Chapter 1 of 8

All 8 chapters
  1. 1Inside the atom, readunderstood
  2. 2How the model of the atom changed, readunderstood
  3. 3Isotopes, readunderstood
  4. 4Electron shells, readunderstood
  5. 5The periodic table, readunderstood
  6. 6Ionic bonding, readunderstood
  7. 7Covalent bonding, readunderstood
  8. 8Metals and alloys, readunderstood

S4.6.1 Chapter 1

Inside the atom

An atom is too small to see. How do we know what is inside it?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Give the charge, mass and place of protons, neutrons and electrons
  • Work out the number of each particle from atomic and mass numbers

Everything around you is made of atoms. They are very small. More than a million atoms side by side would fit across 1 mm. No ordinary microscope can show one.

Scientists found what is inside an atom by experiments, and the next chapter tells how. This chapter gives the result. An atom has a tiny centre called the nucleus. Even smaller particles called electrons move outside it.

The nucleus holds two kinds of particle. A proton has a positive charge. A neutron has no charge. An electron has a negative charge.

Draw it first

Build a helium atom

  1. First the nucleus of a helium atom, with two protons and two neutrons. A key labels a proton, which is hatched with a plus sign, and a neutron, which is plain. Then the whole atom: a nucleus labelled nucleus, with He for helium, and one shell labelled shell. Two electrons on the shell, one of them labelled electron.protonneutron

    1 Start with the nucleus. Helium has 2 protons and 2 neutrons, packed tightly together.

  2. First the nucleus of a helium atom, with two protons and two neutrons. A key labels a proton, which is hatched with a plus sign, and a neutron, which is plain. Then the whole atom: a nucleus labelled nucleus, with He for helium, and one shell labelled shell. Two electrons on the shell, one of them labelled electron.Heelectronnucleusshell

    2 Add the electrons. Two of them move round the nucleus on a shell. The picture is a model, and the shell is not a real ring.

Charge and mass are the two things that tell the particles apart. Scientists compare them with a proton, so the numbers stay small. These are relative values, not kilograms.

A table of three particles with three columns: particle, charge and mass. A proton has charge +1 and mass 1. A neutron has charge 0 and mass 1. An electron has charge −1 and mass ≈ 0, which means about 0.ParticleChargeMassproton+11neutron01electron−1≈ 0
Protons and neutrons are in the nucleus. Electrons are outside it. The sign ≈ means about. An electron has about 1/1840 of the mass of a proton.

An electron is so light that we count its mass as almost nothing. Nearly all the mass of an atom is in the nucleus.

A whole atom has no charge. We say it is neutral. The protons carry the positive charge and the electrons the negative charge, and the two must cancel. So a neutral atom has the same number of protons as electrons. Helium has 2 protons and 2 electrons: +2 and −2 make 0.

The nucleus of a carbon atom, enlarged. Six hatched circles with a plus sign are protons, labelled proton. Six plain circles are neutrons, labelled neutron.protonneutron
A carbon nucleus, enlarged. It has 6 protons and 6 neutrons.

Every atom of one element has the same number of protons. This is the atomic number. Carbon always has 6 protons, so its atomic number is 6. An atom with 7 protons is nitrogen, never carbon. The number of protons decides which element an atom is.

The mass number is the number of protons plus the number of neutrons. These are the particles in the nucleus. A carbon atom with 6 protons and 6 neutrons has mass number 12. Electrons are left out because their mass is almost nothing.

See it three ways

As a picture

Sodium written in nuclide notation. The symbol Na is large. The number 23 at the top left is labelled mass number. The number 11 at the bottom left is labelled atomic number.2311Namass numberatomic number

In words

Chemists write the mass number above the atomic number, to the left of the symbol. This sodium atom has 11 protons and 23 particles in its nucleus.

In symbols

neutrons = mass number − atomic number = 23 − 11 = 12

Worked example

A sodium atom has atomic number 11 and mass number 23. How many protons, electrons and neutrons does it have?

  1. The atomic number is the number of protons. Sodium has 11 protons.
  2. An atom is neutral, so it has as many electrons as protons. Sodium has 11 electrons.
  3. Neutrons = mass number − atomic number = 23 − 11 = 12.

Answer: 11 protons, 11 electrons and 12 neutrons

Worked example

A chlorine atom has atomic number 17 and mass number 35. How many protons, electrons and neutrons does it have?

  1. Protons = atomic number = 17.
  2. Electrons = protons = 17, because the atom is neutral.
  3. Neutrons = mass number − atomic number = 35 − 17 = 18.

Answer: 17 protons, 17 electrons and 18 neutrons

A periodic table shows the same numbers. Each box has the symbol and the name. It also has the atomic number and a number called the relative atomic mass. Chapter 3 explains why that mass is sometimes not a whole number.

A box from the periodic table for sodium. The number 11 at the top is labelled atomic number. The large symbol Na is labelled symbol. The word sodium is labelled name. The number 23 at the bottom is labelled relative atomic mass.11Nasodium23atomic numberrelative atomic masssymbolname
The periodic table box for sodium.

Try it

Make a paper model of a lithium atom

You need: a sheet of paper and a pencil, 3 dark beans or seeds, for protons, 4 light beans, seeds or small stones, for neutrons, 3 tiny seeds or grains of rice, for electrons.

Be safe: Do not eat the beans or seeds you use for the model. Keep small seeds away from young children.

  1. Draw a small circle in the middle of the paper. It is the nucleus.
  2. Put the 3 dark beans and the 4 light ones inside it. Count them.
  3. Draw two rings round the nucleus, one inside the other.
  4. Put 2 tiny seeds on the inner ring and 1 on the outer ring.
  5. Count the particles in the nucleus. Count the electrons.

What you will see: The nucleus holds 7 beans, which is the mass number. It holds 3 dark beans, which is the atomic number. There are 3 electrons, as many as protons. The model is not to scale. If the nucleus were a bean 1 cm wide, the whole atom would be between 100 m and 1 km across.

If you cannot try it, this is what happens.
A model of a lithium atom. The nucleus is labelled nucleus and shows 3p, for three protons, and 4n, for four neutrons. Two shells, labelled shell, hold 2 electrons and 1 electron. One electron is labelled electron.3p4nshellnucleuselectron

A wrong idea many people have

An atom is a solid ball, full all the way through.

Why it feels right: A stone feels solid, so it seems that the atoms in it must be solid too.

The test that breaks it: Think of the bean model. If the nucleus is a bean 1 cm wide, the atom is between 100 m and 1 km across. The electrons are tens or hundreds of metres away. Almost all the atom is empty space between the nucleus and the electrons.

What is true: An atom is mostly empty space. A tiny, heavy nucleus sits at the centre. The stone feels solid because of the forces between atoms, not because the atoms are full.

Key words

nucleus
the tiny centre of an atom, made of protons and neutrons.
proton
a particle in the nucleus with a charge of +1 and a mass of 1.
neutron
a particle in the nucleus with no charge and a mass of 1.
electron
a particle outside the nucleus with a charge of −1 and almost no mass.
neutral
with no overall charge.
atomic number
the number of protons in an atom.
mass number
the number of protons plus the number of neutrons.

Back to your guess

The answer is B: A tiny centre called the nucleus, with even tinier electrons outside it.

An atom has a tiny nucleus made of protons and neutrons. Electrons move outside it. The atom is neutral because the charges cancel.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. This is the nucleus of a lithium atom. How many electrons does the whole atom have?

    The nucleus of a lithium atom, enlarged. Hatched circles with a plus sign are protons, labelled proton. Plain circles are neutrons, labelled neutron.protonneutron
    Tap your answer.

    1. Yes, that is right. The hatched circles are protons, and there are 3. A neutral atom has as many electrons as protons.

    2. Not this one. You may have counted the neutrons. Neutrons have no charge. The electrons must cancel the protons, which are the hatched circles. Try another.

    3. Not this one. You may have counted every particle in the nucleus. Only the protons need electrons to cancel their charge. Try another.

    See the answer

    3 electrons

    The hatched circles are protons, and there are 3. A neutral atom has as many electrons as protons.

    • If you chose B: You may have counted the neutrons. Neutrons have no charge. The electrons must cancel the protons, which are the hatched circles.
    • If you chose C: You may have counted every particle in the nucleus. Only the protons need electrons to cancel their charge.

  2. Which particle in an atom has no charge?

    Tap your answer.

    1. Not this one. A proton is in the nucleus, but it carries a positive charge of +1. Try another.

    2. Yes, that is right. A neutron has a charge of 0. That is why it is called neutral.

    3. Not this one. An electron is very light, but it carries a negative charge of −1. Try another.

    See the answer

    The neutron

    A neutron has a charge of 0. That is why it is called neutral.

    • If you chose A: A proton is in the nucleus, but it carries a positive charge of +1.
    • If you chose C: An electron is very light, but it carries a negative charge of −1.

  3. How does an electron compare with a proton?

    Tap your answer.

    1. Not this one. The charges are opposite, but an electron has only about 1/1840 of the mass of a proton. Try another.

    2. Not this one. A proton is positive and an electron is negative. The charges are opposite. Try another.

    3. Yes, that is right. A proton is +1 and an electron is −1. An electron has about 1/1840 of the mass of a proton.

    See the answer

    It has the opposite charge and much less mass.

    A proton is +1 and an electron is −1. An electron has about 1/1840 of the mass of a proton.

    • If you chose A: The charges are opposite, but an electron has only about 1/1840 of the mass of a proton.
    • If you chose B: A proton is positive and an electron is negative. The charges are opposite.

  4. Where is almost all of the mass of an atom?

    Tap your answer.

    1. Yes, that is right. Protons and neutrons each have a mass of 1. Electrons have almost none. Protons and neutrons are in the nucleus.

    2. Not this one. Electrons have almost no mass. The shells are mostly empty space. Try another.

    3. Not this one. That is the solid ball idea. The mass is packed into the tiny nucleus. Try another.

    See the answer

    In the nucleus

    Protons and neutrons each have a mass of 1. Electrons have almost none. Protons and neutrons are in the nucleus.

    • If you chose B: Electrons have almost no mass. The shells are mostly empty space.
    • If you chose C: That is the solid ball idea. The mass is packed into the tiny nucleus.

  5. A fluorine atom has 9 protons and mass number 19. How many neutrons does it have?

    Tap your answer.

    1. Not this one. The mass number counts protons and neutrons together. Take away the protons. Try another.

    2. Not this one. That is the number of protons. Neutrons = mass number − atomic number. Try another.

    3. Not this one. Adding gives too many. The mass number already includes the protons, so you take the protons away. Try another.

    4. Yes, that is right. Neutrons = mass number − atomic number = 19 − 9 = 10.

    See the answer

    10 neutrons

    Neutrons = mass number − atomic number = 19 − 9 = 10.

    • If you chose A: The mass number counts protons and neutrons together. Take away the protons.
    • If you chose B: That is the number of protons. Neutrons = mass number − atomic number.
    • If you chose C: Adding gives too many. The mass number already includes the protons, so you take the protons away.

  6. Why is a whole atom neutral?

    Tap your answer.

    1. Not this one. Neutrons have no charge, so they cannot cancel anything. The electrons do that. Try another.

    2. Yes, that is right. Each proton is +1 and each electron is −1. With equal numbers the total is 0.

    3. Not this one. It does. It has protons (+1) and electrons (−1). They cancel. Try another.

    See the answer

    It has as many electrons as protons, and their charges cancel.

    Each proton is +1 and each electron is −1. With equal numbers the total is 0.

    • If you chose A: Neutrons have no charge, so they cannot cancel anything. The electrons do that.
    • If you chose C: It does. It has protons (+1) and electrons (−1). They cancel.

  7. A magnesium atom has atomic number 12 and mass number 24. How many neutrons does it have?

    See the answer

    12 neutrons

    24 − 12 = 12.

  8. An atom of silicon has 14 protons and 14 neutrons. What is its mass number?

    See the answer

    28

    The mass number is protons plus neutrons: 14 + 14 = 28.

Keep it

Copy this into your exercise book.

An atom has a tiny nucleus of protons (+1) and neutrons (0). Electrons (−1) move outside. Atomic number = protons. Mass number = protons + neutrons. A neutral atom has equal protons and electrons.

Draw: A sodium atom with its symbol, mass number 23 and atomic number 11, and the number of each particle beside it.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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