Chapter 3:
Electron Structure
and the Periodic Law
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PERIODIC LAW
• This is a statement about the behavior of the elements
when they are arranged in a specific order.
• In its present form the statement is: Elements with
similar chemical properties occur at regular (periodic)
intervals when the elements are arranged in order of
increasing atomic numbers.
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PERIODIC TABLE
• A periodic table is a tabular arrangement of the elements
based on the periodic law.
• In a modern periodic table, elements with similar
chemical properties are found in vertical columns called
groups or families.
group/family
period
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PERIODIC TABLE GROUP OR FAMILY
• A vertical column of elements that have similar chemical
properties.
• Traditionally designated by a Roman numeral and a letter
(either A or B) at the top of the column.
• Designated only by a number from 1 to 18 in a modern but
as yet not universally-used designation.
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PERIODIC TABLE PERIOD
• A horizontal row of elements arranged according to
increasing atomic numbers.
• Periods are numbered from top to bottom of the periodic
table.
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APPEARANCE OF A MODERN PERIODIC TABLE
• In a modern table, elements 58-71 and 90-103 are not
placed in their correct periods, but are located below the
main table.
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ELEMENTS AND THE PERIODIC TABLE
• Each element belongs to a group and period of the
periodic table.
EXAMPLES OF GROUP AND PERIOD LOCATION FOR
ELEMENTS
• Calcium, Ca, element # 20: group IIA, period 4
• Silver, Ag, element # 47: group IB, period 5
• Sulfur, S, element # 16: group VIA, period 3
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Coached Problem
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THE BOHR THEORY OF ELECTRON BEHAVIOR IN
HYDROGEN ATOMS
• Bohr proposed that the electron in a hydrogen atom moved
in any one of a series of circular orbits around the nucleus.
• The electron could change orbits only by absorbing or
releasing energy.
• This model was replaced
by a revised model of
atomic structure in 1926
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THE QUANTUM MECHANICAL MODEL OF
ELECTRON BEHAVIOR IN ATOMS
• According to the quantum mechanical model of electron
behavior, the precise paths of electrons moving around the
nucleus cannot be determined accurately.
• Instead of circular orbits, the location and energy of
electrons moving around the nucleus is specified using the
three terms shell, subshell and orbital.
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SHELL
• The location of electrons in a shell is indicated by
assigning a number n to the shell and all electrons located
in the shell.
• The value of n can be 1, 2, 3, 4, etc.
• The higher the n value, the higher is the energy of the shell
and the contained electrons.
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SUBSHELL
• Each shell is made up of one or more subshells that are
designated by a letter from the group s, p, d, or f.
• The number of the shell to which a subshell belongs is
combined with the letter of the subshell to clearly identify
subshells.
• For example, a p subshell located in the third shell (n = 3)
would be designated as a 3p subshell.
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• The number of subshells located in a shell is the same as
the number of the shell. Thus, shell number 3 (n = 3)
contains three subshells, designated 3s, 3p, and 3d.
• Electrons located in a subshell are often identified by using
the same designation as the subshell they occupy. Thus,
electrons in a 3d subshell are called 3d electrons.
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ATOMIC ORBITALS
• The description of the location and energy of an electron
moving around a nucleus is completed in the quantum
mechanical model by specifying an atomic orbital in which
the electron is located.
• Each subshell consists of one or more atomic orbitals,
which are specific volumes of space around the nucleus in
which electrons move.
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• Atomic orbitals are designated by the same number and
letter used to designate the subshell to which they belong.
Thus, an s orbital located in a 2s subshell would be called
a 2s orbital.
• All s subshells consist of a single s orbital.
• All p subshells consist of three p orbitals.
• All d subshells consist of five d orbitals.
• All f subshells consist of seven f orbitals.
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• According to the quantum mechanical model, all types of
atomic orbitals can contain a maximum of two electrons.
• Thus, a single d orbital can contain a maximum of 2
electrons, and a d subshell that contains five d orbitals can
contain a maximum of 10 electrons.
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ATOMIC ORBITAL SHAPES
• Atomic orbitals of different types have different shapes.
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THE ENERGY OF ELECTRONS IN ATOMS
• Electron energy increases with increasing n value. Thus, an
electron in the third shell (n = 3) has more energy than an
electron in the first shell (n = 1).
• For equal n values but different orbitals, the energy of
electrons in orbitals increases in the order s, p, d and f. Thus,
a 4p electron has more energy than a 4s electron.
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RELATIONSHIPS BETWEEN SHELLS, SUBSHELLS,
ORBITALS AND ELECTRONS
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ELECTRONS AND CHEMICAL PROPERTIES
• The valence shell of an atom is the shell that contains
electrons with the highest n value.
• Atoms with the same number of electrons in the valence
shell have similar chemical properties.
Members of Group IIA(2)
magnesium
calcium
strontium
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ELECTRON OCCUPANCY OF SHELLS
• What do magnesium and
calcium have in common?
2 electrons in valence shell
• What predictions can be
made about the number of
electrons in strontium’s
valence shell?
Sr has similar chemical
properties to Mg and Ca,
so it likely has 2 electrons
in its valence.
• What other element on this
chart has similar properties
to Mg, Ca, and Sr?
Beryllium
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ELECTRONIC CONFIGURATIONS
• Electronic configurations give details of the arrangements
of electrons in atoms.
• The notation used to represent electronic configurations is
1s22s22p6…, where the occupied subshells are indicated
by their identifying number and letter such as 2s and the
number of electrons in the subshell is indicated by the
superscript on the letter. Thus, in the example above, the
2s2 notation indicates that the 2s subshell contains two
electrons.
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THE ORDER OF SUBSHELL FILLING
• Electrons will fill subshells in the order of increasing
energy of the subshells. Thus, a 1s subshell will fill before
a 2s subshell.
• The order of subshell filling must obey Hund's rule and
the Pauli exclusion principle.
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HUND'S RULE
• According to Hund's rule, electrons will not join other
electrons in an orbital of a subshell if an empty orbital of
the same energy is available in the subshell.
• Thus, the second electron entering a p subshell will go into
an empty p orbital of the subshell rather than into the
orbital that already contains an electron.
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THE PAULI EXCLUSION PRINCIPLE
• Electrons behave as if they spin on an axis.
• According to the Pauli exclusion principle, only electrons
spinning in opposite directions (indicated by ↑ and ↓) can
occupy the same orbital within a subshell.
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FILLING ORDER FOR THE FIRST 10 ELECTRONS
• When it is remembered that each orbital of a subshell can
hold a maximum of two electrons, and that Hund's rule and
the Pauli exclusion principle are followed, the following
filling order for the first 10 electrons in atoms results.
H
He
Li
Be
B
C
N
Ne
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FILLING ORDER FOR ALL SUBSHELLS IN ATOMS
• The filling order for any number
of electrons is obtained by
following the arrows in
the diagram.
• Shells are represented
by large rectangles.
• Subshells are represented
by small colored rectangles.
• Orbitals within the subshells
are represented by circles.
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AID TO REMEMBER SUBSHELL FILLING ORDER
• The diagram provides a compact way to remember the
subshell filling order.
• The correct order is given by
following the arrows from top to
bottom of the diagram, going
from the arrow tail to the head,
and then from the next tail to
the head, etc.
• The maximum number of
electrons each subshell can
hold must also be remembered:
s subshells can hold 2,
p subshells can hold 6,
d subshells can hold 10,
and f subshells can hold 14.
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SUBSHELL FILLING ORDER AND THE PERIODIC
TABLE
• Notice the order of subshell filling matches the order of the
subshell blocks on the periodic table, if the fill occurs in the
order of increasing atomic numbers.
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EXAMPLES OF ELECTRON CONFIGURATIONS FOR
ATOMS OF VARIOUS ELEMENTS
• The following electronic configurations result from the
correct use of any of the diagrams given earlier.
• Magnesium, Mg, 12 electrons: 1s22s22p63s2
• Silicon, Si, 14 electrons: 1s22s22p63s23p2
• Iron, Fe, 26 electrons: 1s22s22p63s23p64s23d6
• Galium, Ga, 31 electrons: 1s22s22p63s23p64s23d104p1
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NOBLE GAS CONFIGURATIONS
• With the exception of helium, all noble gases (group VIIIA)
have electronic configurations that end with completely
filled s and p subshells of the highest occupied shell.
These configurations are called noble gas configurations.
• Noble gas configurations can be used to write electronic
configurations in an abbreviated form in which the noble
gas symbol enclosed in brackets is used to represent all
electrons found in the noble gas configuration.
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EXAMPLES OF THE USE OF NOBLE GAS
CONFIGURATIONS
• Magnesium: [Ne]3s2. The symbol [Ne] represents the
electronic configuration of neon, 1s22s22p6.
• Iron: [Ar]4s23d6. The symbol [Ar] represents the electronic
configuration of argon, 1s22s22p63s23p6.
• Galium: [Ar]4s23d104p1. The symbol [Ar] represents the
electronic configuration of argon, 1s22s22p63s23p6.
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PERIODIC TABLE CLASSIFICATIONS OF THE
ELEMENTS
• The periodic table can be used to classify elements in
numerous ways:
• by Distinguishing Electron.
• by status as Representative, Transition, or Inner-Transition
Element.
• by status as Metal, Nonmetal, or Metalloid.
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CLASSIFICATION ACCORDING TO DISTINGUISHING
ELECTRONS
• The distinguishing electron is the last electron listed in the
electronic configuration of the element.
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REPRESENTATIVE, TRANSITION AND INNERTRANSITION ELEMENTS
• Elements are, again, classified according to the type of
distinguishing electron they contain.
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METALS, METALLOIDS AND NONMETALS
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PROPERTY TRENDS WITHIN THE PERIODIC TABLE
• Properties of elements change in a systematic way within
the periodic table.
The Elements of Group VA(15)
arsenic antimony
nitrogen phosphorous
bismuth
METALLIC AND NONMETALLIC PROPERTIES
• Most metals have the following properties: high thermal
conductivity, high electrical conductivity, ductility,
malleability and metallic luster.
• Most nonmetals have properties opposite those of metals
and generally occur as brittle, powdery solids or as gases.
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• Metalloids are elements that form a diagonal separation
zone between metals and nonmetals in the periodic table.
Metalloids have properties between those of metals and
nonmetals, and often exhibit some characteristic properties
of each type.
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TRENDS IN METALLIC PROPERTIES
• Elements in the same period of the periodic table become
less metallic and more nonmetallic from left to right across
the period.
• Elements in the same group of the periodic table become
more metallic and less nonmetallic from top to bottom
down the group.
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TRENDS IN THE SIZE OF ATOMS
• For representative elements in the same period, atomic
size decreases from left to right in the period.
• For representative elements in the same group, atomic
size increases from top to bottom down the group.
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SCALE DRAWINGS OF REPRESENTATIVE ELEMENT
ATOMS
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TRENDS IN FIRST IONIZATION ENERGY
• The first ionization energy is the energy required to remove
one electron from a neutral gaseous atom of an element.
• For representative elements in the same period, the
general trend is an increase from left to right across the
period.
• For representative elements in the same group, the
general trend is a decrease from top to bottom down the
group.
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TRENDS IN CHEMICAL REACTIVITY
• Based on the photo, what is the trend for chemical
reactivity with ethyl alcohol in group 1A(1)?
lithium
sodium
potassium
• As the atomic number increases in group 1A(1), the
chemical reaction becomes more vigorous. The rate of
gas formation and the size of the bubbles indicate that
reactivity increases from top to bottom in this family.
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