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Ancient Philosophers' Views on Matter

• Divisibility of Matter

  • Ancient Indian and Greek philosophers explored the concept of matter.

  • In India, around 500 BC, Maharishi Kanad theorized that dividing matter would lead to smaller particles called Parmanu.

  • Pakudha Katyayana expanded this idea, suggesting these particles usually exist in a combined form.

  • Greek philosophers Democritus and Leucippus proposed that matter could be divided until reaching indivisible particles, termed atoms (meaning indivisible).

Development of Chemical Laws

Laws of Chemical Combination

• Two major laws identified by scientists like Antoine L. Lavoisier:

  • Law of Conservation of Mass

    • Mass neither created nor destroyed during a chemical reaction.

    • Activity: Conduct an experiment measuring mass before and after a reaction using two sets of chemicals (e.g., copper sulphate and sodium carbonate).

  • Law of Constant Proportions

    • Compounds contain elements in fixed ratios by mass regardless of their source.

    • Example: Water (H2O) always exhibits a mass ratio of 1g of hydrogen to 8g of oxygen.

Dalton's Atomic Theory

• British chemist John Dalton built on previous ideas:

  • Postulates of Dalton’s Theory:

    1. Matter composed of tiny particles called atoms.

    2. Atoms are indivisible and preserved in chemical reactions.

    3. Atoms of the same element are identical in mass and properties.

    4. Atoms of different elements vary in mass and properties.

    5. Atoms combine in small whole number ratios to form compounds.

    6. The types and amounts of atoms remain constant in a compound.

  • Dalton’s work was a key turning point in chemistry.

Understanding Atoms

• Size of Atoms:

  • Atoms are incredibly small, measured in nanometers; millions can fit on a paper's surface.

• Symbols of Elements:

  • Dalton used symbols to represent elements, relating them to the atomic mass.

  • Modern symbols approved by IUPAC (first letter capitalized, second lower case).

Atomic Mass and Composition

Atomic Mass Unit

• The atomic mass unit (amu) is a standard measurement based on carbon-12 isotopes, defined as 1/12 the mass of a carbon-12 atom.

• Examples of atomic masses include:

  • Hydrogen: 1 u

  • Carbon: 12 u

  • Oxygen: 16 u

Molecules and Ions

• Molecule Definition: A molecule consists of two or more atoms bonded together; it is the smallest unit retaining properties of a substance.

• Ions: Charged particles formed from atoms (cations and anions).

  • Example: Sodium chloride (NaCl) is composed of Na+ (cation) and Cl- (anion).

Writing Chemical Formulae

• Chemical Formula: Represents the composition of a compound.

  • Binary Compounds: Involve two elements.

  • Example formulae:

    • Magnesium chloride: MgCl2

    • Water: H2O

Molecular Mass and Calculations

• Molecular Mass: Total of atomic masses in a molecule. Example - Water: H₂O = 2(1) + 16 = 18 u.

• Formula Unit Mass: The sum of atomic masses in an ionic compound, similar to molecular mass but for ionic entities.

Key Takeaways

• During chemical reactions, total mass is conserved.

• Pure compounds have elements in constant proportions by mass.

• Atoms are fundamental units of matter, forming the basis for molecules which characterize substances.

• Chemical formulae illustrate how elements combine in compounds.

Crisscross Method for Writing Chemical Formulas

1. Determine the Charges:

   • Identify the charges of the ions involved.

   • Example: Sodium (Na) = +1, Chloride (Cl) = -1

2. Write the Symbols:

   • Write the symbol for the cation first, followed by the anion.

   • Example: Na and Cl

3. Crisscross the Charges:

   • Swap the charges of the ions to use as subscripts.

   • If Na (+1) and Cl (-1):

     • Na gets Cl's charge as subscript (1) and vice versa, but remains NaCl since both are 1.

   • If working with Magnesium (Mg = +2) and Chloride (Cl = -1):

     • You would get MgCl₂ after crisscrossing.

4. Form the Chemical Formula:

   • Write the final formula using the determined subscripts.

   • NaCl for sodium chloride or MgCl₂ for magnesium chloride.

The crisscross method provides a straightforward way to derive the correct chemical formulas for ionic compounds.