Year 8 Science · Unit 2 · Lesson 07

Atomic Number and Mass Number

Foundation Worksheet

Name

Date

Class

This worksheet practises mass number for the isotopes lesson. Your teacher or task instructions define what is included in your Year 8 assessment.

Learning Goals

State what the atomic number (Z) and the mass number (A) count.
Calculate mass numbers and electron numbers from particle counts.
Use proton numbers to identify the element in a set of atoms.

Warm Up

Fill the gap

Choose the correct word from the word bank. Each word is used once at most. Two words will not be used.

mass number ions electrons element protons isotopes neutral atomic number neutrons

The nucleus of an atom holds two kinds of particles: and . The is the number of protons, and it tells you which the atom is. Any atom with 6 protons is carbon. The is the number of protons plus neutrons. In a atom, the number of outside the nucleus equals the number of protons.

1. Complete the two rules. Number of neutrons = − . For an atom with no overall charge, number of electrons = .

Recall 2 marks

Your Turn

Sort it

Write each fact in the correct box. One fact is true of both numbers.

includes the neutrons names the element top left of the symbol equals electrons in a neutral atom needed to work out neutrons bottom left of the symbol

Atomic number (Z) only

Mass number (A) only

Both

Go Deeper

Scenario: what is in the rock?

A geologist crushes a rock sample and tests it. The instrument counts the protons and neutrons in four atoms it finds (Table 1). Row P is done for you. From the periodic table, the atomic numbers are: oxygen 8, magnesium 12, aluminium 13, silicon 14, iron 26.

Table 1. Particles in the nucleus of four atoms from the rock

Atom	Protons	Neutrons	Mass number (A)	Element
P	8	8	16	oxygen
Q	14	14
R	13	14
S	26	30

Source: sample data made up for this worksheet, using real atoms.

2. Complete the last two columns of Table 1. Show your working for the mass number of atom S.

Apply 3 marks

S: A =

3. Atoms Q and R both have 14 neutrons. A student says, “Q and R must be the same element, because they have the same number of neutrons.” Explain why the student is wrong, using the particle numbers.

Analyse 3 marks

Show What You Know

4. A classmate looks at the symbol ³⁹₁₉K for a potassium atom and says, “This atom has 39 protons.” Explain the mistake. In your answer, say what each number in the symbol counts, work out the protons, neutrons and electrons in the atom, and say which number tells you it is potassium.

Analyse 5 marks

Answer Key (for teachers)

Fill the gap

protons; neutrons (either order); atomic number; element; mass number; neutral; electrons. Not used: ions, isotopes.

1. Neutrons = mass number − atomic number (A − Z). Electrons = number of protons (the atomic number). [2 marks]

Sort it

Atomic number only: names the element; bottom left of the symbol; equals electrons in a neutral atom. Mass number only: includes the neutrons; top left of the symbol. Both: needed to work out neutrons.

Go Deeper

2. Q: 28, silicon. R: 27, aluminium. S: 26 + 30 = 56, iron. [3 marks: 1 mass numbers; 1 elements; 1 working for S]

3. The element depends only on the number of protons, not neutrons. Q has 14 protons, so it is silicon; R has 13 protons, so it is aluminium. Having the same number of neutrons does not make atoms the same element. [3 marks: 1 protons decide the element; 1 correct proton numbers; 1 different elements named]

Show What You Know

4. Marking guide (points expected): 1 the top number, 39, is the mass number, which counts protons plus neutrons; 1 the bottom number, 19, is the atomic number, which counts protons only, so there are 19 protons, not 39; 1 neutrons = 39 − 19 = 20; 1 electrons = 19 because the atom is neutral; 1 the atomic number (19 protons) identifies the element: any atom with 19 protons is potassium. [5 marks]

Common error to watch for: reading the larger number in a symbol as the proton count. The atomic number is always the smaller (bottom) number, except in hydrogen-1, where both are 1.