The Science Book

A CENTURY OF PROGRESS 193

other than uranium). This led her
to the belief that the radioactivity
came from the uranium atoms
themselves, and not from any
reactions between uranium and
other elements.
Curie soon found that some
minerals that contained uranium
were more radioactive than
uranium itself, and wondered
whether these minerals contained
another substance—one that was
more active than uranium. By 1898,
she had identified thorium as
another radioactive element. She
rushed to present her findings in
a paper to the Académie des
Sciences, but the discovery of
thorium’s radioactive properties
had already been published.

Science double
Curie and her husband Pierre
worked together to discover the
additional radioactive elements
responsible for the high activity
of the uranium-rich minerals
pitchblende and chalcolite. By the
end of 1898 they had announced
the discovery of two new elements,
which they called polonium (after
her native country, Poland) and
radium. They attempted to prove
their discoveries by obtaining pure
samples of the two elements, but it
was not until 1902 that they obtained
0.003 oz (0.1 g) of radium chloride
from a metric ton of pitchblende.
During this time, the Curies
published dozens of scientific
papers, including one outlining their
discovery that radium could help to
destroy tumors. They did not patent
these discoveries, but in 1903, they
were jointly awarded the Nobel Prize
in Physics, along with Becquerel.
Marie continued her scientific work
after her husband’s death in 1906,

and succeeded in isolating a pure

sample of radium in 1910. In 1911,

she was awarded the Nobel Prize in

Chemistry, becoming the first

person to win or share in two prizes.

New model of the atom

The Curies’ discovery of radiation

paved the way for the two New

Zealand-born physicists Ernest

Rutherford and Ernest Marsden to

formulate their new model of the

atom in 1911, but it was not until

1932 that English physicist James

Chadwick discovered neutrons and

the process of radiation could be

fully explained. Neutrons and

positively charged protons are

subatomic particles that make up

the nucleus of an atom, which also

has negatively charged electrons

buzzing around it. The protons and

neutrons contribute almost all the

mass of the atom. Atoms of a

See also: Wilhelm Röntgen 186–87 ■ Ernest Rutherford 206–13 ■ J. Robert Oppenheimer 260–65

particular element always have the

same number of protons but may

have different numbers of neutrons.

Atoms with different numbers of

neutrons are called isotopes of the

element. For example, an atom of

uranium always has 92 protons in

its nucleus, but may have between

140 and 146 neutrons. These ❯❯

Uranium minerals emit radiation that darkens photographic

plates even when there is no light.

The amount of radiation from the uranium minerals depends

only on the quantity of uranium present.

Radiation is an atomic property

of the elements.

The radiation must therefore come from the uranium atoms.

Marie and Pierre Curie did not have

a dedicated laboratory. Most of their

work was done in a leaking shed next

to the University of Paris’s School of

Physics and Chemistry.