Each individual atom is made up of
smaller particleselectrons, 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
particlesprotons, neutrons, and
electronsare 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,
were unhappy with British
rule. When a crowd gathered on March
5 to challenge British troops, Attucks
quickly came to the front of the group.
As the Americans waved their weapons
and shouted, the British opened fire.
Attucks was shot twice. He was the first
to die, but four others were also killed
and six were injured.
Attucks was immediately recognized as
a hero. It is said that 10,000 people
joined his funeral procession. In 1888
Attucks was honored with a monument
in Boston.
..More to explore
American Revolution Boston
Augusta
Population
(2000 census)
18,560; (2007
estimate)
18,367
Augusta is the capital of the U.S. state of
Maine. The city lies along both banks of
the Kennebec River. Ships travel on the
Crispus Attucks
Buildings of Augusta, Maine, look out onto
the Kennebec River.
226 Augusta BRITANNICA STUDENT ENCYCLOPEDIA
river from Augusta to the Atlantic
Ocean, about 40 miles (65 kilometers)
away.
Some of Augustas factories process foods.
Others make paper products, computer
products, steel, and shoes.Many people
in Augusta work for the state government
or in education or tourism.
Traders first came to the area in the
1600s. They established a trading post
on the site because of its location on the
river. In 1754 a company built Fort
Western to protect the site from Indian
attacks. A village soon grew up around
the fort. In 1797 the village was named
Augusta. Augusta was chosen as the
capital of Maine in 1827.
#More to explore
Maine
Augustus
The first emperor of Rome was Augustus.
During his long reign the Roman
world entered an era of wealth, peace,
and cultural achievement that became
known as the Augustan Age.
Augustus was born on September 23, 63
BC. His original name was Gaius
Octavius. He was related to Julius Caesar,
the Roman leader who was murdered
in 44 BC. After Caesars death he
found out that Caesar had chosen him
to be the next leader. From then on he
was called Octavian.
Other leaders challenged Octavian for
power. He finally defeated the last of
theseMark Antonyin 31 BC.
According to historians, Octavians victory
over Antony marked the beginning
of the Roman Empire. Octavian then
controlled the whole Roman world. In
27 BC the Senate gave him the title
Augustus (the exalted or sacred one).
Augustus armies conquered more territory
for the Roman Empire until the
Germans stopped them in AD 9. Augustus
then focused on improving Rome.
He founded cities, built roads, encouraged
agriculture, and promoted the arts.
Augustus died on August 19, AD 14.
Tiberius, his adopted son, then took
over as emperor.
#More to explore
Caesar, Julius Rome, Ancient
Aurora
Auroras are dazzling displays of colored
light that sometimes appear in the night
sky. They occur in Earths far northern
and far southern regions. In the Northern
Hemisphere such a display is known
as aurora borealis, or the northern lights.
An aurora lights up the sky over Churchill,
Manitoba, a city in northern Canada.
BRITANNICA STUDENT ENCYCLOPEDIA Aurora 227
In the Southern Hemisphere it is called
aurora australis, or the southern lights.
Auroras take many shapes and forms,
with arcs and rays of colored light being
the most common. The light may also
look like shimmering curtains, bands,
waves, or clouds. An aurora constantly
changes shape as the light moves across
the sky. The light also may brighten and
fade.
Auroras are caused by the sun. The sun
sends out a stream of electrically charged
particles called the solar wind. The solar
wind travels from the sun toward Earth
at great speed.
Earth is a huge magnet surrounded by a
magnetic field. Earths magnetic field
forces the charged particles in the solar
wind toward the planets northern and
southern polar regions. The particles
move downward through Earths upper
atmosphere. As they do, they crash into
atoms and molecules of gases such as
oxygen and nitrogen. These collisions
cause the atoms and molecules to give