Topic: Interpretation of chemical formulae

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Elementary school. Chemistry.

II. Internal structure of matter. Pupil:

8) describes how the atom differs from the molecule; interprets entries, e.g. HIndeks dolny 2₂, 2H, 2HIndeks dolny 2₂.

General aim of education

The student interprets the chemical formulas

Key competences

• communication in foreign languages;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

• explain how the atom differs from the molecule, point out the index and stoichiometric ratio in the given chemical record;

• interpret the records of type: H Indeks dolny 2₂, 2H, 2H Indeks dolny 2₂

• draw models based on records of type: HIndeks dolny 2₂, 2H, 2HIndeks dolny 2₂

• specify the number of atoms in the given chemical records of type: HIndeks dolny 2₂O, 2HIndeks dolny 2₂O, AlIndeks dolny 2₂SIndeks dolny 3₃, 2AlIndeks dolny 2₂SIndeks dolny 3₃, HNOIndeks dolny 3₃, 3HNOIndeks dolny 3₃, HIndeks dolny 3₃POIndeks dolny 4₄, 2HIndeks dolny 3₃POIndeks dolny 4₄, Ca(OH)Indeks dolny 2₂, 3Ca(OH)Indeks dolny 2₂, CaIndeks dolny 3₃(POIndeks dolny 4₄)Indeks dolny 2₂, 2CaIndeks dolny 3₃(POIndeks dolny 4₄)Indeks dolny 2₂

Methods/techniques

• activating

  • discussion.

• expository

  • talk.

• exposing

  • film.

• programmed

  • with computer;

  • with e‑textbook.

• practical

  • exercices concerned.

Forms of work

• individual activity;

• activity in pairs;

• activity in groups;

• collective activity.

Teaching aids

• e‑textbook;

• notebook and crayons/felt‑tip pens;

• interactive whiteboard, tablets/computers;

• methodician or green, yellow and red cards;

• periodic table of elements;

• ball‑and‑ball models.

Lesson plan overview

Introduction

1. The teacher hands out Methodology Guide or green, yellow and red sheets of paper to the students to be used during the work based on a traffic light technique. He presents the aims of the lesson in the student's language on a multimedia presentation and discusses the criteria of success (aims of the lesson and success criteria can be send to students via e‑mail or posted on Facebook, so that students will be able to manage their portfolio).

2. The teacher together with the students determines the topic – based on the previously presented lesson aims – and then writes it on the interactive whiteboard/blackboard. Students write the topic in the notebook.

3. Health and safety – before starting the experiments, students familiarise themselves with the safety data sheets of the substances that will be used during the lesson. The teacher points out the need to be careful when working with them.

Realization

1. Classes start with a reminder of the concept of a summary formula - an eager student interprets them on a given formula of a chemical compound, eg AlIndeks dolny 2₂SIndeks dolny 3₃

2. The teacher discusses, on the example of a molecule of water and ammonia, the principles of determining the molecular structure of a covalent compound based on its summary formula. Presents on the multimedia board the formula of the water molecule from the abstract, introduces the concept of the stoichiometric index. He asks those interested in the board and instructs them to determine the number of individual atoms in the following substances: HIndeks dolny 2₂SOIndeks dolny 4₄, HIndeks dolny 3₃PO Indeks dolny 4₄

3. The lecturer displays on the multimedia board an illustration of „Three molecules of water” from the abstract, discussing the principles of describing the sets of molecules, and then introduces the concept of stoichiometric coefficient. It presents the illustration „Number of atoms of particular elements in three molecules of water” and discusses the principles of calculating atoms. Displays the „Number of atoms of particular elements in the five molecules of ammonia” and asks you to interpret and give the number of nitrogen and hydrogen atoms. Presenting and analyzing the table „Interpretation of the recordings of sets of example molecules” from the abstract, initiates discussion.

4. The teacher, using abstract illustrations of „Six atoms of chlorine” and simultaneously „Six molecules of chlorine”, discusses the principles of describing them. Eager students draw on the board models for example chemical records: HIndeks dolny 2₂, 2H, 2HIndeks dolny 2₂, 3N, 4NIndeks dolny 2₂, OIndeks dolny 3₃, OIndeks dolny 2₂, 2OIndeks dolny 3₃, 2OIndeks dolny 2₂. The lecturer displays the table „Number of individual atoms of different chemical records” - analysis and discussion are under way.

5. The teacher discusses the rules for interpreting the records of ionic compounds based on the „Collection described as 10NaCl ...” displayed on the multimedia board. Invites people to the table and instructs them to calculate the individual ions in the following formulas: 4CuClIndeks dolny 2₂, 3NaIndeks dolny 2₂S. Next, he presents the table „Interpretation of chemical records regarding ionic compounds” - ongoing analysis, discussion.

6. The lecturer discusses the interpretation of chemical records using simple and complex chemical formulas. Students, while training the skills of calculating individual atoms of chemical elements, write, for example, 3HIndeks dolny 2₂COIndeks dolny 3₃, 4FeIndeks dolny 2₂OIndeks dolny 3₃, 5SOIndeks dolny 3₃, 3MgIndeks dolny 3₃(POIndeks dolny 4₄)Indeks dolny 2₂. The teacher displays the table „Interpretation of sample chemical records” on the multimedia board - analysis and discussion are under way.

7. At the end of the lesson, the teacher asks students to do an interactive exercise - individual work.

Summary

1. The teacher asks the students to finish the following sentences:

   • Today I learned ...

   • I understood that …

   • It surprised me …

   • I found out ...

   The teacher can use the interactive whiteboard in the abstract or instruct students to work with it

Homework

1. Listen to the abstract recording at home. Pay attention to pronunciation, accent and intonation. Learn to pronounce the words learned during the lesson.

2. Make at home a note from the lesson using the sketchnoting method.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Interpretation of chemical formulae

The chemical formula of a covalent compound describes the structure of its molecules. Symbols of elements used in the formulae inform about types of atoms creating a molecule and numbers placed on at the bottom right of the symbol inform about their number. The numbers we use to express the number of atoms in a molecule are called stoichiometric indices. If stoichiometric index is equal to one, number 1 is not written down. In a chemical formulae of water ${\text{H}}_{\text{2}}\text{O}$ stoichiometric index for hydrogen is 2 and the one for oxygen is – 1. On this basis, we can say that water molecule consists of two atoms of hydrogen and one atom of oxygen.

If we want to present a set consisting of more than one molecule, for example of three molecules of water, the number of these molecules is placed before its formula. Three molecules of water (${\text{H}}_{\text{2}}\text{O}$, ${\text{H}}_{\text{2}}\text{O}$, ${\text{H}}_{\text{2}}\text{O}$) will be described as $\text{3}{\text{H}}_{\text{2}}\text{O}$. The number placed before the compound’s formulae is called stoichiometric number.

Let’s try to determine the number of individual atoms of elements in the set of three molecules of water: ${\text{3H}}_{\text{2}}\text{O}$. One molecule contains two atoms of hydrogen and one atoms of oxygen, while three molecules contain in total $\text{3 · 2}$, that is six atoms of hydrogen and $\text{3 · 1}$, that is three atoms of oxygen.

Five molecules of ammonia, ${\text{5NH}}_{\text{3}}$, consist in total of ($\text{5 · 1 = 5}$) five atoms of nitrogen and ($\text{5 · 3 = 15}$) fifteen atoms of hydrogen.

In OIndeks dolny 3₃ molecule a stoichiometric index with an element’s symbol equal to 3 indicates that ozone molecules are made up of three atoms of oxygen.

A stoichiometric number written before the symbol (for example $\text{6Cl}$ – 6 atoms of chlorine) or a formula (for example ${\text{6Cl}}_{\text{2}}$ – 6 molecules of chlorine) denotes the number of atoms or molecules. $\text{6Cl}$ indicates 6 free unbound atoms of chlorine, while ${\text{6Cl}}_{\text{2}}$ – indicates that there are 6 diatomic chlorine molecules, a total of 12 atoms.

Ionic compounds are formed when metals bind with non‑metals. Such compounds are made of ions that are arranged alternately and form structures known as crystals. In a crystal there are no isolated groups of atoms that could be called molecules. A molecular formula of an ionic compound thus represents the smallest set of repetitive ions in a crystal. Stoichiometric indices denote a number of ions of given element in this set.

In ionic compounds consisting of two elements, metal is always a cation and non‑metal is – an anion. Sodium chloride is an example of such a compound. Its formula, $\text{NaCl}$, indicates that a crystal of sodium chloride is made of sodium cations (${\text{Na}}^{\text{+}}$) and chloride anions (${\text{Cl}}^{-}$), and their quantitative ratio is 1:1.

Numbers placed before the formula of an ionic compound (stoichiometric numbers) denote a number of the smallest sets making up for a crystal. For example $\text{10NaCl}$ denotes ten sets which consist of one sodium cation (${\text{Na}}^{\text{+}}$) and one chloride anion (${\text{Cl}}^{-}$).

• A certain number of molecules, atoms and ions can be represented using chemical formulae.

• The number of atoms in a molecule or ions in a unit is denoted using a stoichiometric index. In a formula this number is written down at the bottom right of the element’s symbol.

• The number of atoms, molecules or units of an ionic compound is denoted using a stoichiometric number – the number written before the molecular formula of a compound.