9.1 Nuclear atom and isotopes — Practise

Work through these on your own. Keep your working neat: the method matters as much as the answer.

Both the questions and the answers below come from a set that is checked by computer every time the book is built.

State the relative electric charge of a proton, a neutron and an electron, and say where the protons and neutrons are found in an atom.
An atom has 15 protons and 16 neutrons. State its mass number and the number of electrons in the neutral atom.
An atom of iron is written $^{56}_{26}\text{Fe}$ . State the number of protons, the number of neutrons and the number of electrons.
Define the term isotope.
Carbon-12 and carbon-14 are both atoms of carbon. Explain why they are isotopes of the same element.
Describe what the alpha particle scattering experiment showed about the mass and the charge of an atom.
Describe the difference between the plum pudding model and the nuclear model of the atom.
An atom of magnesium loses two outer electrons. State the charge on the ion that is formed, and explain your answer.
The radius of an atom is about $1 \times 10^{-10}\,\text{m}$ . The radius of its nucleus is about $1 \times 10^{-14}\,\text{m}$ . Work out how many times smaller the nucleus is than the atom.
Describe what happens to an electron when an atom absorbs electromagnetic radiation, and what happens when the atom emits electromagnetic radiation.
In 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.
A pupil says that the mass number of an atom is the number of neutrons in it. Explain what is wrong with this statement, and give the correct definition.

Answers and concise solutions

1. The three particles and their charges

A proton has a relative charge of +1.
A neutron has no charge; its relative charge is 0.
An electron has a relative charge of -1.
The protons and neutrons are packed together in the nucleus at the centre of the atom.

2. Mass number and electrons

Answer: Mass number $= 15 + 16 = 31$ . A neutral atom has 15 electrons, the same as the number of protons.

3. Protons, neutrons and electrons in iron-56

Answer: The lower number is the atomic number, so there are 26 protons. The upper number is the mass number, so the number of neutrons is $56 - 26 = 30$ . A neutral atom has 26 electrons.

4. Definition of an isotope

Isotopes are atoms of the same element, so they have the same number of protons.
They have different numbers of neutrons, so they have different mass numbers.

5. Why carbon-12 and carbon-14 are isotopes

Both have 6 protons, because the number of protons decides which element an atom is.
Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. The different numbers of neutrons give them different mass numbers.

6. What the scattering experiment showed

Most alpha particles passed straight through, but a few were repelled back. This showed that almost all of the mass is concentrated in a very small volume at the centre.
The particles that were repelled were pushed back by a concentrated positive charge, so the centre of the atom is positively charged. This centre is the nucleus.

7. Plum pudding compared with the nuclear model

In the plum pudding model the atom is a ball of positive charge with negative electrons spread through it.
In the nuclear model the positive charge and almost all of the mass are concentrated in a tiny nucleus, with the electrons around the outside.

8. The charge on the ion

The atom loses two negative electrons, so the number of protons is now greater than the number of electrons.
The atom had no overall charge. Removing two electrons leaves a charge of $+2$ .

9. How many times smaller the nucleus is

Answer: $\dfrac{1 \times 10^{-10}}{1 \times 10^{-14}} = 1 \times 10^{4}$ . The atom is about 10 000 times wider than its nucleus.

10. Absorption and emission of radiation

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

11. Atomic number, mass number and electrons of the tracer

Answer: Atomic number $Z = 43$ . Mass number $A = 43 + 56 = 99$ . A neutral atom has 43 electrons.

12. Correcting the definition of mass number

The mass number is not the number of neutrons on its own. It counts both kinds of particle in the nucleus.
The mass number is the total number of protons and neutrons in the nucleus.