Relative mass | Structure strip

Structure strips can be used to promote independent writing. In this resource learners write about relative mass using key words, explaining how to calculate relative atomic and relative formula masses.  

Learning objectives

1. Explain why relative, rather than actual, masses are more commonly used in chemistry.
2. Combine writing with calculations to give fuller explanations of chemical concepts.
3. Describe how the relative masses of individual atoms, relative atomic mass, relative formula mass and percentage composition are linked.

Introduction

This resource asks learners to demonstrate their thought process, rather than simply to use methods they have memorised for carrying out calculations. In chemistry we compare the masses of subatomic particles, atoms, elements and compounds using relative masses, rather than stating the actual mass of the species in grams. The relative mass of any particle is its mass relative (compared) to 1/12 of the mass of a single Carbon-12 (¹²C) atom. Relative masses are important because we count atoms using their masses as they are far too small to see.

Scaffolding

• Encourage learners to use the suggested key words and phrases in their answers.
• To further support learners, include additional prompts in the structure strip. If learners are struggling to engage with the task, supply them with sentence starters created from the model answers.
• As learners grow in confidence, ask them to attempt the extension question first and then use the structure strip to improve or self-assess their answer.

Key words

Proton, neutron, electron, standard form, isotope, relative atomic mass, weighted average, relative formula mass, percentage. 

Metacognition

This resource supports learners to develop their metacognitive skills in three key areas.

• Planning: the strips provide scaffolding to plan the written response. Learners will decide where to gather information from (textbooks, own notes, revision websites). Ask learners: is the source of information you are using reliable?
• Monitoring: learners are prompted by the questions in the structure strip and can check their own answer against the prompts. Ask learners: have you covered all of the prompts in the space provided? Do you need to change anything to complete the task?
• Evaluation: learners can self-assess or ask a peer to check their work against the answers. Ask learners: did you achieve what you meant to achieve? What might you do differently another time?

Extended answer question

Instruct learners to answer the question after they have attempted the structure strip. The structure strip activates the required knowledge, which learners can then apply to the question.

Consider re-framing the context of this question to one that your learners are more familiar with, to empower them to unlock their existing science capital. Read more about science capital. 

Answers

Suggested answers for the structure strip activity are given in the downloadable teacher notes.

Answers to extended answer question

The relative formula mass of bromine is 80 + 80 = 160.

Bromine exists as two isotopes. Bromine atoms with a relative mass of 79 contain 35 protons and 44 neutrons in their nuclei, whereas bromine atoms with a relative mass of 81 still contain 35 protons but have 46 neutrons.

The relative atomic mass of an element is calculated as a weighted average based on the naturally occurring percentages of its isotopes. The fact that bromine’s relative atomic mass falls right in the middle of the relative masses of its two isotopes suggests that equal amounts of these exist in nature.

The relative formula mass for bromine is different to its relative atomic mass because bromine atoms are never found on their own. Instead, in the element, the atoms always pair up to make molecules, so the masses of the two atoms are added to give the relative formula mass.