The Science Book

176 DMITRI MENDELEEV

I

n 1661, Anglo-Irish physicist

Robert Boyle defined elements

as “certain primitive and

simple, or perfectly unmingled

bodies; which not being made

of any other bodies, or of one

another, are the ingredients of

which all those called perfectly

mixt bodies are immediately

compounded, and into which

they are ultimately resolved.”

In other words, an element cannot

be broken down by chemical

means into simpler substances.

In 1803, British chemist John

Dalton introduced the idea of

atomic weights (now called relative

atomic masses) for these elements.

Hydrogen is the lightest element,

and he gave it the value 1, which

we still use today.

Law of eight

In the first half of the 19th century,

chemists gradually isolated more

elements, and it became clear that

certain groups of elements had

similar properties. For example,

sodium and potassium are silvery

solids (alkali metals) that react

violently with water, liberating

hydrogen gas. In fact, they are

so similar that British chemist

Humphry Davy did not distinguish

between them when he first

discovered them. Similarly,

the halogen elements chlorine

and bromine are both pungent,

poisonous oxidizing agents,

even though chlorine is a gas

and bromine a liquid. British

chemist John Newlands noticed

that when the known elements

were listed in order of increasing

IN CONTEXT

BRANCH

Chemistry

BEFORE

1803 John Dalton introduces

the idea of atomic weights.

1828 Johann Döbereiner

attempts first classification.

1860 Stanislao Cannizzaro

publishes an extensive table of

atomic and molecular weights.

AFTER

1913 Lothar Meyer shows the

periodic relationship between

elements by plotting atomic

weight against volume.

1913 Henry Moseley redefines

the periodic table using atomic

numbers—the number of

protons in an atom’s nucleus.

1913 Niels Bohr suggests

a model for the structure of

the atom. It includes shells

of electrons that explain the

relative reactivity of the

different groups of elements.

The first to attempt a classification

of the elements was German chemist

Johann Döbereiner. By 1828, he had

found that some elements formed

groups of three with related properties.

The periodic table

can be used to

guide experiments.

The elements can be arranged in

a table according to their

atomic weights.

Assuming a periodicity

of properties, predictions can be

made from the gaps in a periodic

table for the discovery of

missing elements.

The discovery of these missing

elements suggests that the periodic

table reveals important features of the

structure of the atom.