The Existence of
Molecules
Year of Discovery: 1811
What Is It? A molecule is a group of attached atoms. An atom uniquely identifies one of the 100+ chemical elements that make up our planet. Bonding a
number of different atoms together makes a molecule, which uniquely
identifies one of the many thousands of substances that can exist.
Who Discovered It? Amedeo Avogadro
Why Is This One of the 100 Greatest?
If atoms are the basic building block of each element, then molecules are the basic
building blocks of each substance on Earth. Scientists were stalled by their inability to accurately imagine—let alone detect—particles as small as an atom or a molecule. Many had
theorized that some tiny particle (that they called an atom) was the smallest possible particle
and the basic unit of each element. However, the substances around us were not made of individual elements. Scientists were at a loss to explain the basic nature of substances.
Avogadro’s discovery created a basic understanding of the relationship between all of
the millions of substances on Earth and the few basic elements. It adjusted existing theory to
conclude that every liter of gas (at the same temperature and pressure) had exactly the same
number of molecules in it. This discovery allowed scientists to make critical calculations
with and about gasses and allowed scientists to understand the nature of all substances.
Avogadro’s discovery (and the related Avogadro’s Number) have become one of the cornerstones of organic and inorganic chemistry as well as the basis for the gas laws and much
of the development of quantitative chemistry.
How Was It Discovered?
In the spring of 1811, 35-year-old college professor Amedeo Avogadro sat in his classroom scowling at two scientific papers laid out on his desk. Avogadro taught natural science
classes at Vercelli College in the Italian mountain town of Turin. Twenty-five students sat
each day and listened to Professor Avogadro lecture, discuss, and quiz them on whatever
aspects of science caught his fancy.
This day he read these two papers to his class, claimed that he saw an important mystery in them, and challenged his students to find it.
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