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Chapter 5
Models of
the Atom
by Christopher Hamaker
© 2011 Pearson Education, Inc.
Chapter 5
1
Dalton Model of the Atom
• John Dalton proposed that all matter is made
up of tiny particles.
• These particles are molecules or atoms.
• Molecules can be broken down into atoms by
chemical processes.
• Atoms cannot be broken down by chemical or
physical processes.
© 2011 Pearson Education, Inc.
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2
Subatomic Particles
•
Three subatomic particles were discovered.
1. Negatively charged electrons, e–.
2. Positively charged protons, p+.
3. neutrons, n0, with no charge
•
An electron has a relative charge of -1, and a
proton has a relative charge of +1. A neutron
carries zero charge
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Mass of Subatomic Particles
– An electron has a mass of 9.11 × 10-28 g.
– A proton has a mass of 1.67 × 10-24 g.
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Types of Radiation
•
There are three types of radiation:
1. Alpha (a)
2. Beta (b)
3. Gamma (g)
•
Alpha rays are composed of helium atoms
stripped of their electrons (helium nuclei).
•
Beta rays are composed of electrons.
•
Gamma rays are high-energy electromagnetic
radiation.
© 2011 Pearson Education, Inc.
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•
Atomic
Notation
Each element has a characteristic number of
protons in the nucleus. This is the atomic number,
Z.
• The total number of protons and neutrons in the
nucleus of an atom is the mass number, A.
• We use atomic notation to display the number of
protons and neutrons in the nucleus of an atom:
mass number (p+ and n0)
atomic number (p+)
© 2011 Pearson Education, Inc.
A
Z
Sy
Chapter 5
symbol of the element
6
Using
Atomic Notation
29
• An example: 14Si
• The element is silicon (symbol Si).
• The atomic number is 14; silicon has 14 protons.
• The mass number is 29; the atom of silicon has 29
protons + neutrons.
• The number of neutrons is A – Z = 29 – 14 = 15
neutrons.
© 2011 Pearson Education, Inc.
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Isotopes
• All atoms of the same element have the same
number of protons.
• Most elements occur naturally with varying
numbers of neutrons.
• Atoms of the same element that have a
different number of neutrons in the nucleus
are called isotopes.
• Isotopes have the same atomic number, but
different mass numbers.
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Isotopes, Continued
• We often refer to an isotope by stating the name
of the element followed by the mass number.
– Cobalt-60 is
– Carbon-14 is
60
37
14
6
Co
C
• How many protons and neutrons does an atom of
lead-206 have?
– The atomic number of Pb is 82, so it has 82 protons.
– Pb-206 has 206 – 82 = 124 neutrons.
© 2011 Pearson Education, Inc.
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Isotopes
• Percent Abundance:
– Percent of atoms in a natural sample of a pure
element that are a particular isotope of the
element.
– Distribution of isotopes in any particular sample is
generally constant.
– Is used to determine an atomic mass unit (amu)
– 23 elements have only one naturally occurring
form.
Simple and Weighted Averages
• A simple average assumes the same number
of each object.
• A weighted average takes into account the
fact that we do not have equal numbers of all
the objects.
• A weighted average is calculated by
multiplying the percentage of the object (as a
decimal number) by its mass for each object
and adding the numbers together.
© 2011 Pearson Education, Inc.
Chapter 5
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Average Atomic Mass
•
Since not all isotopes of an atom are present in
equal proportions, we must use the weighted
average.
•
Copper has two isotopes:
1.
2.
•
63Cu,
with a mass of 62.930 amu and 69.09% abundance.
65Cu, with a mass of 64.928 amu and 30.91% abundance.
The average atomic mass of copper is:
(62.930 amu)(0.6909) + (64.928 amu)(0.3091)
= 63.55 amu
© 2011 Pearson Education, Inc.
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Periodic Table
• We can use the periodic table to obtain the
atomic number and atomic mass of an element.
• The periodic table shows the atomic number,
symbol, and atomic mass for each element.
© 2011 Pearson Education, Inc.
Chapter 5
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Electrons
• Remember:
– Electrons are the smallest of the three
subatomic particles we will deal with.
– They have very little mass.
– They reside in the electron cloud that
surrounds the nucleus.
– Their movement rapidly about the nucleus
defines the size of the atom
Electrons
– Electrons are the high energy part of the atom.
– The faster the electron moves the more energy it
has and the farther from the nucleus it tends to
travel.
– Electrons have spin.
Electrons
• Electron shell – a defined region of space
about a nucleus that contains electrons
with approximately the same energy.
• Shell number – ‘n’ used to identify the
electron shell.
– These are numbered 1-7
– Electrons in higher number shells have more
energy.
Electrons
How many electrons in an electron shell?
• Depends –
– Not all shells are equal.
– Lower shells have less electrons, less energy.
– Number of electrons in a shell follows the rule
2n2 where ‘n’ is the electron shell level.
– Lower level shells fill before higher ones.
• So how many electrons are in shell 3?
18
Electrons – Subshells and Orbitals
• Within a shell there are subshells and orbitals.
– A subshell ( or sublevel) is defined region of space
within an electron shell that contain electrons of the
same energy.
– These sublevels are given the designations s, p, d,
and f.
– Number of subshells is equal to the number ‘n’ where
‘n’ is the shell number.
– Subshells are written with a number, ‘n’, and a letter.
– The superscript represents the number of electrons in
the subshell. Ex: 1s2
Electron Shells and Subshells
Energy Levels and Sublevels, Continued
• The first energy level has one sublevel designated
1s.
• The second energy level has two sublevels
designated 2s and 2p.
• The third energy level has three sublevels
designated 3s, 3p, and 3d.
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Electrons – Subshells and Orbitals
• Subshells
– Subshells are written with a number, ‘n’, and a
letter, s, p, d, f (always in lower case).
– Number of electrons per subshell is defined
and independent of shell number.
s=2
p=6
d = 10
f = 14
Electron Occupancy in
Sublevels
• The maximum number of electrons in each of
the energy sublevels depends on the
sublevel:
–
–
–
–
The s sublevel holds a maximum of 2 electrons.
The p sublevel holds a maximum of 6 electrons.
The d sublevel holds a maximum of 10 electrons.
The f sublevel holds a maximum of 14 electrons.
• The maximum electrons per level is obtained
by adding the maximum number of electrons
in each sublevel.
© 2011 Pearson Education,
Inc.
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Electrons per Energy Level
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Electrons – Subshells and Orbitals
• Electron orbital – a region of space within an
electron subshell where an electron with a
specific energy is most likely to be found.
s=1
p=3
d=5
f=7
• Each orbital can hold two electrons.
Quantum Mechanical Model
• An orbital is the region of space where there is a
high probability of finding an atom.
• In the quantum mechanical atom, orbitals are
arranged according to their size and shape.
• The higher the energy of an orbital, the larger its
size.
• All s orbitals
have spherical
shapes.
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Electron Configurations
• Electrons are arranged about the nucleus in a
regular manner. The first electrons fill the energy
sublevel closest to the nucleus.
• Electrons continue filling each sublevel until it is
full, and then start filling the next closest
sublevel.
• A partial list of sublevels in order of increasing
energy is as follows:
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d …
© 2011 Pearson Education, Inc.
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Filling Electron Shells
• Electron shells fill
according to the
Aufbau Principle.
Electrons normally
occupy electron
subshells in an atom
in order of increasing
subshell energy.
Energy of subshells can
overlap
Filling Diagram for Energy Sublevels
• The order does
not strictly
follow 1, 2, 3,
etc.
• For now, use
Figure 5.16 to
predict the
order of
sublevel filling.
© 2011 Pearson Education, Inc.
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Electron Configurations
• The electron configuration of an atom is a
shorthand method of writing the location of
electrons by sublevel.
• The sublevel is written followed by a
superscript with the number of electrons in
the sublevel. For example, if the 2p sublevel
contains two electrons, it is written 2p2.
• The electron sublevels are arranged according
to increasing energy.
© 2011 Pearson Education, Inc.
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Writing Electron Configurations
• First, determine how many electrons are in
the atom. Bromine has 35 electrons.
• Arrange the energy sublevels according to
increasing energy:
–1s 2s 2p 3s 3p 4s 3d …
• Fill each sublevel with electrons until you have
used all the electrons in the atom:
–Fe: 1s2 2s2 2p6 3s2 3p6 4s2 3d 10 4p5
• The sum of the superscripts equals the atomic
number of bromine (35).
© 2011 Pearson Education, Inc.
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Writing Electron Configuration
While this may seem tedious, you can learn a lot
from the written electron configuration.
This is a shorthand notation designating the
subshells in an atom that are occupied by
electrons.
Start at the beginning and keep filling subshells
until the correct number of electron is
represented:
• Remember the nuclear notation and ‘z’ is the
number of protons which equals the number of
electrons in a neutral atom:
Element
Hydrogen
z
1
Electron Configuration
1s1
Helium
2
1s2
Carbon
6
1s22s22p2
Neon
10
1s22s22p6
Sodium
11
1s22s22p63s1
Shapes of p Orbitals
• Recall that there are three different p sublevels.
• All p orbitals have dumbbell shapes.
• Each of the p orbitals has the same shape, but
each is oriented along a different axis in space.
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Shapes of d Orbitals
• Recall that there are five different d sublevels.
• Four of the d orbitals have a clover-leaf shape and
one has a dumbbell and doughnut shape.
© 2011 Pearson Education, Inc.
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Chapter Summary
• Atoms are composed of protons, neutrons,
and electrons.
• The protons and neutrons are located in the
nucleus, and the electrons are outside the
nucleus.
• Atoms are mostly empty space.
• The number of protons is referred to as the
atomic number for the atom.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
• All atoms of the same element have the same
number of protons.
• Isotopes are atoms with the same number of
protons, but differing numbers of neutrons.
• The mass number for an isotope is the total
number of protons plus neutrons.
© 2011 Pearson Education, Inc.
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Chapter Summary, Continued
• The atomic mass of an element is the
weighted average of the masses of all the
naturally occurring isotopes.
• Electrons exist around the nucleus of atoms in
discrete, quantized energy levels.
• Electrons fill energy sublevels, starting with
the lowest energy sublevel and filling each
successive level of higher energy.
© 2011 Pearson Education, Inc.
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