1st Term Week 11 Lesson Note: Chemical Symbols, Formulae, and Equations for SSS 1

 

Term: First Term

Week: [Insert Week Number]

Class: SSS 1 (Senior Secondary School 1)

Subject: Chemistry

Topic: Chemical Symbols, Formulae, and Equations

Sub-Topics:

• Writing Chemical Symbols

• Writing Chemical Formulae (Valency Method)

• Balancing Chemical Equations

1. Chemical Symbols

A chemical symbol is a shorthand notation or abbreviation used to represent a specific chemical element. Instead of writing out the full name of an element, chemists use these universally accepted abbreviations.

This system was introduced by the Swedish chemist Jöns Jacob Berzelius.

Rules for Writing Symbols

1. Single Letter: If an element is represented by a single letter, it must always be a capital letter.

   • Examples: Hydrogen (H), Carbon (C), Oxygen (O), Sulfur (S).

2. Two Letters: If an element's name shares a starting letter with another, a second lowercase letter is added. The first letter is always capital, and the second is always lowercase.

   • Examples: Calcium (Ca}), Chlorine (Cl), Sodium Na - from Latin Natrium), Iron Fe - from Latin Ferrum).

Important Note: Always write symbols clearly. For example, Co represents the element Cobalt, whereas CO represents the compound Carbon Monoxide.

2. Chemical Formulae

A chemical formula is a representation of a chemical substance using symbols to show the proportions of atoms that make up its molecules or formula units.

To write a chemical formula correctly, you must understand Valency. Valency is the combining power of an element, determined by the number of electrons an atom loses, gains, or shares to attain a stable structure.

Valencies of Common Elements and Radicals

Element / Radical	Symbol / Formula	Valency
Hydrogen	H	1
Sodium	Na	1
Calcium	Ca	2
Aluminum	Al	3
Chlorine	Cl	1
Oxygen	O	2
Ammonium radical	NH4	1
Hydroxide radical	OH	1
Carbonate radical	CO3	2
Tetraoxosulfate(VI) radical	SO4	2

Steps to Write Chemical Formulae (The Interchanging/Criss-Cross Method)

1.Write the Symbols side-by-side:Step 1.

Place the positive element (usually the metal or hydrogen) first, followed by the negative element or radical.

Example: For Calcium Chloride, write Ca and Cl

2.Write the Valencies above the symbols:Step 2.

Place the respective combining power directly above each symbol.

Example: Calcium has a valency of 2, Chlorine has 1. Write: Ca^2 and Cl^1.

3.Cross over the valency numbers:Step 3.

Move the valency number of the first element to become the subscript of the second, and vice versa. Drop any positive or negative signs.

Example: The 2 from goes to Cl, and the 1 from Cl goes to Ca.

4.Write the final formula:Step 4.

Combine them. Note that the subscript '1' is omitted in the final formula. If the numbers can be simplified (like 2:2 turning into 1:1), reduce them to the lowest terms.

Final Result:CaCl2

Handling Radicals

If a radical (a group of atoms acting as a single unit) requires a subscript greater than 1, enclose the radical in a bracket before writing the subscript.

• Example: Magnesium Hydroxide =Mg^2+OH1-=Mg(OH)2

3. Balancing Chemical Equations

A chemical equation uses symbols and formulae to show the chemical change taking place. It consists of Reactants (starting substances on the left) and Products (new substances formed on the right).

Reactants = Products

The Law of Conservation of Mass

A chemical equation must be balanced because of the Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction. Therefore, the total number of atoms of each element on the reactant side must equal the total number of atoms of that same element on the product side.

Key Rules for Balancing

1. Never change the subscripts in a correct chemical formula (e.g., changing H2O to H2O2 to balance oxygen changes the substance completely).

2. Only change the coefficients (the big numbers written in front of the formulae, such as H2O.

Worked Example: Balancing the Combustion of Hydrogen

Let's balance the skeleton equation:

H2 +O2 = H2O

• Count the atoms initially:

  • Left (Reactants):H = 2, O = 2

  • Right (Products): H = 2,O = 1

• Balance the Oxygen:

  Put a coefficient of 2 in front  =H2O on the right side.

  H2 + O2  = H2O

  • Now, Right side: H = 4O = 2. Oxygen is balanced, but Hydrogen is now unbalanced.

• Balance the Hydrogen:

  Put a coefficient of 2 in front of H2 on the left side.

  2H2 + O2 = H2O

• Final Verification:

  • Left: H = 4, O = 2

  • Right: H = 4, O = 2

  • The equation is perfectly balanced.

Classwork / Evaluation Exercises

1. Write down the chemical symbols for the following elements: Potassium, Copper, Iron, Carbon, and Zinc.

2. Deduce the chemical formulae for:

   • Aluminum Oxide

   • Sodium Tetraoxosulfate(VI)

   • Ammonium Carbonate

3. Balance the following chemical equations:

   • Mg + O2 =MgO

   • KClO3 = KCl + O2

   • NaOH +HCl = NaCl + H2O