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Notes on Atoms and Molecules

Ancient Philosophical Concepts of Matter

  • Ancient Indian and Greek philosophers contemplated the nature of matter, surmising its unseen forms and divisibility.

  • Around 500 BC, Indian philosopher Maharishi Kanad proposed that continuously dividing matter leads to infinitesimal particles called "Parmanu".

  • Greek philosophers Democritus and Leucippus suggested that repeated division yields indivisible particles known as "atoms."

  • Empirical validation emerged only in the eighteenth century.

Foundations of Chemical Science

  • By the late 18th century, chemists differentiated between elements and compounds.

  • Antoine Lavoisier established key principles of chemical science, particularly the Laws of Chemical Combination:

Law of Conservation of Mass

  • Mass remains constant in a chemical reaction; it is neither created nor destroyed.

  • Demonstrated through an experiment with solutions like copper sulfate and sodium carbonate.

Law of Constant Proportions (Law of Definite Proportions)

  • Compounds consist of elements in fixed mass ratios (e.g., water: 1:8 mass ratio of hydrogen and oxygen).

Dalton’s Atomic Theory

  • John Dalton introduced his atomic theory in 1808, merging philosophical insights with empirical data:

  1. All matter comprises atoms.

  2. Atoms are indivisible during chemical processes.

  3. Atoms of an element are identical in mass and properties.

  4. Atoms combine in simple whole-number ratios to form compounds.

  5. The number and types of atoms in a compound remain consistent.

Structure of Atoms

  • Atoms are incredibly minute, measured in nanometres (1 nm = 10^-9 m).

Chemical Symbols

  • Symbols represent elements, standardized by the International Union of Pure and Applied Chemistry (IUPAC); e.g., H for hydrogen and Na for sodium (Latin name: natrium).

Atomic Mass

  • Introduced by Dalton; based on the carbon-12 isotope, establishing the atomic mass unit (u).

Molecules and Ions

  • A molecule is the smallest unit of an element or compound that retains its chemical identity.

  • Molecular Types: Can consist of identical atoms (e.g., O2) or different atoms (e.g., H2O).

  • Ions: Charged particles (cations: positive, anions: negative); for example, sodium chloride consists of Na+ and Cl− ions.

Writing Chemical Formulas

  • Represent composition using symbols and valencies.

  • The cation's symbol precedes the anion's.

  • Ensure charges balance; use parentheses for polyatomic ions.

Molecular and Formula Unit Mass

  • Molecular mass: sum of atomic masses; e.g., water (H2O = 18 u).

  • Formula unit mass: applies to ionic compounds calculated based on empirical formulas.

Conclusion

  • Understanding principles of matter composition provides a foundation for modern chemistry, crucial for exploring chemical behavior and interactions.