This chapter, "Atoms and Molecules," is the "alphabet" of Chemistry. If the previous chapter was about ingredients (Mixtures), this chapter is about the specific recipes (Formulas) and the math of cooking (The Mole Concept). In the Science and Technology section, questions here are usually numerical or logic-based.

1. The History (Ancient Roots)

Long before modern labs, philosophers pondered the nature of matter:

• Maharishi Kanad (500 BC): An Indian philosopher who postulated that if you divide matter (padarth), you eventually reach a particle that cannot be divided further. He named it Parmanu.
• Democritus (Greek): He called these indivisible particles Atoms (Greek for "indivisible").

The Shift: For centuries, this was just philosophy. In the 18th century, Antoine Lavoisier turned it into science, becoming the Father of Modern Chemistry.

2. The Two Laws of Chemical Combination

Lavoisier and Joseph Proust established two laws that govern how atoms behave.

A. Law of Conservation of Mass (Lavoisier)

Statement: Mass can neither be created nor destroyed in a chemical reaction.
Equation: Mass of Reactants = Mass of Products.
Real-world Logic: If you burn 1kg of wood, the ash + smoke + gases produced will weigh exactly 1kg.

B. Law of Constant Proportions (Proust)

Statement: In a chemical substance, elements are always present in definite proportions by mass, regardless of the source.

3. Dalton’s Atomic Theory

John Dalton provided the theory to explain the two laws above. His key postulates were:

• All matter is made of tiny particles called atoms.
• Atoms are indivisible (Note: Modern science discovered protons/electrons later, proving this wrong, but historically, this postulate is important).
• Atoms of the same element are identical in mass and properties.
• Atoms combine in whole numbers to form compounds.

4. Atoms: Symbols and Mass

Symbols: The IUPAC (International Union of Pure and Applied Chemistry) approves element names. The rule is: First letter Capital, second letter small (e.g., Cobalt is Co, not CO).

Atomic Mass

Atoms are too small to weigh on a scale, so we use a "relative" scale based on the Carbon-12 isotope.

• Definition: One atomic mass unit (u) is exactly 1/12th the mass of one Carbon-12 atom.
• Analogy: Imagine a watermelon is Carbon-12. Cut it into 12 slices. One slice is your standard weight unit (1u).

5. Molecules and Atomicity

A molecule is a group of atoms bonded together. Atomicity is the number of atoms constituting a molecule.

• Monoatomic: Argon (Ar), Helium (He). (Noble gases exist alone).
• Diatomic: Oxygen (O₂), Nitrogen (N₂), Chlorine (Cl₂).
• Triatomic: Ozone (O₃).
• Tetra-atomic: Phosphorus (P₄). (Memorize this).
• Poly-atomic: Sulphur (S₈). (Memorize this).

6. Ions and Chemical Formulae

When metals and non-metals combine, they form charged species called Ions.

• Cation: Positively charged (loses electrons). E.g., Na⁺, Ca²⁺.
• Anion: Negatively charged (gains electrons). E.g., Cl^-, O^2-.

How to Write Formulas (The "Criss-Cross" Method)

You must match the Valency (combining capacity).
Example: Magnesium Chloride

1. Write symbols: Mg and Cl
2. Write valency below: Mg (2) and Cl (1)
3. Criss-Cross: Give Mg's number to Cl, and Cl's number to Mg.
4. Result: MgCl₂

7. The Mole Concept (Simplified)

Do not fear the math. The "Mole" is just a counting unit, like a "Dozen".

Formula to Remember:

Number of Moles (n) = Given Mass / Molar Mass

Example: How many moles are in 52g of Helium? (He mass = 4u).
n = 52 / 4 = 13 moles.

8. Mentor’s Final Drill (Exam-Ready Questions)

Action Plan: Memorize the Latin names of elements (Iron, Sodium, Potassium) and the Atomicity of Phosphorus (P₄) and Sulphur (S₈). These are the most common "trap" questions in General Science sections.