Britannica Student Encyclopedia - 2010

Other planets, and some of their larger

moons, also have atmospheres.

Earth’s atmosphere consists of several

different gases. The gases are held close

to Earth by a force called gravity. Near

Earth’s surface, the atmosphere is about

three fourths nitrogen and one fifth oxygen.

At higher elevations, the atmosphere

is mostly hydrogen and helium.

Surprisingly, air has weight. It is heaviest

at sea level. There the gas particles are

pressed together by the weight of the air

above them. Air becomes lighter away

from Earth’s surface.

Scientists divide the atmosphere into

five regions, or layers. The layer closest

to Earth is the troposphere. It extends

up to about 6 miles (10 kilometers)

above Earth’s surface. Most of Earth’s

weather, including the wind and most

clouds, exists in the troposphere.

The second layer, the stratosphere,

extends to about 30 miles (50 kilometers)

above Earth’s surface. The stratosphere

includes the ozone layer. Ozone

blocks much of the sun’s radiation that

would harm plants and animals if it

reached Earth.

The third layer is the mesosphere. It

extends up to about 50 miles (80 kilometers)

above Earth’s surface.

The fourth layer, the thermosphere,

ranges from about 50 to 300 miles (80

to 480 kilometers) above Earth.

The fifth and highest layer of the atmosphere

is the exosphere. This layer ends

An atlas can teach children about the world.

The atmosphere

protects

Earth’s

surface from

being hit by

objects from

outer space.

These objects

are called

meteoroids.

Most burn up

as they enter

the atmosphere.

BRITANNICA STUDENT ENCYCLOPEDIA Atmosphere 223

 

where Earth’s gravity is too weak to prevent

particles of gas from drifting into

space.

Atom

The tiny particles called atoms are the

basic building blocks of all matter.

Atoms can be combined with other

atoms to form molecules, but they cannot

be divided into smaller parts by

ordinary means.

The word atom comes from the Greek

word atomos, meaning “indivisible.” The

ancient Greeks were the first to think of

the atom as the basic unit of all matter.

It was not until the early 1800s, though,

that scientists began to understand how

atoms work.

Structure

Each individual atom is made up of

smaller particles—electrons, protons,

and neutrons. These are called

subatomic particles. At the center of an

atom is a nucleus. The nucleus consists

of protons and neutrons. Protons carry

a positive electrical charge, while

neutrons carry no electrical charge.

Together, protons and neutrons are

called nucleons. Surrounding the

nucleus is a cloud of negatively charged

electrons.

Scientists believe that subatomic

particles—protons, neutrons, and

electrons—are themselves made up of

smaller substances. The substances are

called quarks and leptons.

224 Atom BRITANNICA STUDENT ENCYCLOPEDIA

 

Properties

The single most important thing to

know about an atom is how many protons

it has in its nucleus. This is known

as its atomic number. The atomic number

determines what kind of atom it is.

Every atom is associated with a specific

chemical element. An atom is the smallest

unit of an element, and each chemical

element has a unique atomic

number. For instance, hydrogen has an

atomic number of 1 because every

hydrogen atom has one proton in its

nucleus. No other element has an

atomic number of 1.

Another property of atoms is their

atomic weight. This is roughly equal to

the total number of protons and neutrons

in an atom. Atoms that have the

same atomic number but different

atomic weights are called isotopes.

Carbon-12, the ordinary form of carbon,

has six protons and six neutrons

per atom. Carbon-14 is an isotope with

eight neutrons per atom. It still has six

protons. If it did not have six protons, it

would not be carbon.

An ordinary atom has an equal number

of protons and electrons. This means that

the positive and negative charges are

balanced. Some atoms, however, lose or

gain electrons in chemical reactions or in

collisions with other particles.Ordinary

atoms that gain or lose electrons are called

ions. If a neutral atom loses an electron, it

becomes a positive ion. If it gains an

electron, it becomes a negative ion.

#More to explore

Chemical Element • Chemistry • Matter

• Molecule

Atsina

#see Gros Ventre.

Attucks, Crispus

An event called the Boston Massacre

helped to turn American colonists

All atoms have the same basic parts. These

are a nucleus and at least one particle

called an electron. The nucleus contains

protons and neutrons. Diagrams of two

kinds of atoms show that each kind of atom

has a different number of electrons.

One proton is

about as

heavy as

2,000 electrons.

BRITANNICA STUDENT ENCYCLOPEDIA Attucks, Crispus 225

 

against British rule, leading to the

American Revolution. Crispus Attucks

was killed by British soldiers during the

massacre, and many consider him to be

the first person to lose his life in the

struggle for independence.

Crispus Attucks was of African and

Native American ancestry. He was born

in about 1723 and probably grew up in

a Natick Indian settlement. He may

have escaped from slavery in 1750. Most

likely he became a sailor on whaling

ships.

By 1770 many people in Boston, Massachusetts,