Unit 2: Chemistry – Symbols, Formulae and Equations
Welcome to one of the most useful topics in chemistry! Think of chemical symbols and formulae as the universal language of scientists. Just like musicians use notes to write music that anyone in the world can play, chemists use symbols and equations to describe reactions so that anyone anywhere can understand them. Don't worry if this seems like a lot of letters and numbers at first — once you learn the simple rules, it becomes just like solving fun puzzles!
---1. Chemical Symbols
Everything in the universe is made of building blocks called elements. Each element has its own unique chemical symbol shown on the Periodic Table.
Rules for Writing Chemical Symbols
• Single-letter symbols: Always use a single capital letter.
Examples: Carbon is \(C\), Hydrogen is \(H\), Oxygen is \(O\), Nitrogen is \(N\), and Sulfur is \(S\).
• Two-letter symbols: The first letter is always a capital, and the second letter is always lowercase.
Examples: Helium is \(He\), Sodium is \(Na\), Calcium is \(Ca\), Magnesium is \(Mg\), and Chlorine is \(Cl\).
Common Mistake to Avoid:
Be very careful with your handwriting! Writing \(CO\) is completely different from writing \(Co\).
• \(Co\) is the element Cobalt (a magnetic metal).
• \(CO\) is Carbon Monoxide (a toxic gas made of one carbon atom and one oxygen atom).
Did You Know?
Some symbols look different from their English names because they come from ancient Latin names! For example, Sodium has the symbol \(Na\) from the Latin word Natrium, and Iron has the symbol \(Fe\) from Ferrum.
Key Takeaway: Always write the first letter as a capital and the second letter (if there is one) in lowercase.
---2. Chemical Formulae and Counting Atoms
A chemical formula tells us which elements are in a substance and exactly how many atoms of each element are joined together.
Understanding the Small Subscript Numbers
A small number written after an element's symbol tells you how many atoms of that specific element are present. If there is no number written, it means there is just one atom of that element.
Example 1: Water is \(H_2O\)
• It has \(2\) Hydrogen atoms (\(H\))
• It has \(1\) Oxygen atom (\(O\))
• Total atoms: \(2 + 1 = 3\) atoms
Example 2: Carbon Dioxide is \(CO_2\)
• It has \(1\) Carbon atom (\(C\))
• It has \(2\) Oxygen atoms (\(O\))
• Total atoms: \(1 + 2 = 3\) atoms
Dealing with Brackets in Formulae
Sometimes you will see brackets in a formula. Just like in maths, you multiply everything inside the brackets by the number outside the bracket.
Example 3: Magnesium Hydroxide is \(Mg(OH)_2\)
• Magnesium (\(Mg\)): \(1\) atom
• Oxygen (\(O\)): \(1 \times 2 = 2\) atoms
• Hydrogen (\(H\)): \(1 \times 2 = 2\) atoms
• Total atoms: \(1 + 2 + 2 = 5\) atoms
Example 4: Calcium Nitrate is \(Ca(NO_3)_2\)
• Calcium (\(Ca\)): \(1\) atom
• Nitrogen (\(N\)): \(1 \times 2 = 2\) atoms
• Oxygen (\(O\)): \(3 \times 2 = 6\) atoms
• Total atoms: \(1 + 2 + 6 = 9\) atoms
Diatomic Molecules
Some non-metal elements do not like to be alone. In their natural state, they always travel in pairs (two atoms chemically bonded together). These are called diatomic molecules.
There are seven diatomic elements you need to remember:
• Hydrogen: \(H_2\)
• Nitrogen: \(N_2\)
• Oxygen: \(O_2\)
• Fluorine: \(F_2\)
• Chlorine: \(Cl_2\)
• Bromine: \(Br_2\)
• Iodine: \(I_2\)
Helpful Mnemonic to Remember the Diatomic Elements:
"Have No Fear Of Ice Cold Beer"
(\(H_2\), \(N_2\), \(F_2\), \(O_2\), \(I_2\), \(Cl_2\), \(Br_2\))
Key Takeaway: Small numbers tell you how many atoms of that element are present. Multiply everything inside brackets by the number outside.
---3. Word Equations
A chemical reaction occurs when chemical bonds are broken and made, turning starting substances into brand new substances.
Parts of a Chemical Equation
• Reactants: The starting substances (written on the left-hand side).
• Arrow (\(\rightarrow\)): Means "react to produce" or "turns into". Never use an equals sign (\(=\)) in chemistry!
• Products: The new substances formed (written on the right-hand side).
General Form:
\(\text{Reactant 1} + \text{Reactant 2} \rightarrow \text{Product 1} + \text{Product 2}\)
Examples of Word Equations
• Burning magnesium in oxygen:
\(\text{Magnesium} + \text{Oxygen} \rightarrow \text{Magnesium oxide}\)
• Neutralisation reaction:
\(\text{Hydrochloric acid} + \text{Sodium hydroxide} \rightarrow \text{Sodium chloride} + \text{Water}\)
• Metal and acid reaction:
\(\text{Zinc} + \text{Sulfuric acid} \rightarrow \text{Zinc sulfate} + \text{Hydrogen}\)
Key Takeaway: Word equations show reactants on the left turning into products on the right using a reaction arrow (\(\rightarrow\)).
---4. Balancing Symbol Equations
In every chemical reaction, the Law of Conservation of Mass applies. This states that atoms cannot be created or destroyed. The atoms you start with in the reactants are simply rearranged to make the products.
This means you must have the exact same number of each type of atom on both sides of the equation. When this is true, the equation is balanced.
The Golden Rules of Balancing
1. NEVER change the small subscript numbers inside a chemical formula (e.g., you cannot change \(O_2\) to \(O\) or \(H_2O\) to \(H_2O_2\)). Changing the formula changes the substance entirely!
2. ONLY add big numbers in front of a formula (called coefficients). A big number multiplies all atoms in that molecule.
Step-by-Step Method to Balance Equations
Worked Example 1: Making Magnesium Oxide
Unbalanced equation: \(Mg + O_2 \rightarrow MgO\)
Step 1: Count atoms on both sides
• Left side: \(Mg = 1\), \(O = 2\)
• Right side: \(Mg = 1\), \(O = 1\)
Step 2: Balance the unbalanced atoms
Oxygen is unbalanced (\(2\) on left, \(1\) on right). Put a big \(2\) in front of \(MgO\):
\(Mg + O_2 \rightarrow 2MgO\)
Step 3: Recount atoms
• Left side: \(Mg = 1\), \(O = 2\)
• Right side: \(Mg = 2\), \(O = 2\)
Step 4: Balance the remaining element
Now Magnesium is unbalanced (\(1\) on left, \(2\) on right). Put a big \(2\) in front of \(Mg\) on the left:
\(2Mg + O_2 \rightarrow 2MgO\)
Final Check: Left (\(2Mg\), \(2O\)) = Right (\(2Mg\), \(2O\)). The equation is balanced!
Worked Example 2: Reaction of Hydrogen and Chlorine
Unbalanced equation: \(H_2 + Cl_2 \rightarrow HCl\)
• Left side: \(H = 2\), \(Cl = 2\)
• Right side: \(H = 1\), \(Cl = 1\)
Place a big \(2\) in front of \(HCl\) on the right:
Balanced equation: \(H_2 + Cl_2 \rightarrow 2HCl\)
Worked Example 3: Combustion of Methane
Unbalanced equation: \(CH_4 + O_2 \rightarrow CO_2 + H_2O\)
Step 1: Count atoms
• Left side: \(C = 1\), \(H = 4\), \(O = 2\)
• Right side: \(C = 1\), \(H = 2\), \(O = 3\) (one in \(CO_2\), one in \(H_2O\))
Step 2: Balance Hydrogen
Put a \(2\) in front of \(H_2O\) to get \(4\) hydrogens on the right:
\(CH_4 + O_2 \rightarrow CO_2 + 2H_2O\)
Step 3: Balance Oxygen
Now count Oxygen on the right: \(2\) (from \(CO_2\)) \(+ 2\) (from \(2H_2O\)) \(= 4\) oxygens.
Put a \(2\) in front of \(O_2\) on the left:
Balanced equation: \(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)
Key Takeaway: Balancing ensures the same number of each atom exists before and after the reaction. Only put big numbers in front of formulas!
---5. State Symbols
State symbols are letters written in brackets after a chemical formula to show the physical state of the substance at room temperature.
• \((s)\) – Solid: Metals, precipitates, powders (e.g., \(Mg(s)\), \(CaCO_3(s)\))
• \((l)\) – Liquid: Pure liquids only, mainly pure water (e.g., \(H_2O(l)\)) or liquid bromine (\(Br_2(l)\))
• \((g)\) – Gas: Gases like oxygen, carbon dioxide, hydrogen (e.g., \(O_2(g)\), \(CO_2(g)\), \(H_2(g)\))
• \((aq)\) – Aqueous solution: A solid, liquid, or gas dissolved in water (e.g., acids like \(HCl(aq)\), dissolved salts like \(NaCl(aq)\))
Common Mistake to Avoid:
Do not confuse \((l)\) and \((aq)\)!
• Pure liquid water is \(H_2O(l)\).
• Salt water or dissolved acids are aqueous, so they get \((aq)\).
Example with State Symbols
\(CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)\)
Reading this equation: Solid calcium carbonate reacts with aqueous hydrochloric acid to produce aqueous calcium chloride, liquid water, and carbon dioxide gas.
Key Takeaway: The four state symbols are \((s)\) for solid, \((l)\) for pure liquid, \((g)\) for gas, and \((aq)\) for substances dissolved in water.
---Quick Exam Revision Checklist
• Have you capitalized only the first letter of each element symbol?
• Did you remember the 7 diatomic molecules (\(H_2\), \(N_2\), \(O_2\), \(F_2\), \(Cl_2\), \(Br_2\), \(I_2\))?
• Did you check that the number of atoms on the left equals the number of atoms on the right?
• Did you remember never to alter the small numbers when balancing?
• Do you know the difference between \((l)\) and \((aq)\)?