The Science Book

112

ASCERTAINING THE

RELATIVE WEIGHTS OF

ULTIMATE PARTICLES

JOHN DALTON (1766–1844)

T

oward the end of the

18th century, scientists

had begun to realize that

the world is made up of a range

of basic substances, or chemical

elements. But no one was certain

what an element was. It was John

Dalton, an English meteorologist,

who, through his study of weather,

saw that each element is made

wholly of its own unique, identical

atoms, and it is this special atom

that distinguishes and defines

an element. In developing the

atomic theory of elements, Dalton

established the basis of chemistry.

The idea of atoms dates back to

ancient Greece, but it had always

been assumed that all atoms were

identical. Dalton’s breakthrough

was to understand that each

element is made from different

atoms. He described the atoms that

made up the elements then known—

including hydrogen, oxygen, and

nitrogen—as “solid, massy, hard,

impenetrable, moveable particles.”

Dalton’s ideas originated in

his study of the way in which

air pressure affected how much

water could be absorbed by air.

He became convinced that air is

IN CONTEXT

BRANCH

Chemistry

BEFORE

c.400 BCE Democritus

proposes that the world is

made of indivisible particles.

8th century CE Persian

polymath Jabir ibn Hayyan (or

Geber) classifies elements into

metals and non-metals.

1794 Joseph Proust shows that

compounds are always made

of elements combined in the

same proportions.

AFTER

1811 Amedeo Avogadro shows

that equal volumes of different

gases contain equal numbers

of molecules.

1869 Dmitri Mendeleev draws

up a periodic table, displaying

elements by atomic weight.

1897 Through his discovery

of the electron, J. J. Thomson

shows that atoms are not the

smallest possible particle.

Elements combine

with each other to make

compounds in simple

fixed ratios.

These fixed ratios must

depend on the relative

weight of the atoms of

each element.

Tables of elements

should be based on

the weights of their

ultimate particles.

Therefore, the atomic

weight of an element

can be calculated from

the weight of each element

involved in a compound.