What makes some things brittle, instead of just hard or soft? 146 (8/9) – Science Mysteries Explained

Lead works so well because it’s very dense. With 82
protons in its nucleus, lead is the densest non-
radioactive element that occurs naturally. Interestingly
enough, uranium itself is an even better shield against
radioactivity—up to five times better at stopping gamma
rays. And because it only gives o alpha particles, we can
coat it in a thinner secondary shield to make it even safer.
But it is very expensive, so lead is more common.
Alpha particles, with both neutrons and protons in
them, are actually pretty easy to block. You can stop most
of them with a sheet of paper. Beta particles, made up of an
electron, penetrate deeper. And while they can be stopped
by a thin sheet of, say, aluminum, this can produce x-rays
as a by-product—which are still dangerous.
If the radiation is a stream of neutrons, that can be
doubly dangerous because those neutrons can hit the
shield and make some of the atoms in it radioactive
Because of its density and stability, lead is immune to
a lot of these eects, and so extremely good at blocking
all forms of radiation. It’s heavy, though, and for really big
sources of radioactivity—like nuclear reactors—it gets
used in conjunction with special super-dense concrete
and even plain water in a clever multilayered shield.
Heading out to stock up on lead plates now? Don’t wor-
ry, in your day-to-day life, you’re pretty well shielded from
radiation. The Earth’s magnetic field and atmosphere
block nearly all the dangerous radiation from the Sun—
and you can block ultraviolent rays with a thin smear of
Anyway, in one of nature’s cruel ironies, lead itself is
very poisonous to humans in other ways, so we need to
keep its use to a minimum. That’s why we don’t line our
homes with lead.
What keeps molecules stuck together?
Everything is made up of atoms, but atoms also stick together in dierent combinations
to make molecules. How does this work?
An electric charge keeps atoms stuck together via their electrons. There are a couple dierent ways this
works, and this has a big eect on how a molecule looks.
We can’t see individual atoms without using a spe-
cial kind of instrument called a scanning electron
microscope, but we can see individual molecules.
While some are still very tiny, other molecules
are huge. Most pure metals—for instance, gold, alu-
minum, and iron—are actually a single giant mole-
cule made up of billions upon billions of atoms.
Atoms are so small they don’t really “look” like
anything—light doesn’t interact with them in the
same way it does with large-scale structures. But
you can think of an atom as a tiny ball of protons
and neutrons surrounded by a fuzzy cloud of elec-
These super-tiny electrons carry a negative
electric charge, while the nucleus has a positive
charge. It’s this electrical attraction that keeps
electrons buzzing around their parent atoms.
However, when two atoms come close together,
the electron is also attracted to the other atom’s
positively charged nucleus.
If the right combination of atoms comes
together, electrons can move between or be
shared between the two. This forms what’s
called a “chemical bond”—and it’s the basis for
all chemical reactions.
After a bond has formed, the two atoms are
stuck together into a molecule. There are two
main types of molecule: one made of the same
kind of element, and one made of two or more
dierent elements. This second kind of mole-
cule is called a “compound” by chemists.
Some of the simplest molecules are the gases
in our atmosphere. Nitrogen and oxygen float
around in molecules made up of just two nitro-
gen and two oxygen atoms. Some of the most
complex molecules are the ones found in living things.
These so-called organic compounds can be made up of
millions or even billions of individual atoms of four or
five dierent elements—usually carbon, oxygen, nitrogen,
hydrogen, phosphorus, and sulfur.
A strand of human DNA, for instance, has more than
200 billion individual atoms—and we still need an elec-
tron microscope to see it!
So individual molecules are still too small to be useful
in making up large chunks of matter like rocks or trees or
kitchen cabinets. Fortunately, molecules of the same type
often stick together with weaker chemical bonds. The
principle is still the same—electric charge attracts the
atoms—but because the bonds are weaker, the substance
can change its appearance or be broken up quite easily.
Water is a perfect example. An ice cube is made up of
billions of water molecules all weakly bonded together.
Add a little heat and those bonds break down, and the ice
melts into liquid water. Add more heat and the individ-
ual water molecules start shooting around at random as
But even at this point you haven’t destroyed an actual
water molecule. If you want to do that, and crack it into
hydrogen and oxygen, you need way more energy and spe-
cialized equipment. In fact, this is how we make hydrogen
for fuel cells—by cracking water molecules.
From molecules and their chemical bonds comes every
physical thing in the world you can touch and use.
Water from Hydrogen and Oxygen
1 2 3 4
What exactly is a flame?
Matter can be solid, liquid, or gas. But which one of these applies to flame?
A flame is light emitted from a whole bunch of chemical reactions that occur as a substance burns in a fire.
And flames aren’t as simple to explain as you might think ….
One of the first chemistry experiments ever done by
humans was when some long-ago ancestor took a
burning ember from, say, a forest fire, and held it to
some dry wood. The wood burst into flame, and so
began our long history with fire.
Wait—chemistry experiment? Yes! A fire is a
chemical reaction, where heat and fuel combine
with oxygen in the air to form new compounds and
release heat. It’s the heat that humans are most
interested in, but fire creates lots of other by-
products as well, depending on the fuel used.
Flames are a handy visual cue for us that
something is burning. But the chemistry of
flame is actually incredibly complex when you
look at it down at the level of individual mole-
cules and atoms.
In a small flame, like from a candle, heat
makes the fuel—in this case, wax—vaporize.
This lets the wax interact with oxygen in the
air in a reaction that releases even more heat.
We only need to supply some starting heat (a
match) to kick o a self-sustaining reaction that
lasts as long as there’s wax and oxygen to react
with each other.
Candle wax is a mix of hydrocarbon mole-
cules that, as it burns, breaks down into smaller
molecules. Each break of a chemical bond
releases heat. As the chain reaction proceeds,
some parts of it get so hot that the electrons in
the individual atoms release
photons—light particles. These photons let us
see the flame.
So really a flame is a glowing zone in a fire made up of
millions of chemical reactions. This zone gets pushed and
pulled around by the air, making those familiar moving
flame shapes we know so well. On the edges of the flame,
the reactions are cooling o and slowing down, so the light
is less bright and less energetic.
The overall color of the flame is determined by the fuel
being burned. Candles burn mostly yellow, but copper
burns mostly green. Natural gas stoves burn blue, and pure
hydrogen actually burns with an ultraviolet flame that
humans can’t see. It’s all down to the molecules that are
involved in the reactions in the heart of the fire.
You’ll also notice that flames don’t actually touch the
thing that’s burning. In a wood fire, there’s always a tiny
gap between the surface of the wood and the flame itself.
That’s because the flame comes from chemical reactions
in the gaseous part of the fire—the wood is supplying a
stream of combustible gas via a process called pyrolysis.
This causes the wood to char and ultimately break down
into ash. All of this is powered by heat.
So really, a flame isn’t a “thing” in the standard sense.
It’s a visible part of a chemical reaction. Without the
reaction, there is no flame. You can’t capture a flame or put
it in a container—though you can contain the reaction that
makes the flame.
Chemistry of a Burning Candle
Liquid Paran
600° C
Zone (Carbon
Dark Red
Main Reaction
Zone (H O,
CO , OH , C )
2 2
Zone, Light
(White 1,400°C)
H O, CO ,
and Unburned