Overview
This lecture covers the basics of atoms and molecules, key laws of chemical combination, atomic theory, atomic and molecular masses, ions, valency, and how to write chemical formulas, with memory tricks and practice questions to help prepare for exams.
Introduction to Atoms and Molecules
- Atoms are the smallest building blocks of matter and cannot be seen with a microscope.
- The radius of an atom is about 1 nanometer (1 nm = 10⁻⁹ meters).
- Everything around us (solid, liquid, gas) is made of atoms.
- Molecules are formed when atoms combine chemically.
Laws of Chemical Combination
- Law of Conservation of Mass: Total mass of reactants equals total mass of products in a chemical reaction.
- Law of Constant Proportion: A pure compound always contains the same elements in the same ratio by mass.
- Examples: Water (H₂O) has hydrogen and oxygen in a constant mass ratio of 1:8.
Dalton’s Atomic Theory
- All matter consists of tiny indivisible particles called atoms.
- Atoms of the same element are identical in mass and properties; atoms of different elements differ.
- Atoms combine in simple whole-number ratios to form compounds.
- Limitations: Atoms can be divided (protons, neutrons, electrons), isotopes exist, and isobars have same mass but are different elements.
Symbols and Atomic Mass
- Chemical elements are represented by symbols (H for hydrogen, O for oxygen).
- Atomic mass is the mass of one atom, measured in atomic mass unit (amu), based on carbon-12 isotope.
- Use simple tricks and mnemonics to remember the first 20 elements and their atomic masses.
Atomicity and Types of Molecules
- Atomicity = number of atoms in one molecule (e.g., O₂ is diatomic, P₄ is tetra-atomic).
- Monoatomic: noble gases (He, Ne), Diatomic: H₂, O₂, Cl₂, Triatomic: O₃, Polyatomic: S₈, P₄.
Ions and Valency
- Ions are atoms or groups of atoms with positive (cations) or negative (anions) charge.
- Valency = combining capacity of an element, corresponds to its ability to bond with other atoms.
- Use tables and mnemonic tricks to remember common valencies and charges.
Writing Chemical Formulas
- Cross-multiply valencies to form correct chemical formulas.
- For polyatomic ions, use brackets when needed.
- Examples: NaCl (sodium chloride), H₂SO₄ (sulphuric acid), NH₄NO₃ (ammonium nitrate).
Molecular Mass and Formula Mass
- Molecular mass = sum of atomic masses of all atoms in a molecule.
- Formula unit mass is used for ionic compounds (sum of atomic masses in the formula unit).
Mole Concept (If covered)
- 1 mole = 6.022 × 10²³ particles (Avogadro's number).
- Molar mass is the mass of 1 mole of a substance in grams.
- Number of moles = Given mass / Molar mass; Number of moles = Number of particles / Avogadro's number.
Practice Questions & Application
- Practice matching names and formulas, calculating mass ratios, atomicity, and writing chemical formulas.
- Assertion-reasoning and MCQ type questions are common for exams.
Key Terms & Definitions
- Atom — smallest unit of matter retaining element properties.
- Molecule — group of two or more atoms chemically bonded.
- Law of Conservation of Mass — mass is neither created nor destroyed in a reaction.
- Law of Constant Proportion — fixed mass ratio of elements in a compound.
- Atomic Mass — mass of a single atom, measured in amu.
- Atomicity — number of atoms in a molecule.
- Ion — atom/group with net electrical charge.
- Valency — combining power of an element.
- Molecular Mass — sum of atomic masses in a molecule.
- Mole — unit representing 6.022 × 10²³ particles.
Action Items / Next Steps
- Memorize the symbols and atomic masses of the first 20 elements.
- Practice writing chemical formulas using cross-multiplication of valencies.
- Review the laws of chemical combination and Dalton’s theory.
- Complete practice questions on formula writing, molecular mass, and atomicity.
- Prepare for assertion-reasoning and numerical questions for exams.