Download Pearson Prentice Hall Physical Science: Concepts in Action

Pearson Prentice Hall
Physical Science: Concepts
in Action
Chapter 4
Atomic Structure
4.1 Studying Atoms
Objectives:
1. Explain Dalton’s atomic
theory
2. Explain the contributions of
Thomson and Rutherford
Democritus (4th century B.C) said the
universe was made of invisible units
called atoms
In 1808, an Englishman named John
Dalton proposed a theory:
1. every element is made of tiny
unique particles called atoms that
cannot be subdivided
2. atoms of the same element have
the same mass & atoms of different
elements have different masses
3. compounds contain atoms of more
than one element
4. in any particular compound, atoms
of different elements always combine
in the same way
Thomson & Rutherford
Q: What did Dalton notice that
all compounds have in
common?
A: Compounds contain atoms
of more than one element
JJ Thomson (1856-1940) used
a sealed tube of gas to show
that particles are charged
In Thomson’s experiment, the
particles repelled the negative
plate and were attracted to
the positive plate
Q: What was the charge on the
particles in the beam of the
gas?
Thomson’s experiments
provided the first evidence
that atoms are made of even
smaller particles
Q: How do objects of the same
charge behave when they
come close to one another?
Thomson developed the “plum
pudding” model of the atom
His model suggested that
negative charges were evenly
scattered throughout an atom
filled with a positively charged
mass of matter
Ernest Rutherford (1871-1937)
discovered fast moving particles in
uranium that he called alpha
particles
Uranium is a radioactive substance
He used a zinc sulfide screen, gold
foil and alpha particles to produce a
very famous experiment in which he
showed that the positive charges
are not evenly dispersed throughout
the atom
Rutherford concluded that atoms
must have a concentrated central
core
Definition: a nucleus is a dense,
positively charged mass located
in the center of the atom
According to Rutherford’s model,
all of an atom’s positive charge is
concentrated in its nucleus
Today, much of an atom consists
of empty space outside the
positively charged nucleus
4.2 The Structure of an Atom
Objectives:
1. Describe the three subatomic
particles
2. Compare protons, electrons and
neutrons in terms of physical
properties
3. Explain how atoms of one
element are different from atoms of
other elements
4. Describe the difference between
2 isotopes of the same element
Three Subatomic Particles
Protons, neutrons & electrons are
subatomic particles
Definition: a proton is a positively
charged subatomic particle that is
found in the nucleus of an atom
Definition: an electron is a negatively
charged subatomic particle found in
the space OUTSIDE THE NUCLEUS
(ORBITALS)
Definition: a neutron is a neutral
subatomic particle found in the
nucleus of an atom
Comparing Protons, Neutrons &
Electrons
Protons, neutrons and electrons
can be distinguished by mass,
charge & location in an atom
The mass of a neutron is almost
exactly equal to the mass of a
proton
Electrons are nearly 2,000 times
less massive than either a
proton or a neutron
Isotopes
Definition: isotopes are atoms of the
same element that have different
numbers of neutrons and different
mass numbers
Isotopes: element has same atomic
number, same # protons, different #
neutrons, different mass #
Q: Why do isotopes have different
mass numbers for the same element?
A: Because they have different
numbers of neutrons
The mass of an electron is
insignificant
Thomson’s experiment with the gas
tube detected electrons
Rutherford discovered the positive
central core
The mass of each proton or neutron
equals one atomic mass unit (amu)
Q: What is the charge of a proton?
(neutron? electron?)
Q: Where is the proton located in an
atom? (neutron? electron?)
Atoms of Different Elements
Atoms of any given element
ALWAYS have the same number of
PROTONS
Definition: atomic number of an
element equals the number of
protons
Atoms of different elements
ALWAYS have different numbers of
PROTONS
The number of PROTONS
determines which element you have
Definition: mass number of an
atom is the sum of the protons
plus the neutrons
Neutrons + Protons = Mass #
Q: Why aren’t electrons considered
in the mass number of an atom?
# neutrons = mass # - atomic #
Q: What subatomic particle could
be substituted for atomic number?
Q: What 2 values represent mass#?
4.3 Modern Atomic Theory
Objectives:
1. Explain what can happen
when atoms gain or lose
energy
2. Explain the model that
scientists use to describe how
electrons behave in atoms
3. Describe the most stable
configuration of electrons in
an atom
Losing and Gaining Energy
Since the time of Democritus &
Dalton the model of the atom has
been revised
In 1913, Niels Bohr theorized that
atoms move in set paths around the
nucleus like planets orbiting the sun
The electron path is an electron’s
energy level
Energy level: any of the possible
energies an electron may have in an
atom
Electrons in atoms must gain
energy to move to a higher energy
level or lose energy to move to a
lower energy level
The amount of energy gained
(absorbed) to go higher or energy
lost (released) can be measured
Each element has its own unique
set of energy levels
The element itself determines the
amount of energy gained or lost
Bohr’s model does not explain all
chemical observations
How Electrons Behave in Atoms
Definition: an electron cloud is
a visual model of the most
likely locations for electrons in
an atom
Scientists use the electron
cloud model to describe the
possible locations of electrons
around the nucleus
Instead of set paths like orbits,
electrons are found in orbitals
which have unique shapes
An electron cloud is a good
approximation of how electrons
behave in their orbitals
Definition: an orbital is a region
of space around the nucleus of
an atom where there is a high
probability of finding an electron
The level with the least energy
has only one orbital
Higher energy levels have more
than one orbital
Electron Configurations
Definition: an electron configuration is
the arrangement of electrons in the
orbitals of an atom
The most stable configuration is the
one in which the electrons are in
orbitals with the lowest possible
energies
Definition: the ground state is when
all electrons in an atom have the
lowest possible energies
Definition: excited state is when an
electron is in an orbital with higher
energy & less stability