2.2 Fundamental Chemical Laws

• Law of Conservation of Mass
  • Mass is neither created nor destroyed in a chemical reaction.
• Law of Definite Proportion
  • A given compound always contains exactly the same proportion of elements by mass.
• Law of Multiple Proportions
  • When two elements form a series of compounds, the ratios of the masses of the second element that combine with $\ce{1 g}$ of the first element can always be reduced to small whole numbers.

2.3 Dalton’s Atomic Theory

Dalton’s Atomic Theory

1. Each element is made up of tiny particles called atoms.
2. The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.
3. Chemical compounds are formed when atoms of different elements combine with each other. A given compound always has the same relative numbers and types of atoms.
4. Chemical reactions involve reorganisation of the atoms — changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

• Dalton prepared the first table of atomic masses.
  • Many of the masses were later proved to be wrong because of Dalton’s incorrect assumptions about the formulas of certain compounds, but the construction of a table of masses was an important step forward.
• Avogadro’s hypothesis
  • at the same temperature and pressure, equal volumes of different gases contain the same number of particles.

2.4 Early Experiments to Characterize the Atom

The Electron

• English physicist J. J. Thomson studied electrical discharges in partially evacuated tubes called cathode-ray tubes during the period from 1898 to 1903.

  • Thomson determined the charge-to-mass ratio of an electron

    $$ \frac{e}{m} = \ce{-1.76 \times 10^8 C/g} $$

    where $e$ represents the charge on the electron in coulombs (C) and represents the electron mass in grams.

  • electrons: negatively charged particles

Radioactivity