Topic: The valence of chemical elements pt 1

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Elementary school. Chemistry.

II. Internal structure of matter. Pupil:

13) determines on the basis of the periodic table the valence (relative to hydrogen and maximal to oxygen) for the elements of groups: 1, 2, 13, 14, 15, 16 and 17;

14) draws the structural formula of a molecule of a two‑element compound (with covalent bonds) with known valences of elements.

General aim of education

The student defines the concept of valence and reads from the periodic table the maximum valence for chemical elements for selected groups

Key competences

• communication in foreign languages;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

• to define the concept of valence;

• to read from the periodic table the maximum valences of chemical elements of groups 1, 2 and from 13 to 17 of the periodic table in their relationship with hydrogen or oxygen;

• to write summary formulas of two‑element chemical compounds on the basis of information on the valences that make up their elements;

• to determine the valence of one chemical element in a relationship, when the valence of the other is known;

• to recognize an oxide based on its total formula;

• to write the total formula of the oxide, knowing its name;

• to draw structural formulas of two‑element chemical compounds, knowing what is the valence of the elements that make them.

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. The teacher initiates the lesson by reminding the students of the concept of a summary formula - eagerly interpreting it on a given formula of a chemical compound, eg HIndeks dolny 2₂O.

2. The structured pattern of water is displayed on the multimedia board. The teacher asks the students the question: „What does the structural formula inform about?” - the discussion continues. Then, after collecting the answers given by the students, the teacher defines the concept - the students write them in notebooks.

3. The teacher divides the students into groups, distributes the ball‑and‑ball models. By displaying on the multimedia board an illustration of „Complex and structural formulas of exemplary substances”, he asks for the construction of the indicated particle models. Monitors the course of work and corrects errors.

4. The lecturer presents an illustration of „Determining the valence of nitrogen in the ammonia molecule”. He explains - on the basis of a structural formula - the concept of valence of an element. Students write the definition in notebooks. Then, in relation to the valence concept, the teacher determines the valence of nitrogen.

5. The lecturer displays an illustration of „Determination of hydrogen valency in the ammonia molecule”. He asks students questions: „How many chemical bonds does one hydrogen atom produce? How much hydrogen is valuable? „- the students answer.

6. The teacher instructs students to determine how many chemical bonds each oxygen atom produces on the basis of the carbon dioxide ball‑and‑stamper model - the discussion continues. There is a summary that each oxygen atom produces one double bond. The lecturer asks another question: „How much oxygen is valuable, and how much is the value of coal?” - the students answer.

7. The lecturer displays the illustration of „Sodium chloride crystal”, and then explains how to determine the valence of chemical elements in ionic compounds. Presenting the illustration of „Crystal of magnesium chloride”, he asks: „How much magnesium is valuable and how much is chlorine worth?” - students answer.

8. The teacher explains that the elements have any valence - he emphasizes that the periodic table of elements provides some help in predicting the maximum valence of some of them in relation to hydrogen and oxygen. This applies to groups: 1., 2., 13., 14., 15., 16. and 17. Then it displays tables „Maximum valence of elements in relation to hydrogen” and „Maximum valence of elements in relation to oxygen”.

9. The lecturer displays the presentation of „Values of chemical elements”, presenting all possible valences of chemical elements recorded in the core curriculum: H, C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Fe, Cu , Zn, Br, Ag, Sn, I, Ba, Au, Hg, Pb.

10. 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

The valence of chemical elements pt 1

On the basis of the combined formula of covalent compounds, the composition of their molecules can be determined: the number and type of atoms of elements. However, we are not able to predict how atoms are connected to each other. This information is provided by a different pattern, called structural formula. It reflects the way atoms are combined in a molecule. In the structural formula, as in the sum formula, we use the symbols of elements to designate the atoms that make up a molecule of a chemical compound. Using dashes, we present bonds between atoms (one dash symbolizes one bond).

For example, a water molecule with the formula sum ${\text{H}}_{\text{2}}\text{O}$ has the following structural formula:

On its basis, we can conclude that in the water molecule, each hydrogen atom is connected to the oxygen atom by a single bond and that hydrogen atoms are not connected with each other. The structural formula does not specify how individual atoms are arranged in space.

Is it possible to describe ionic compounds using structural formulas?

Due to the lack of the possibility to distinguish isolated structures (molecules) of ionic compounds in ionic crystals, we do not describe them using structural formulas.

The number of bonds that an atom of a given chemical element creates, merging with other atoms, is called valence. When describing it, we use Roman numerals.

The valence of the elements forming the covalent compound can be easily determined on the basis of the structural formula of the compound.

How do we determine the valence of chemical elements in ionic compounds?

In the case of ionic compounds, the valence of an element is equal to the number of charges of its ion which occurs in the crystal of a compound. When determining the valence of ions, we omit positive and negative signs.

There are elements whose valence in most of the chemical compounds they create is constant. For example, sodium or hydrogen always have a valence equal to one (I). There are also elements, which depending on the type of compound may have different valences. For example, a carbon in a compound of the formula $\text{CO}$ it takes the value of two (II) and in carbon dioxide ${\text{CO}}_{\text{2}}$its valence is four (IV).

The periodic table can provide some help in predicting the maximum valence of certain elements in compounds with hydrogen or oxygen.

Table 1. The maximum valence of elements relative to hydrogen

Table 2. The maximum valence of elements relative to oxygen

On the basis of the data contained in the tables, it can be concluded that the elements belonging to the groups: 1., 2., 13. and 14. have the same highest valences in the compounds with both oxygen and hydrogen. On the other hand, the numbers defining the valence of elements from other groups in relation to oxygen and hydrogen are different.
In all compounds, the elements belonging to group 1 have a valence equal to one (I), while the elements from group 2 show a valence of two (II).

• Valence is a feature of all the forming elements and compounds is the number of bonds that forms the atom of an element linking with other atoms. It is described using Roman numerals.

• The valence of an element in covalent compounds is equal to the number of bonds that make up its atoms with other atoms.

• The valence of the element in the ionic compounds is equal to the charge of its ions, omitting the negative and positive signs.