http://www.ck12.org Chapter 4. Atomic Structure
Beginnings of Atomic Theory
By the late 1700s, chemists had accepted the definition of an element as a substance that cannot be broken down
into simpler substances by ordinary chemical means. It was also clear that elements combine with one another to
form more complex substances called compounds. The chemical and physical properties of these compounds are
different than the properties of the elements from which they were formed. There was some disagreement, however,
about whether elements always combined in exactly the same ratio when forming a particular compound.
In the 1790s, a greater emphasis began to be placed on the quantitative analysis of chemical reactions. Accurate
and reproducible measurements of the masses of reacting elements and the compounds that they form led to the
formulation of several basic laws. One of these is called thelaw of conservation of mass,which states that during
a chemical reaction, the total mass of the products must be equal to the total mass of the reactants. In other words,
mass cannot be created or destroyed during a chemical reaction, but it must always be conserved.
The law of conservation of mass is demonstrated in this video: http://www.youtube.com/watch?v=J5hM1DxaPLw
(1:05).
MEDIA
Click image to the left for more content.The discovery that mass is always conserved in chemical reactions was soon followed by thelaw of definite
proportions,which states that a given chemical compound always contains the same elements in the exact same
proportions by mass. As an example, any sample of pure water contains 11.19% hydrogen and 88.81% oxygen by
mass. It does not matter where the sample of water came from or how it was prepared. Its composition, like that of
every other compound, is fixed (Figure4.2).
FIGURE 4.2
Regardless of its source or its form (solid, liquid, or gas), water always has
the exact same elemental composition: 11.19% of its mass is hydrogen,
and 88.81% of its mass is oxygen.Experiments also began to demonstrate that the same pairs of certain elements could combine to form more than
one compound. Consider the elements carbon and oxygen. Combined in one way, they form the familiar compound
called carbon dioxide. In every sample of carbon dioxide, there is 32.0 g of oxygen present for every 12.0 g of
carbon. By dividing 32.0 by 12.0, this simplifies to an oxygen/carbon mass ratio of 2.66 to 1. Another compound