Atomic Structure, Valency, Formulae and Chemical Equations for SS 1

Class: SS 1
Term: First Term
Week: 10
Age: 15 years
Duration: 60 minutes
Subject: Chemistry
Curriculum Theme: The Chemical World
Focal competence: Using symbols, formulae and equations to express the composition and interactions between elements and compounds
Key competencies/values: Critical Thinking
Skills:

• Calculating mass of atomic numbers and valencies of elements from symbols and formulae
• Balancing of chemical equations

Previous Lesson: Chemical Symbols,
Topic: Symbols, Formulae And Equations: Atomic Structure, Atomic Number, Mass Number And Valencies
Subject Matter: Empirical and molecular formulae of simple compounds. Chemical equations

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Define atomic number, mass number, and valency.
• Explain the relationship between empirical and molecular formulae.
• State the rules for writing and balancing chemical equations.

Affective Domain

• Appreciate the importance of accurate chemical representation.
• Demonstrate critical thinking in solving chemical problems.

Psychomotor Domain

• Write simple chemical equations.
• Balance simple chemical equations.
• Determine the valencies of common elements from their formulae.

Reference Materials

The following resources were used in planning this lesson:

• 2025 New Revised Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• A suitable Chemistry textbook for SS 1
• The HeadTeacher Scheme of work For The New Revised Senior Secondary Education Curriculum (SSEC)

Instructional Materials

The teacher will teach this lesson with the aid of:

• Periodic Table of elements
• Atomic structure models
• Projector
• Internet access (if available)
• Cardboard (for making models or flashcards)

Rationale for the Lesson

This lesson is foundational to understanding how chemical substances are represented and how they interact. A solid grasp of atomic structure, valency, and chemical equations is essential for comprehending chemical reactions, stoichiometry, and the quantitative aspects of chemistry. It equips students with the necessary tools to interpret and predict chemical behaviour.

Prerequisite/Previous Knowledge

Students should have a basic understanding of atoms, elements, compounds, and mixtures from their Junior Secondary School (JSS) science classes.

Lesson Content/Board Summary

Symbols, Formulae And Equations: Atomic Structure, Atomic Number, Mass Number And Valencies

Atomic Structure

An atom is the smallest unit of an element that retains the chemical identity of that element. It consists of a central nucleus containing protons and neutrons, surrounded by electrons orbiting in shells.

Components of an Atom

1. Protons: Positively charged particles found in the nucleus. They determine the atomic number of an element.
2. Neutrons: Neutral particles (no charge) found in the nucleus. They contribute to the mass of the atom.
3. Electrons: Negatively charged particles that orbit the nucleus in specific energy levels or shells. They are involved in chemical bonding.

Atomic Number (Z)

The atomic number (Z) of an element is the number of protons in the nucleus of an atom of that element. It uniquely identifies an element. In a neutral atom, the number of electrons is equal to the number of protons, hence equal to the atomic number.

For example, Carbon (C) has an atomic number of 6, meaning it has 6 protons and 6 electrons.

Mass Number (A)

The mass number (A) of an atom is the total number of protons and neutrons in its nucleus. It is also known as the nucleon number.

Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N)

Number of Neutrons (N) = Mass Number (A) – Atomic Number (Z)

For example, an atom of Oxygen with 8 protons and 8 neutrons has a mass number of 16 (8 protons + 8 neutrons).

Valency

Valency is the combining power of an element. It represents the number of electrons an atom can gain, lose, or share to achieve a stable electron configuration (usually a full outermost shell, like noble gases).

Determining Valency

1. For metals: Valency is usually the number of electrons in the outermost shell that they can lose.
2. For non-metals: Valency is often the number of electrons they need to gain to complete their outermost shell, or the number of electrons they share.
3. From chemical formulae: In a binary compound (two elements), the valency of one element can often be deduced from the subscript of the other element (by criss-cross method).

Valencies of Common Elements (First 20 and others)

1. Hydrogen (H): 1
2. Helium (He): 0 (noble gas)
3. Lithium (Li): 1
4. Beryllium (Be): 2
5. Boron (B): 3
6. Carbon (C): 4
7. Nitrogen (N): 3
8. Oxygen (O): 2
9. Fluorine (F): 1
10. Neon (Ne): 0 (noble gas)
11. Sodium (Na): 1
12. Magnesium (Mg): 2
13. Aluminium (Al): 3
14. Silicon (Si): 4
15. Phosphorus (P): 3 or 5
16. Sulphur (S): 2, 4 or 6
17. Chlorine (Cl): 1
18. Argon (Ar): 0 (noble gas)
19. Potassium (K): 1
20. Calcium (Ca): 2
21. Iron (Fe): 2 or 3
22. Copper (Cu): 1 or 2
23. Zinc (Zn): 2

Chemical Formulae

A chemical formula uses symbols and subscripts to represent the types and numbers of atoms present in a compound.

Empirical Formula

The empirical formula shows the simplest whole-number ratio of atoms of each element in a compound. It represents the lowest possible ratio.

Example: For hydrogen peroxide, the molecular formula is H₂O₂, but its empirical formula is HO (simplest ratio of 1:1).

Molecular Formula

The molecular formula shows the actual number of atoms of each element present in a molecule of a compound. It is a multiple of the empirical formula.

Molecular Formula = (Empirical Formula)_n, where ‘n’ is a whole number.

Example: For glucose, the empirical formula is CH₂O. If n=6, the molecular formula is (CH₂O)₆ = C₆H₁₂O₆.

Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction, showing the reactants (starting materials) on the left side and the products (substances formed) on the right side, separated by an arrow.

Reactants (rightarrow) Products

Writing Chemical Equations

1. Write the correct chemical formulae for all reactants and products.
2. Use a plus sign (+) to separate reactants and products.
3. Use an arrow ((rightarrow)) to indicate the direction of the reaction.
4. Indicate the physical states of reactants and products using subscripts: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution.

Example (unbalanced): H₂(g) + O₂(g) (rightarrow) H₂O(l)

Balancing Chemical Equations

Balancing a chemical equation ensures that the Law of Conservation of Mass is obeyed, meaning the number of atoms of each element is the same on both sides of the equation (reactants and products). This is done by placing coefficients (whole numbers) in front of the chemical formulae.

Steps for Balancing:

1. Write the unbalanced equation with correct formulae.
2. Count the number of atoms of each element on both sides of the equation.
3. Balance elements one by one, usually starting with metals, then non-metals (excluding H and O), then oxygen, and finally hydrogen.
4. Adjust coefficients in front of the formulae, never change the subscripts within a formula.
5. Recount atoms after each adjustment to ensure balance.
6. Ensure all coefficients are the smallest possible whole numbers.

Worked Example: Balancing the reaction of Hydrogen and Oxygen

Question: Balance the equation: H₂(g) + O₂(g) (rightarrow) H₂O(l)

Solution:

Step 1: Write the unbalanced equation.

H₂(g) + O₂(g) (rightarrow) H₂O(l)

Step 2: Count atoms.

• Reactants: H = 2, O = 2
• Products: H = 2, O = 1

Step 3: Balance Oxygen.

To balance oxygen, place a coefficient of 2 in front of H₂O on the product side:

H₂(g) + O₂(g) (rightarrow) 2H₂O(l)

Step 4: Recount atoms and balance Hydrogen.

• Reactants: H = 2, O = 2
• Products: H = 2 x 2 = 4, O = 2

Now, hydrogen is unbalanced. Place a coefficient of 2 in front of H₂ on the reactant side:

2H₂(g) + O₂(g) (rightarrow) 2H₂O(l)

Step 5: Final count.

• Reactants: H = 2 x 2 = 4, O = 2
• Products: H = 2 x 2 = 4, O = 2

The equation is now balanced.

Answer: 2H₂(g) + O₂(g) (rightarrow) 2H₂O(l)

Example 2: Reaction of Methane with Oxygen

Question: Balance the equation: CH₄(g) + O₂(g) (rightarrow) CO₂(g) + H₂O(l)

Solution:

Step 1: Unbalanced equation.

CH₄(g) + O₂(g) (rightarrow) CO₂(g) + H₂O(l)

Step 2: Count atoms.

• Reactants: C = 1, H = 4, O = 2
• Products: C = 1, H = 2, O = 2 (from CO₂) + 1 (from H₂O) = 3

Step 3: Balance Carbon (already balanced).

Step 4: Balance Hydrogen.

There are 4 H atoms on the reactant side and 2 on the product side. Place a coefficient of 2 in front of H₂O:

CH₄(g) + O₂(g) (rightarrow) CO₂(g) + 2H₂O(l)

Step 5: Recount atoms and balance Oxygen.

• Reactants: C = 1, H = 4, O = 2
• Products: C = 1, H = 2 x 2 = 4, O = 2 (from CO₂) + 2 (from 2H₂O) = 4

Now, oxygen is unbalanced. There are 2 O atoms on the reactant side and 4 on the product side. Place a coefficient of 2 in front of O₂:

CH₄(g) + 2O₂(g) (rightarrow) CO₂(g) + 2H₂O(l)

Step 6: Final count.

• Reactants: C = 1, H = 4, O = 2 x 2 = 4
• Products: C = 1, H = 2 x 2 = 4, O = 2 + 2 = 4

The equation is now balanced.

Answer: CH₄(g) + 2O₂(g) (rightarrow) CO₂(g) + 2H₂O(l)

Teaching Methods/Instructional Techniques

Discussion, Explanation, Guided Practice, Question and Answer, Pair Work, Group Work, Demonstration

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Questioning/Recall

Teacher’s Activity: The teacher greets the students and asks questions to recall their prior knowledge on elements, compounds, and the basic structure of an atom. For example, “What is the smallest unit of an element?” or “Can you name some common elements?”

Pupils’ Activity: Pupils respond to the teacher’s questions, sharing their previous knowledge.

Learning Point: Review of basic chemical concepts

Step 2: Atomic Structure, Atomic Number, and Mass Number

Time: 10 minutes

Teaching Skill: Explanation/Demonstration

Teacher’s Activity: The teacher explains the components of an atom (protons, neutrons, electrons) using atomic structure models. The teacher then defines atomic number (Z) and mass number (A), explaining how they relate to the number of protons, neutrons, and electrons. The teacher uses the Periodic Table to show examples.

Pupils’ Activity: Pupils observe the models, listen to the explanation, and ask questions for clarification. They locate elements on the Periodic Table.

Learning Point: Understanding atomic composition

Step 3: Valency and its Determination

Time: 10 minutes

Teaching Skill: Explanation/Group Work

Teacher’s Activity: The teacher defines valency as the combining power of an element. The teacher guides students to work in small groups to deduce the valencies of elements by examining the chemical formulae of simple compounds (e.g., H₂O, NaCl, CO₂). The teacher provides a list of the first 20 elements and other common elements, discussing their typical valencies.

Pupils’ Activity: Pupils participate in group discussions, deduce valencies from given formulae, and note down the valencies of common elements.

Learning Point: Deducing element valencies

Step 4: Empirical and Molecular Formulae

Time: 7 minutes

Teaching Skill: Explanation/Examples

Teacher’s Activity: The teacher defines empirical and molecular formulae, explaining the difference between them using simple examples like hydrogen peroxide (H₂O₂ vs HO) and glucose (C₆H₁₂O₆ vs CH₂O).

Pupils’ Activity: Pupils listen to the explanation, understand the difference, and copy the examples.

Learning Point: Formulae types and differences

Step 5: Introduction to Chemical Equations

Time: 5 minutes

Teaching Skill: Explanation

Teacher’s Activity: The teacher introduces chemical equations as a way to represent chemical reactions. The teacher explains the components of an equation (reactants, products, arrow, state symbols) and the importance of correct chemical formulae.

Pupils’ Activity: Pupils listen and understand the basic structure and components of a chemical equation.

Learning Point: Structure of chemical equations

Step 6: Balancing Chemical Equations

Time: 10 minutes

Teaching Skill: Guided Practice/Pair Work

Teacher’s Activity: The teacher explains the principle of conservation of mass in chemical reactions and the need for balancing equations. The teacher demonstrates step-by-step how to balance simple chemical equations on the board. The teacher then guides students to work in pairs to write and balance other simple chemical equations.

Pupils’ Activity: Pupils pay close attention to the balancing steps, ask questions, and then practice balancing equations in pairs.

Learning Point: Steps for balancing equations

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Define atomic number and mass number.
2. What is the valency of Oxygen in H₂O?
3. Write the empirical formula for C₆H₆.
4. Balance the following equation: N₂(g) + H₂(g) (rightarrow) NH₃(g)

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Assessment of key concepts

Step 8: Note-Taking

Time: 10 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on atomic structure, valency, formulae, and chemical equations into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson content

Step 9: Conclusion

Time: 3 minutes

Teaching Skill: Consolidation

Teacher’s Activity: The teacher summarises the key points of the lesson, reiterating the importance of understanding atomic structure, valency, and the ability to write and balance chemical equations for future chemistry studies. The teacher encourages students to practice more balancing equations.

Pupils’ Activity: Pupils listen attentively and ask any final questions.

Learning Point: Reinforcement of lesson objectives

Continuous Assessment/Further Study

Type: Homework

Instruction: Answer the following questions in your notebook:

1. An element has an atomic number of 11 and a mass number of 23. State the number of protons, neutrons, and electrons in a neutral atom of this element.
2. Determine the valency of the underlined element in each of the following compounds:
   1. NaCl
   2. MgO
   3. Al₂O₃
3. Balance the following chemical equations:
   1. Fe(s) + O₂(g) (rightarrow) Fe₂O₃(s)
   2. C₂H₆(g) + O₂(g) (rightarrow) CO₂(g) + H₂O(l)
   3. K(s) + H₂O(l) (rightarrow) KOH(aq) + H₂(g)

Lesson Keywords

• Atomic Number – Number of protons in an atom.
• Mass Number – Total number of protons and neutrons in an atom.
• Valency – Combining power of an element.
• Empirical Formula – Simplest whole-number ratio of atoms in a compound.
• Molecular Formula – Actual number of atoms of each element in a molecule.
• Chemical Equation – Symbolic representation of a chemical reaction.
• Balancing Equations – Ensuring equal number of atoms on both sides of a chemical equation.

Differentiation

For students who may be struggling, provide additional simplified examples for balancing equations and determining valencies. Use physical models more extensively to illustrate atomic structure. For advanced learners, introduce slightly more complex compounds for valency deduction or challenge them with equations involving polyatomic ions.

Suggested Lesson Videos

YouTube search for “atomic structure valency chemical equations SS1 chemistry”