The Science Book

162 AUGUST KEKULÉ

T

he early years of the

19th century saw huge

developments in chemistry

that fundamentally changed the

scientific view of matter. In 1803,

John Dalton suggested that each

element was made of atoms that

are unique to that element, and

used the concept of atomic weight

to explain how elements always

combine with each other in whole-

number proportions. Jöns Jakob

Berzelius studied 2,000 compounds

to investigate these proportions.

He invented the naming system we

use today—H for hydrogen, C for

carbon, and so on—and compiled

a list of atomic weights for all 40

elements that were then known.

He also coined the term “organic

chemistry” for the chemistry of

living organisms—the term later

came to mean most chemistry

involving carbon. In 1809, French

chemist Joseph Louis Gay-Lussac

explained how gases combine in

simple proportions by volume,

and two years later the Italian

Amedeo Avogadro suggested that

equal volumes of gas contain equal

numbers of molecules. It was

clear that there were strict rules

governing the combination of the

IN CONTEXT

BRANCH

Chemistry

BEFORE

1852 Edward Frankland

introduces the idea of valency

—the number of bonds an atom

can form with other atoms.

1858 Archibald Couper

suggests that carbon atoms

can link directly to one

another, forming chains.

AFTER

1858 Italian chemist Stanislao

Cannizzaro explains the

difference between atoms

and molecules, and publishes

atomic and molecular weights.

1869 Dmitri Mendeleev lays

out the periodic table.

1931 Linus Pauling elucidates

the structure of the chemical

bond in general, and that of

the benzene molecule in

particular, using the ideas

of quantum mechanics.

I spent a part of the night

putting at least sketches of

those musings down on paper.

This is how the structural

theory came into being.

Friedrich August Kekulé

elements. Atoms and molecules

remained essentially theoretical

concepts that nobody had seen

directly, but they were concepts

with growing explanatory power.

Valency

In 1852, the first step toward

an understanding of how atoms

combine with each another was

taken by English chemist Edward

Frankland, who introduced the idea

of valency—which is the number of

atoms each atom of an element

can combine with. Hydrogen

has a valency of one; oxygen has

This structure came

to Kekulé in a vision

of a snake grabbing

its own tail.

The atoms of each element

can combine with other atoms in

a set number of ways. This is

called valency.

Carbon atoms have

a valency of four.

In the molecules of

benzene, carbon atoms bond with

each other to form rings, onto

which hydrogen atoms bond.