9.1 Nuclear atom and isotopes — Learn

Prerequisites: the idea from earlier science that all matter is made of atoms; writing very small numbers in standard form; using a table to read values.

By the end of this section you can: describe the structure of an atom; state the atomic number, the mass number and the number of electrons; explain what an isotope is; and describe how the model of the atom changed.

1. Atoms are mostly empty space

An atom is the smallest part of an element. It is far too small to see. The radius of an atom is about $1 \times 10^{-10}\,\text{m}$ .

Almost all of the mass of an atom sits in a tiny central nucleus. The radius of the nucleus is less than $\dfrac{1}{10\,000}$ of the radius of the atom. If the atom were the size of a sports hall, the nucleus would be a grain of sand at its centre.

2. The particles inside an atom

An atom is built from three particles.

Particle	Relative charge	Where it is
Proton	$+1$	In the nucleus
Neutron	$0$	In the nucleus
Electron	$-1$	In energy levels around the nucleus

The nucleus is positively charged, because the protons in it are positive and the neutrons are neutral. The electrons are negatively charged, and they are arranged at different distances from the nucleus. Each distance is an energy level.

An atom has no overall charge, because the number of electrons equals the number of protons.

3. Atomic number and mass number

Two numbers describe the nucleus of an atom.

The atomic number is the number of protons in the nucleus. Every atom of the same element has the same atomic number.
The mass number is the total number of protons and neutrons in the nucleus.

An atom is written with these numbers beside its symbol.

^{A}_{Z}\text{X}

Here $Z$ is the atomic number and $A$ is the mass number. So $^{23}_{11}\text{Na}$ has 11 protons and $23 - 11 = 12$ neutrons.

Worked example 1. An atom has 11 protons and 12 neutrons. State its atomic number, its mass number and the number of electrons in the neutral atom.

\begin{aligned} \text{atomic number} &= 11 \\ \text{mass number} &= 11 + 12 = 23 \\ \text{electrons} &= 11 \end{aligned}

The atomic number is the number of protons. The mass number counts protons and neutrons together. A neutral atom has the same number of electrons as protons.

4. Isotopes

Atoms of the same element can have different numbers of neutrons. These atoms are called isotopes of that element.

Carbon-12 has 6 protons and 6 neutrons, so $A = 12$ .
Carbon-14 has 6 protons and 8 neutrons, so $A = 14$ .

Both are carbon, because both have 6 protons. Their mass numbers are different, because their numbers of neutrons are different. Isotopes of an element have the same chemical behaviour, because the number of electrons is the same.

Worked example 2. Write the symbols for carbon-12 and carbon-14, and explain why they are isotopes.

Carbon-12 is $^{12}_{6}\text{C}$ and carbon-14 is $^{14}_{6}\text{C}$ . Both have 6 protons, so both are atoms of carbon. They have 6 and 8 neutrons, so their mass numbers are 12 and 14. Atoms with the same number of protons but different numbers of neutrons are isotopes.

5. Ions

An atom can lose or gain outer electrons. It then becomes a charged particle called an ion.

An atom that loses one or more electrons has more protons than electrons, so it becomes a positive ion.
An atom that gains electrons has more electrons than protons, so it becomes a negative ion.

For example, a magnesium atom can lose two outer electrons. It then has 12 protons and 10 electrons, so its charge is $+2$ .

6. Electron energy levels

An electron can move between energy levels. It does this by absorbing or emitting electromagnetic radiation.

When an atom absorbs electromagnetic radiation, an electron moves to a higher energy level, further from the nucleus.
When an atom emits electromagnetic radiation, an electron moves to a lower energy level, closer to the nucleus.

The radiation absorbed or emitted has an energy that matches the gap between the two levels.

Watch out. The mass number is not the number of neutrons. It counts the protons and the neutrons together. A carbon-14 atom has mass number 14, but it has only 8 neutrons. Also take care not to change the proton number when you write an isotope: isotopes of carbon always have 6 protons. Re-check by adding the protons and neutrons to see whether you get the mass number.

7. How the model of the atom changed

Scientific models change when new evidence does not fit them. The model of the atom changed several times.

A solid sphere. Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided.
The plum pudding model. The discovery of the electron led to a model in which the atom was a ball of positive charge with negative electrons spread through it.
The nuclear model. In the alpha particle scattering experiment, most alpha particles passed straight through a thin gold foil, but a few were repelled back. This showed that the mass and the positive charge are concentrated in a tiny nucleus. The nuclear model replaced the plum pudding model.
Electron shells. Niels Bohr suggested that electrons orbit the nucleus at specific distances, and his calculations agreed with experiments.
The proton. Later experiments showed that the positive charge of the nucleus comes in whole numbers of a smaller particle, which was named the proton.
The neutron. About 20 years later, James Chadwick found evidence for the neutron in the nucleus.

Worked example 3. Explain how the alpha particle scattering experiment changed the model of the atom.

Most alpha particles passed straight through the foil, so most of the atom is empty space. A small number were repelled back, so the positive charge is concentrated in a very small region. This evidence produced the nuclear model: a tiny, positively charged nucleus holding almost all of the mass, with the electrons around the outside.

Worked example 4. The radius of a nucleus is about $1 \times 10^{-14}\,\text{m}$ . The radius of an atom is about $1 \times 10^{-10}\,\text{m}$ . Work out how many times smaller the nucleus is than the atom.

\begin{aligned} \frac{1 \times 10^{-10}}{1 \times 10^{-14}} &= 1 \times 10^{4} \end{aligned}

The atom is about $10\,000$ times wider than its nucleus.

8. Try it: build an atom

The figure lets you add protons and neutrons and watch the atom change.

Predict first. Do not move the sliders yet. Write down your answers.

If you add a neutron, which number changes: the atomic number or the mass number?
If you add a proton, what happens to the name of the element?
If you add a neutron, does the atom stay the same element?

Test. Set the protons to 3 and the neutrons to 4. Read the element. Now add one neutron and read the numbers again. Then move the protons one at a time and watch the element name.

Explain. Adding a neutron changes the mass number but not the element. Adding a proton changes the element. Explain why, using the words proton, neutron, atomic number and mass number. Write your answer in two or three sentences.

9. Progress check

Return to the chapter problem. The tracer isotope has 43 protons and 56 neutrons. State its atomic number, its mass number, and the number of electrons in a neutral atom.

You should now be able to

describe the structure of an atom and the three particles inside it;
state the relative charge and the position of a proton, a neutron and an electron;
state the atomic number, the mass number and the number of electrons of an atom;
work out the numbers of protons, neutrons and electrons from an isotope symbol;
explain what an isotope is, and write the symbols of two isotopes;
explain how the model of the atom changed from a solid sphere to the nuclear model.