Chapter 2.2: The Observations That Led to an Atomic View of Matter

Flashcards from Chapter 2.2 of Chemistry: The Molecular Nature of Matter and Change.

Mass laws

Includes three:

1. The law of mass conservation

2. The law of definite composition

3. The law of multiple proportions

Law of mass conservation

States that the total mass of substances does not change during a chemical reaction

• The number of substances may change, but the total amount of matter remains constant

• _{Some mass may be converted to energy, but this is often too small to measure}

Composition calculation

A fixed proportion of elements remain a compound, either via:

1. Fraction by mass (mass fraction), calculating the part of the compound’s mass that each element contributes

2. Percent by mass (mass percent), expressing fraction by mass as a percentage

These should equal 1 part (100%) and can be compared to determine substance similarity via the law of definite composition

Law of definite composition

States that no matter what its source, a particular compound is composed of the same elements in the same parts by mass

• Mass percents, for example, can be used to find the mass of each substance

Law of multiple proportions

States that if elements A and B react to form two compounds, the different masses of B that combine with a fixed mass of A can be expressed as a ratio of small whole numbers

• For example, given two compounds with 57.1% O / 42.9% C and 72.7% O / 27.3% C, when applied to a fixed mass of the two compounds like 100 g, a ratio of 2 results between the two compound ratios ((57.1 / 42.9) / (72.7 / 27.3) = 2).

  • As a result, we can see that increments based on small whole numbers are used to determine element changes.

  • Put more simply: the mass ratio of A₁ / B₁ to A₂ / B₂ will be a clean, small ratio.

John Dalton

Scientist who presented the atomic theory of matter, which stated:

1. All matter consists of atoms.

2. Atoms of one element cannot be converted into atoms of another, just rearranged.

3. Atoms of an element are identical in mass and other properties, and are different from other elemental atoms.

4. Compounds result from the combination of a specific ratio of atoms.