Elements, Formulae, and Equations

This chapter introduces the basic building blocks of Chemistry – elements, their symbols, and how they combine to form chemical compounds. Students will learn how to write chemical formulas using valency rules and practice balancing chemical equations to represent reactions accurately. Mastery of this topic builds a strong foundation for understanding all types of chemical reactions.

ELEMENTS:

Elements are pure substances having identical atoms.

There are 118 elements in the world. Some are metals, many of them are non-metal. We can express the elements in short form is called symbol of elements.

• The symbol of hydrogen is H                                      
• The symbol of carbon is C

Rules for symbol writing:

The first letter of the element is considered a symbol.

• The symbol of hydrogen is H
• The symbol of carbon is C
• The symbol of Nitrogen is N

If the first letter is same, another letter is taken with first letter as symbol. Here, the first letter will be capital and the second letter will be smaller shaped.

• The symbol of helium is He
•  The symbol of Chlorine is Cl
• The symbol of neon is Ne

The name of some elements are comes from their lateen names. i.e., sodium from its Latin name Natrium.   

VALENCY:

Valency is the combining power of an element. It tells us how many electrons an atom can gain, lose, or share to achieve a full outer shell and form chemical bonds.

Valency is usually equal to the number of electrons in the outer shell (for metals) or the number needed to complete the outer shell (for non-metals).

It helps in writing correct chemical formulas.

Valences of some elements:

POLYATOMIC IONS:

The polyatomic ion is a group of atoms that behaves as a single unit and carries a charge (positive or negative). Radicals do not exist independently, but they combine with other ions to form compounds.

🔹 Key Features of Radicals:

• A radical has a fixed composition (same atoms, same arrangement).
• It carries a net positive or negative charge.
• Radicals are used to form chemical formulas of compounds.

Common Examples:

CHEMICAL FORMULAE:

A chemical formula is a symbolic way of representing the elements in a compound and the number of atoms of each element present.

It tells you:

• Which elements are in the compound.
• How many atoms of each element are involved.
• In the case of ionic compounds, it shows the ratio of ions that combine to form a neutral compound.

🔹 Types of Chemical Formulae:

1. Empirical Formula

• The simplest whole number ratio of atoms in a compound.
• Example: CH₂O is the empirical formula of glucose (C₆H₁₂O₆).

2. Molecular Formula

• The actual number of atoms of each element in a molecule.
• Example: C₆H₁₂O₆ (glucose), H₂O (water)

3. Ionic Formula

• Shows the ratio of positive and negative ions in an ionic compound.
• Example: NaCl (1 Na⁺ and 1 Cl⁻), MgCl₂ (1 Mg²⁺ and 2 Cl⁻)

🔸 Rules for Writing Chemical Formulas:

1. Write the symbols of the elements or ions.
2. Determine their valencies (combining powers).
3. Balance the charges to form a neutral compound.
4. Write the formula without showing charges, using subscripts to indicate the number of atoms/ions.

⚙️ Example 1: Sodium Chloride

• Sodium (Na⁺), valency = 1
• Chloride (Cl⁻), valency = 1
  ✅ Formula: NaCl

⚙️ Example 2: Magnesium Chloride

• Magnesium (Mg²⁺), valency = 2
• Chloride (Cl⁻), valency = 1
  ✅ Formula: MgCl₂ (2 Cl⁻ ions needed to balance 1 Mg²⁺)

⚙️ Example 3: Ammonium Sulfate

• Ammonium (NH₄⁺), valency = 1
• Sulfate (SO₄²⁻), valency = 2
  ✅ Formula: (NH₄)₂SO₄

📘 Common Mistakes to Avoid:

• Don’t write charges in the final formula.
• Use brackets for radicals if there’s more than one of them.
  ✅ e.g., Ca(NO₃)₂, not CaNO₃₂

💡 Why Chemical Formulae Matter:

• They are essential for writing and balancing chemical equations.
• They help understand the composition and structure of substances.
• They form the basis of chemical naming, calculations, and laboratory work.

CHEMICAL EQUATIONS:

A chemical equation is a symbolic representation of a chemical reaction. It shows how reactants are converted into products, using chemical formulas.

It provides essential information:

• What substances are reacting (reactants)
• What substances are produced (products)
• The quantities involved in the reaction
• The physical states of substances (solid, liquid, gas, aqueous)

🔹 Parts of a Chemical Equation

General Format:
Reactants → Products

For example:
H₂ + O₂ → H₂O

This means Hydrogen reacts with Oxygen to produce Water.

🔸 Balanced Chemical Equation

In a balanced chemical equation, the number of atoms of each element is the same on both sides of the equation.

This follows the Law of Conservation of Mass:
➡️ Matter cannot be created or destroyed in a chemical reaction.

🔧 Example (Balanced):

2H₂ + O₂ → 2H₂O

• Reactants: 4 Hydrogen atoms, 2 Oxygen atoms
• Products: 4 Hydrogen atoms, 2 Oxygen atoms ✅

🧪 State Symbols (Physical States)

To give more detail, we use state symbols:

Example:
NaCl (s) + H₂SO₄ (aq) → NaHSO₄ (aq) + HCl (g)

🔄 Types of Chemical Equations

1. Word Equation

Shows names only:
Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen

2. Symbol Equation

Shows chemical formulas:
Mg + 2HCl → MgCl₂ + H₂

3. Ionic Equation

Shows only the ions involved in the reaction (especially for reactions in solution).

✍️ Steps to Write a Chemical Equation

1. Write down the correct formulas of the reactants and products.
2. Balance the equation by adding coefficients (never change subscripts!).
3. Add state symbols, if required.

🔥 Example: Combustion of Methane

Word Equation:
Methane + Oxygen → Carbon dioxide + Water

Balanced Symbol Equation:
CH₄ + 2O₂ → CO₂ + 2H₂O

❗ Common Student Mistakes

• Not balancing the equation properly.
• Changing subscripts (which changes the substance).
• Forgetting state symbols (in structured questions).
• Not using correct chemical formulas of compounds or radicals.

🎓 Why Is This Important in Exams?

Understanding chemical equations helps in:

• Predicting products of reactions
• Balancing equations for marks
• Performing stoichiometric calculations
• Understanding experimental procedures

IONIC EQUATIONS:

An ionic equation shows only the chemical species that are involved in a reaction as ions, especially in aqueous (water-based) solutions. It omits the spectator ions, which do not take part in the actual chemical change.

It is especially useful in understanding reactions that occur in solutions, like precipitation reactions, acid-base reactions, and redox reactions.

🧪 Types of Chemical Equations

1. Molecular Equation: Shows all the reactants and products in their complete molecular form.
   • Example:
     Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
2. Full Ionic Equation: Breaks all soluble ionic compounds into their individual ions.
   • Example:
     2Na⁺(aq) + SO₄²⁻(aq) + Ba²⁺(aq) + 2Cl⁻(aq) → BaSO₄(s) + 2Na⁺(aq) + 2Cl⁻(aq)
3. Net Ionic Equation: Removes spectator ions (those that do not change) to show the actual reaction taking place.
   • Final Net Ionic Equation:
     Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

⚙️ Steps to Write an Ionic Equation

1. Write a balanced molecular equation.
2. Break all aqueous (soluble) compounds into ions.
3. Cancel out the spectator ions on both sides.
4. Write the net ionic equation showing only the species involved in the reaction.

🎯 Why Ionic Equations Are Important

• Help in understanding which substances are actually reacting.
• Highlight the ionic nature of reactions in water.
• Are essential in explaining precipitation, acid-base, and redox reactions.

📌 Example: Precipitation Reaction

Molecular Equation:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

Ionic Equation:
Ag⁺(aq) + NO3⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO3⁻(aq)

Net Ionic Equation:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

🧠 Spectator Ions

Spectator ions appear on both sides of the equation and do not change in the reaction.

In the above example:
Na⁺ and NO3⁻ are spectator ions.

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