9.1 Nuclear atom and isotopes — Practise with help

Work each problem first. Open a hint only if you are stuck, and the full working only to check.

Warm-up: remember

Answer these from memory. They use ideas from earlier science and from writing numbers in standard form.

Write 0.0001 in standard form.
A neutral atom has 8 protons. How many electrons does it have?

Answers

$1 \times 10^{-4}$ .
8 electrons.

If question 1 went wrong, count the places the decimal point moves to the right. If question 2 went wrong, remember that a neutral atom has no overall charge, so the positive and negative charges must balance.

Practise with help

1. State the relative charge of a proton, a neutron and an electron, and say where the protons and neutrons are found.

Hint

One of the three particles has no charge at all. The nucleus is the centre of the atom.

Next step

Write the three particles in a list and put a charge beside each one: $+1$ , $0$ or $-1$ .

Full working

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 the neutrons are packed together in the nucleus at the centre of the atom.

2. An atom has 17 protons and 18 neutrons. State its atomic number, its mass number and the number of electrons, and write its symbol in the form $^{A}_{Z}\text{X}$ .

Hint

The atomic number is the number of protons. The mass number counts the protons and the neutrons together.

Next step

Write the atomic number as the number of protons, then add the protons and the neutrons for the mass number.

Full working

The atomic number is the number of protons, so $Z = 17$ . The mass number is $A = 17 + 18 = 35$ . A neutral atom has 17 electrons. The symbol is $^{35}_{17}\text{Cl}$ .

3. The symbol $^{235}_{92}\text{U}$ describes a uranium atom. State the number of protons, the number of neutrons and the number of electrons in the neutral atom.

Hint

The lower number is the atomic number and the upper number is the mass number.

Next step

Write down the number of protons from the lower number, then work out the neutrons by subtracting it from the upper number.

Full working

The atomic number is 92, so there are 92 protons. The number of neutrons is $235 - 92 = 143$ . The neutral atom has 92 electrons.

4. Carbon-12 and carbon-14 are both atoms of carbon. Explain why they are isotopes of the same element.

Hint

The element is decided by the number of protons, not by the number of neutrons.

Next step

State the number of protons in each atom, then state the number of neutrons in each atom.

Full working

Both atoms have 6 protons, so both are atoms of carbon. Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons, so their mass numbers are different. Atoms of the same element with different numbers of neutrons are isotopes.

5. Explain why the nuclear model replaced the plum pudding model of the atom.

Hint

The alpha particle scattering experiment gave new evidence that the old model could not explain.

Next step

Say what happened to most of the alpha particles, then say what happened to the small number that were repelled.

Full working

In the plum pudding model, the positive charge is spread through the whole atom. If that were true, alpha particles would pass through with only small deflections. In the experiment, most alpha particles did pass straight through, but a few were repelled back. The only way to explain the repelled particles is a tiny, concentrated positive charge. This evidence gave the nuclear model, so the plum pudding model was replaced.

6. Explain how a neutral atom becomes a positive ion.

Hint

Think about which particle, when it is removed, leaves more positive charge than negative charge.

Next step

Say which particle leaves the atom, then compare the number of protons with the number of electrons that are left.

Full working

The atom loses one or more outer electrons. It then has more protons than electrons, so the positive charge is greater than the negative charge. The atom becomes a positive ion.