that has all the properties of that
substance. For instance, a water molecule
is the smallest unit that is still
water. A water molecule can be divided
into tiny parts called atoms. This produces
two hydrogen atoms and one oxygen
atom. But these atoms alone do not
have the properties of water.
Molecules and Elements
The chemical elements are the
substances of which all matter is made.
That is, everything in the world is
A mole’s wide, flat front feet and sharp
claws help it dig through soil.
A water molecule contains two atoms of
hydrogen (H) and one atom of oxygen (O).
Scientists have different ways of drawing
molecules to show how they are made.
BRITANNICA STUDENT ENCYCLOPEDIA Molecule 155
either a pure element or a combination
of two or more elements. The
molecules of pure elements contain
only one type of atom. For example,
the molecules of the element iron
consist only of iron atoms.
A few elements have molecules that are
made of a single atom. For instance, a
molecule of helium gas consists of one
helium atom. But most elements have
molecules that contain at least two
atoms joined together. For example, a
molecule of oxygen gas consists of two
oxygen atoms.
Molecules and Compounds
Other molecules contain atoms of two
or more different elements. A substance
made of such molecules is called a
chemical compound.Water is a chemical
compound because its molecules
have two hydrogen atoms and one oxygen
atom. The sugar called glucose is
another chemical compound. Each glucose
molecule contains 6 carbon atoms,
12 hydrogen atoms, and 6 oxygen
atoms. Some very complex molecules in
living cells are made up of hundreds of
thousands of atoms.
How Molecules Are Formed
Strong chemical bonds hold the atoms
in a molecule together. The bonds are
formed when atoms share particles
called electrons. Electrons are found
either alone or in pairs in the outer part
of atoms. When two atoms with
unpaired electrons approach each other,
the unpaired electrons may form a pair.
Both atoms then share the pair. This
holds the atoms together.
#More to explore
Atom • Chemical Element • Chemistry
• Matter
Mollusk
A mollusk is a kind of animal with a
soft body. Most mollusks have a hard
shell that protects the body. There are
more than 100,000 species, or types, of
mollusk. Octopuses, oysters, snails, and
squid are just a few examples.
Where Mollusks Live
Mollusks can be found throughout the
world. Most live in the sea, but others
live in freshwater. Some like shallow
water along the coast. Others prefer
deep water. Many mollusks dig into
sandy or muddy bottoms. Some attach
themselves to rocks.
Some snails and slugs live on land. They
prefer cool, moist places. But they also
can be found in cold regions and in dry
deserts.
Squid are awake and active at night in the
water.
A molecule is
constantly in
motion. Its
atoms rotate
and vibrate
around one
another.
156 Mollusk BRITANNICA STUDENT ENCYCLOPEDIA
Physical Features
Mollusks vary a lot in their features.
Even mollusks of the same type can be
very different. For example, the giant
squid can be more than 60 feet (18
meters) long. But some other squid are
less than an inch (2.5 centimeters) long.
Mollusks are invertebrates, meaning that
they do not have a backbone. They have
a soft body that holds the heart, liver,
digestive system, and other important
organs. Most mollusks have a mouth
structure called a radula. The radula is
sort of like a rough tongue with lots of
tiny teeth.
A mollusk’s soft body has a covering
called the mantle. In most kinds of mollusk,
the mantle makes a shell. Some
mollusks, such as snails, have a single,
solid shell. Other mollusks, called
bivalves, have a shell with two halves
that can close together for protection.
Oysters and clams are bivalves. Some
mollusks, such as octopuses and squid,
do not have a shell.
Most mollusks with shells have a “foot.”
The foot moves them forward by
expanding and contracting muscles.
Some bivalves, however, do not move.
They stay attached to rocks or other
hard surfaces. Octopuses, squid, and
cuttlefish move by forcing water out of a
funnel-shaped structure on their body.
Behavior
Many mollusks eat mostly algae. They
use the radula to scrape algae off rocks.
But larger mollusks have a greater
appetite. Snails and slugs eat plants and
fruit. Squid may eat fish and shrimp.
An octopus can go after prey as large as
a shark.
Mollusks usually are active at night. For
instance, octopuses stay in the deep
parts of the ocean during the day. At
dawn and dusk they swim closer to the
surface looking for food.
Many mollusks hide from enemies in
their shells. But mollusks without shells
need other ways to protect themselves.
For example, squid and octopuses
change color and patterns to blend into
their surroundings.
Reproduction
Some mollusks bear live young, but
most reproduce by laying eggs. There
may be one egg, several eggs, or thousands
of eggs. The babies of some mollusks
look like small adults when they
hatch. Others hatch in a wormlike form
called larvae.
#More to explore
Bivalve • Octopus • Shell • Snail and
Slug • Squid
Mussels have hard shells with two halves
