomson’s atomic model.
  6. Describe the gold foil experiment Rutherford conducted.
  7. Explain how the gold foil experiment showed that Thomson’s theory was wrong.
  8. Describe what atoms are like in Rutherford’s model.
  9. Challenge: Find out how the atomic model developed further. You could look at the work of Niels Bohr, Werner Heisenberg and James Chadwick.

Active reading exercise: How do muscles work?

Image: D. Keeno, 17 November 2017.

How do muscles work?

Muscles work by getting shorter. We say that they contract.

Muscles are attached to bones by strong tendons. When a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint.

Muscles can only pull, but not push. This would be a problem if a joint was controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. The problem is solved by having muscles in pairs, called antagonistic pairs.

To move the bone back the second muscle of the antagonistic pair contracts. Examples of antagonistic pairs are biceps and triceps.

How does your forearm move?

The elbow joint lets our forearm move up or down. It is controlled by two muscles, the biceps on the front of the upper arm, and the triceps on the back of the upper arm.

The biceps and the triceps are an antagonistic pair.

  • When the biceps muscle contracts, the forearm moves up.
  • When the triceps muscle contracts, the forearm moves down.This solves the problem. To lift the forearm, the biceps contracts and the triceps relaxes. To lower the forearm again, the triceps contracts and the biceps relaxes.


1. Box how muscles work.

2. Circle what attaches muscles to bones.

3. Underline what happens when muscles contract.

4. Underline why muscles need to work in antagonistic pairs.

5. Describe in your own words how antagonistic pairs work.

6. Explain in your own words how your forearm moves.

Reading exercise: Antibiotics and bacteria resistance

The following text is written for pupils with low reading ages to help them study the topic of antibiotics and bacteria resistance.

Before antibiotics

Think back to the last time you cut yourself. Can you imagine that cut becoming infected with bacteria – so seriously infected that you would die?

Before the discovery of antibiotics, there was nothing anybody could do. Bacteria could kill 80 percent of people with infected wounds.

Who would have thought a mouldy plate would lead to this?

In 1928, Alexander Fleming, a doctor at London’s St. Mary’s Hospital, found that a mould on a discarded plate had antibacterial properties. This mould was ‘penicillin’. Penicillin is an antibiotic.

Antibiotics kill bacteria and slow down their growth. A bacterium consists of one single cell and antibiotics disturb their cell functions. Antibiotics do not work against viruses because a virus consists of a DNA fragment instead of a cell.

Human life expectancy increased rapidly by eight years when antibiotics were first introduced in the 1930s.

Bacteria resistance

Within four years of penicillin being introduced onto the market, bacteria resistance was being reported. Bacteria resistance means that an antibiotic no longer kills the bacteria.

Today bacteria resistance against commonly used antibiotics is increasing rapidly around the world and a growing problem.

Active Reading Exercise: What is an echo?

The following active reading exercise includes a short test and tasks suitable for students aged 11 to 14 when studying waves and sound.

When waves hit a surface, they are reflected. This means they bounce off the surface and come back. For example, light is reflected by the surface of a mirror.

When you are high up in the mountains and call out in a loud voice. Your sound waves will be reflected by a mountain surface nearby and you can hear it coming back after a few seconds. This is called an echo.

You can measure the distance from where you stand to the mountain surface and time how long it takes until your hear the echo. With this information you can calculate the speed of sound.

This principle is used by animals like dolphins and bats for navigation. They send out sound waves and listen for their echo. This helps them to work out how far away predators or food are. When used like this echoes are called sonar.

Sonar is also used by submarines for navigation. Submarines send our sound waves and listen for the echo to know their own position. They can also detect other submarines and ships.


  1. Box the words in the text you do not know.
  2. Highlight what happens to waves when they hit a surface.
  3. Highlight the word that describes the reflection of sound waves.
  4. Circle the two types of waves that are mentioned in the text.
  5. a) Underline the animals that use sonar.
  6. b) Underline how submarines use sonar.

*Extension: Explain what a dolphin needs to know to work out the distance to a fish when using sonar.

Can we see atoms?

Image credit: Pixabay 2017.

photos of atoms


There is a large number of physical and chemical techniques available today for the analysis of substances. They provide the opportunity to gain information about the properties of atoms, molecules and ions. Infrared spectroscopy, for example, measures the vibrations of molecules. Mass spectrometry, which is widely known from TV crime dramas, gives information about the weight of molecules or ions. This information about mass can then be used to identify poisons, drugs and other substances.

However, most techniques cannot give us a direct picture of what atoms, molecules or ions really look like. Normally, conclusions about their looks are drawn from measured properties. Imagine you are drawing a picture of a house while somebody is telling you about its appearance, but you cannot see it yourself. This is how physicists and chemists use the information from their measurements to figure out what atoms, molecules and ions look like. The measurements are giving them information about substances, which corresponds to the information about the house, such as colours; size or where doors and windows are.

But are there any methods that can provide us with – let us say – a photo of an atom? The main problem is that atoms are extremely tiny. One atom in an apple is as small as the apple itself is compared to our entire Earth. Nevertheless, there are a few methods that can indeed capture pictures of atoms with the help of quantum mechanics and other cool science. A few of these methods are listed below.

Transmission electron microscopy (TEM)

Atoms cannot be seen under normal light microscopes. The reason is that the wavelength of visible light is much larger than atoms themselves are. To be able to see a sample in a microscope, the wavelength has to be smaller than the sample itself. For example, light waves are smaller than cells which is why cells observed in light microscopes. Transmission electron microscopes use electrons instead of visible light. Thanks to the wave-particle duality, electrons can behave as both waves and particles. As waves, they have a much smaller wavelength than light which makes it possible to see atoms in a transmission electron microscope. Besides the use of electrons, the working principle of a TEM is the same as that of a light microscope.

Scanning tunneling microscopy (STM)

Scanning tunneling microscopy is another microscopic technique. It is based on a quantum mechanical phenomenon called ”tunneling”. Electrons can ”tunnel”, or in other words transtition, from an atom on the tip of an extremely sharp needle to atoms on a sample surface. Needle and surface have to be at a very short distance from each other to enable tunneling. The probability of tunneling increases when the gap between the needle and the surface gets smaller. This means that electron tunneling is more likely to occur when the needle tip is above the center of an atom as compared to the tip being above the space between two atoms. In consequence, a topographic picture of the atoms on the sample surface can be obtained.

Atom probe tomography (APT)

Atom probe tomography is a very powerful technique that can provide three-dimensional images of a sample’s atomic structure. In APT, the magnification is caused by a highly curved electric field instead of electron properties like wavelength or tunneling. For this technique, atoms have to be removed from the sample surface and turned into ions, in other words they have to be ionized. To obtain three-dimensional information, the atoms are removed from the sample layer by layer, which means that, unlike the previous two methods, this technique is destructive.