4.1. Evolution of the Atomic Model http://www.ck12.org
FIGURE 4.2
Democritusmass, no matter the amount. Based off of this idea, Proust developed thelaw of definite proportionswhich was
published in 1799. To illustrate this, suppose compound AB is made of element A and element B. Regardless of
how much AB is present, the ratio between the mass of A and the mass of B will be the same for any sample. In
other words, elements combine in fixed ratios based on their mass. Water, H 2 O, is always 1/9 by mass hydrogen and
8/9 by mass oxygen, regardless of whether we are looking at one drop or an entire lake.
Lavoisier’s Law of Conservation of Mass
At the same time, another French scientist named Antoine Lavoisier was studying mass relations in chemical
reactions. He noticed that for an isolated system, the mass of the reactants must equal the mass of the products
for any chemical process. This discovery was later called thelaw of conservation of mass. This law greatly
influenced chemistry because it quantified gaseous chemical species, which were often viewed as "missing matter"
that was not involved in chemical processes.
FIGURE 4.3
John DaltonWhile it must be assumed that many more scientists, philosophers and others studied the composition of matter after
Democritus, a major leap forward in our understanding of the composition of matter took place in the 1800s with
the work of John Dalton (Figure4.3). John Dalton, a school teacher from England, studied the weights of various
elements and compounds. He noticed that matter always combined in fixed ratios based on weight (and volume,