I’m Sam. And, hello, I’m Rob.

On August the sixth 1945, the US aircraft, Enola Gay,

dropped an atomic bomb on the city of Hiroshima,

instantly killing 70,000 people.

When Japan refused to surrender, a second bomb

was dropped on Nagasaki three days later.

Many believe the bombings quickened the end of the Second World War.

But it came at a terrible human cost,

which some have called a crime against humanity.

The invention of the atomic bomb, which

resulted from the cooperation between

the US military and some of the world’s

leading scientific minds, was known as The Manhattan Project.

In this programme we’ll take a look into

the science and the politics of The Manhattan Project,

and as usual, we’ll learn some new vocabulary as well.

Even before World War Two, scientists had known

about the potential energy inside uranium, the

heaviest metal in the periodic table - a diagram which groups

the chemical elements into rows and columns according

to their atomic number and symbol.

The challenge for science was learning how to unleash

this potential energy in a controlled way.

We’ll hear more soon, but first I have a question

for you, Rob.

I mentioned that uranium is the

heaviest element in the periodic table,

but which is the lightest?

a) hydrogen b) carbon

c) oxygen

Well, oxygen is a gas, so it must pretty light.

I’ll say c) oxygen.

OK, Rob, we’ll find out the answer later in

the programme.

First, let’s find out a bit more

about the science of uranium from Frank Close,

an Oxford professor of theoretical physics,

in conversation with BBC Radio 4 programme,

In Our Time:

In 1938 the discovery was made that if you use uranium,

the atoms of uranium, which are the heaviest that

occur naturally in the periodic table, they’re very fragile…

and the discovery was that if you just almost touched

them with a single neutron, that’s a nuclear particle,

the uranium was like a drop of water,

it would just break apart, split in two… and this action

of splitting the uranium has become known as fission.

Atoms of uranium are very fragile – easily broken or damaged.

In 1938, it was discovered that when nuclear particles

called neutrons were fired at uranium atoms,

they would split, or break in two.

This process of splitting uranium, or fission, did two things.

First, it released huge amounts of energy,

a billion times more than would be released

in a normal chemical reaction.

Secondly, the act of splitting atoms released

two more neutrons.

These new neutrons were freed to hit more uranium,

creating four neutrons, which in turn were freed

and created eight, then sixteen and so on,

making what’s known as a chemical chain reaction.

In everyday English, a chain reaction is

a series of events where each event

becomes the cause of the next.

The politics behind the development of the atomic bomb

was no less complex than the science.

In the same year that Hitler invaded Poland,

two Jewish scientists exiled from Nazi Germany,

Rudolf Peierls, and Otto Frish, first realised uranium’s

power as a weapon of war.

Listen as Professor Frank Close takes up the story for BBC Radio 4 programme In Our Time.

Now, what happened at that moment was, having had the idea and the shock of the discovery,

you immediately then think, ‘maybe scientists in Germany

have already had the same idea and come to the same

conclusions – could Hitler already be building such a weapon?

And in their memorandum which they wrote and reached the British

government they said it’s conceivable that Germany is

in fact developing this weapon, and the only defence

against it is to have one yourself.

After their discovery, Peierls and Frish were worried

that the Nazis had already found out how to weaponise uranium.

It was conceivable, or believable, that Germany

was building an atomic bomb.

They shared this terrifying thought in their famous memorandum

– a short written report on a specific topic.

As soon as US President Franklin Roosevelt read it,

he started the Manhattan Project, and the race to build

an atomic bomb began.

In a strange twist of history, it turned out that Hitler

hadn’t been building atomic bombs at all.

And Hiroshima, the Japanese city destroyed in 1945,

was rebuilt and stands as a symbol of peace today.

Let’s end on a lighter note, Sam, with your question.

Yes, I asked which is the lightest element

in the periodic table.

It’s A, hydrogen, the lightest of all gases which come at

the very start of the periodic table, having the atomic number 1.

Ah, if only I’d remembered what our chemistry teacher

taught us about the periodic table – a chart grouping all the chemical

elements according to their atomic number.

Let’s recap the rest of the vocabulary, too.

If something is fragile it’s easily broken.

To split something means to break it into two parts.

A chain reaction happens when one event becomes

the cause of the next.

A memorandum is a short, written report

on a specific topic.

And finally, the adjective conceivable means believable.

That brings us to the end of our programme!

We hope you’ll join us again soon for more interesting issues and useful vocabulary.

Bye for now! Goodbye!