Download Chapter 4 What Are Atoms?

Chapter 4
Section 1 Atomic Structure
What Are Atoms?
• Our understanding of atoms required many centuries.
• The idea of an atom—which means “unable to be
divided”—dates back to the Greek philosopher
Democritus, who lived in the fourth century BCE.
• John Dalton developed an atomic theory in 1808.
• Like Democritus, Dalton proposed that atoms
could not be divided. Dalton’s was the first
atomic theory with a scientific basis.
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Chapter 4
Section 1 Atomic Structure
What Are Atoms? continued
• An atom is the smallest part of an element that still
has the element’s properties.
• Atoms are the building blocks of molecules.
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Chapter 4
Section 1 Atomic Structure
Atom
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Chapter 4
Section 1 Atomic Structure
What’s in an Atom?
• Atoms are made of protons, neutrons, and electrons.
• At the center of each atom is a small, dense
nucleus with a positive electric charge.
• The nucleus is made of protons (a subatomic
particle that has a positive charge) and neutrons
(a subatomic particle that has no charge).
• Moving around outside the nucleus is a cloud
of electrons: subatomic particles with
negative charges.
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Chapter 4
Section 1 Atomic Structure
What’s in an Atom? continued
• Unreacted atoms have no overall charge.
• Although atoms are made of charged particles,
they do not have an overall charge because they
have an equal number of protons and electrons
whose charges exactly cancel.
• To the right is shown a helium
atom, which is made of two
protons, two neutrons, and
two electrons.
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Chapter 4
Section 1 Atomic Structure
Models of the Atom
• Bohr’s model compares electrons to planets.
• In 1913, the Danish scientist Niels Bohr suggested
that electrons in an atom move in set paths around
the nucleus much like the planets orbit the sun in
our solar system.
• In Bohr’s model, electrons can only be in certain
energy levels. Bohr’s model of electrons is
illustrated on the following slide.
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Chapter 4
Section 1 Atomic Structure
Building Model
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Chapter 4
Section 1 Atomic Structure
Models of the Atom, continued
• Electrons act more like waves.
• By 1925, Bohr’s model of the atom no longer
explained electron behavior.
• A new model was proposed, in which electrons
behave more like waves on a vibrating string than
like particles.
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Chapter 4
Section 1 Atomic Structure
Models of the Atom, continued
• An electron’s exact location cannot be determined.
• It is impossible to determine both the exact
location of an electron in an atom and the
electron’s speed and direction.
• The best scientists can do is calculate the chance
of finding an electron in a certain place within
an atom.
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Chapter 4
Section 1 Atomic Structure
Models of the Atom, continued
• Electrons exist in energy levels.
• The number of filled energy levels an atom
has depends on the number of electrons.
• The figure to the right
shows how the first four
energy levels are filled.
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Chapter 4
Section 1 Atomic Structure
Models of the Atom, continued
• Electrons are found in orbitals within energy levels.
• An orbital is a region in an atom where there is a high
probability of finding electrons.
• An s orbital is shaped like a sphere:
• A p orbital is dumbbell shaped and
can be oriented three different ways in space:
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Chapter 4
Section 1 Atomic Structure
Models of the Atom, continued
• Every atom has between one and eight
valence electrons.
• Valence electrons are found in the outermost
shell of an atom and determine the atom’s
chemical properties.
• Valence electrons are the electrons in an atom
that participate in chemical bonding.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Objectives
• Relate the organization of the periodic table to the
arrangement of electrons within an atom.
• Explain why some atoms gain or lose electrons to
form ions.
• Determine how many protons, neutrons, and
electrons an atom has, given its symbol, atomic
number, and mass number.
• Describe how the abundance of isotopes affects an
element’s average atomic mass.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Organization of the Periodic Table
• The periodic table groups similar elements together.
• This organization makes it easier to predict the
properties of an element based on where it is in
the periodic table.
• Elements are listed in order of number of protons,
because the periodic law states that when
elements are arranged this way, similarities in their
properties will occur in a regular pattern.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Organization of the Periodic Table, continued
• The periodic table helps determine electron
arrangement.
• Horizontal rows in the periodic table are
called periods.
• Just as the number of protons an atom has
increases as you move from left to right across a
period, so does its number of electrons.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Organization of the Periodic Table, continued
• Elements in the same group have similar properties.
• A group is a vertical column of elements in the
periodic table.
• Atoms of elements in the same group have the
same number of valence electrons, so these
elements have similar properties.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Some Atoms Form Ions
• An ion is an atom or group of atoms that has lost
or gained one electrons and has a negative or
positive charge.
• A lithium atom loses one electron to form a 1+ charged ion:
• A fluorine atom gains one electron to form a 1 charged ion:
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
How Do the Structures of Atoms Differ?
• The atomic number, Z, of an atom equals the
number of protons in the nucleus.
• The mass number, A, of an atom equals the
number of protons plus the number of neutrons in
the nucleus.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Nucleus
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Atomic Number
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Mass Number
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
How Do the Structures of Atoms Differ?
continued
• An isotope is an atom that has the same number of
protons as other atoms of the same element do but
that has a different number of neutrons.
• Example: Hydrogen has three isotopes, shown
below.
• Some isotopes are more common than others.
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
How Do the Structures of Atoms Differ?
continued
• If you know the atomic number and mass number
of an atom, you can calculate the number of
neutrons it has.
• Example: uranium-235 has a mass number of 235.
Like all uranium atoms, it has an atomic number of
92. The number of neutrons it has is therefore:
Mass number (A):
Atomic number (Z):
Number of neutrons:
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235
–92
143
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
Isotopes
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Chapter 4
Section 2 A Guided Tour of the
Periodic Table
How Do the Structures of Atoms Differ? continued
• Because the mass of a single atom is so tiny, atomic
masses are usually expressed in atomic mass units.
• An atomic mass unit (amu) is equal to one twelfth of
the mass of a carbon-12 atom.
• The average atomic mass for an element is a
weighted average of the masses of all naturallyoccurring isotopes of an element.
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Chapter 4
Section 3 Families of Elements
Objectives
• Locate alkali metals, alkaline-earth metals, and
transition metals in the periodic table.
• Locate semiconductors, halogens, and noble gases
in the periodic table.
• Relate an element’s chemical properties to the
electron arrangement of its atoms.
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Chapter 4
Section 3 Families of Elements
How Are Elements Classified?
• The elements are classified into three groups.
• Most elements are metals, elements that are
shiny and conduct heat and electricity well.
• Nonmetals, all except hydrogen of which are
found on the right side of the periodic table, may
be solids, liquids, or gases at room temperature.
• Between these groupings are semiconductors,
elements that can conduct electricity under
certain conditions.
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Chapter 4
Section 3 Families of Elements
How Are Elements Classified? continued
• The periodic table below shows the distribution of
metal, nonmetals, and semiconductors in the
periodic table.
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Chapter 4
Section 3 Families of Elements
Metals
• The alkali metals, found in Group 1
of the periodic table, are very reactive.
• The alkaline-earth metals, which
include calcium, are found in
Group 2 of the periodic table, and
are somewhat less reactive than the
alkali metals.
• The transition metals, such as gold,
iron, and mercury, occupy Groups
3–12 of the periodic table.
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Chapter 4
Section 3 Families of Elements
Nonmetals
•
Carbon is found in three different forms and can form many
compounds.
•
Nonmetals and their compounds
are plentiful on Earth.
•
Halogens, such as chlorine, are
located in Group 17 of the
periodic table.
•
Noble gases, such as neon, make
up Group 18 of the periodic table.
They are unreactive.
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Chapter 4
Section 3 Families of Elements
Nonmetals, continued
• Semiconductors are intermediate
conductors of heat and electricity.
• Silicon is the most familiar semiconductor.
• Silicon is an important part of computer chips, as
well as other semiconductor devices such as
transistors, LED display screens, and solar cells.
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Chapter 4
Section 4 Using Moles to
Count Atoms
Objectives
• Explain the relationship between a mole of a
substance and Avogadro’s constant.
• Find the molar mass of an element by using the
periodic table.
• Solve problems converting the amount of an element
in moles to its mass in grams, and vice versa.
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Chapter 4
Section 4 Using Moles to
Count Atoms
Counting Things
• There are many different counting units: for example,
eggs are packaged by the dozen.
• The mole is useful for counting small particles.
• A mole (abbreviation: mol) is the number of
particles that is the same as the number of atoms
of carbon in 12 g of carbon-12.
• Avogadro’s constant is the number of particles
per mole of a substance: 6.022 × 1023
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Chapter 4
Section 4 Using Moles to
Count Atoms
Counting Things, continued
• Moles and grams are related.
• The mass in grams of 1 mol of a substance is called
its molar mass.
• For example, 1 mol of carbon-12 atoms has a
molar mass of 12.00 g.
• The molar mass of an element
is its average atomic mass, which
is listed in the periodic table.
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Chapter 4
Section 4 Using Moles to
Count Atoms
Molar Mass
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Chapter 4
Section 4 Using Moles to
Count Atoms
Calculating with Moles
• To convert between moles and grams and vice versa,
you can use a conversion factor: a ratio that is
derived from the equality of two different units.
• Let’s say that a shopkeeper knows that exactly
10 gumballs have a total mass of 21.4 g. This
relationship can be written as either one of two
equivalent conversion factors:
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Chapter 4
Section 4 Using Moles to
Count Atoms
Calculating with Moles, continued
• An element’s molar mass can be used as a
conversion factor.
• The diagram below shows how to set up the
conversion factor, depending on whether you
want to convert from amount to mass or the other
way around.
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