The Science Book

A PARADIGM SHIFT 263

but instead of creating heavier
elements by neutron absorption,
or lighter elements by emission
of one or more nucleons (protons
or neutrons), the pair found that
the lighter element barium was
released, which had 100 fewer
nucleons than the uranium nucleus.
No nuclear process understood at
the time could account for the loss
of 100 nucleons.
Perplexed, Hahn sent a letter to
colleagues Lise Meitner and Otto
Frisch in Copenhagen. Within the
month, Meitner and Frisch had
figured out the basic mechanism
of nuclear fission, recognizing how
uranium was split into barium and
krypton, the missing nucleons were
converted into energy, and a chain
reaction could follow. In 1939,
Danish physicist Niels Bohr took
the news to the US. His account,
along with the publication of the
Meitner–Frisch paper in the journal
Nature, set the East Coast scientific
community ablaze with excitement.
Conversations between Bohr and

John Archibald Wheeler at

Princeton after the annual

Theoretical Physics Conference,

led to the Bohr–Wheeler theory

of nuclear fission.

All the atoms of the same

element have nuclei with the same

number of protons in them, but

the number of neutrons can vary,

making different isotopes of the

same element. In the case of

uranium, there are two naturally

occurring isotopes. Uranium-238

(U-238) makes up 99.3 percent of

natural uranium. Its nuclei contain

See also: Marie Curie 190–95 ■ Ernest Rutherford 206–13 ■ Albert Einstein 214–21

We knew the world would not

be the same. A few people

laughed. A few people cried.

Most people were silent. I

remembered the line from

the Hindu scripture: “Now

I am become Death, the

destroyer of worlds.”

J. Robert Oppenheimer

92 protons and 146 neutrons. The

remaining 0.7 percent is made up

of uranium-235 (U-235), whose

nuclei contain 92 protons and

143 neutrons. The Bohr–Wheeler

theory incorporated the finding that

low-energy neutrons could cause

fission in U-235, causing the atom

to split and releasing energy in

the process.

When the news reached the

West Coast, Oppenheimer, now at

Berkeley, was captivated. He gave

a series of lectures and seminars on

the brand new theory and quickly ❯❯

The nuclear fission of uranium-235

(U-235) is started when a neutron

hits a U-235 nucleus. The uranium

atom splits to form an atom of barium

(Ba), an atom of krypton (Kr), and three

more neutrons. The three neutrons can

then go on to cause fission in more

atoms, causing a chain reaction. Each

time an atom splits, energy is released.

This one

neutron starts

the chain

reaction

Krypton

Barium

Neutron

Uranium-235