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Chapter 2 - Cont.
30 January 2014
The Structure of the Atom
and the Periodic Table
Element Information in the
Periodic Table
20
Ca
calcium
40.08
atomic number
symbol
name
atomic mass
1
Electron Arrangement
and the Periodic Table
• The electron arrangement is the primary
factor in understanding how atoms join
together to form compounds
• Electron configuration - describes the
arrangement of electrons in atoms
• Valence electrons - outermost electrons
– The electrons involved in chemical bonding
Valence Electrons
• The number of valence electrons is the
group number for the representative
elements
• The period number gives the energy level
(n) of the valence shell for all elements
2
Valence Electrons and Energy
Level
• How many valence electrons does fluorine
have?
– 7 valence electrons
• What is the energy level of these electrons?
– Energy level is n = 2
Valence Electrons - Detail
• What is the total number of electrons in
fluorine?
– Atomic number = 9
– 9 protons and 9 electrons
• 7 electrons in the valence shell, (n = 2 energy
level), so where are the other two electrons?
– In n = 1 energy level
– Level n=1 holds only two electrons
3
Determining Electron Arrangement
List the total number of electrons, total number of valence
electrons, and energy level of the valence electrons for
silicon.
1. Find silicon in the periodic table
•
•
•
Group IVA
Period 3
Atomic number = 14
2. Atomic number = number of electrons in an atom
•
Silicon has 14 electrons
Determining Electron Arrangement #2
List the total number of electrons, total number of valence electrons,
and energy level of the valence electrons for silicon.
3. As silicon is in Group IV, only 4 of its 14
electrons are valence electrons
•
Group IVA = number of valence electrons
4. Energy levels:
•
•
•
n = 1 holds 2 electrons
n = 2 holds 8 electrons (total of 10)
n = 3 holds remaining 4 electrons (total = 14)
4
Determining Electron Arrangement
Practice
List the total number of electrons, total
number of valence electrons, and energy
level of the valence electrons for:
•
•
Na
Ar
Determining Electron Arrangement
Practice
List the total number of electrons, total
number of valence electrons, and energy
level of the valence electrons for:
•
•
Na total: 11
Ar
5
Determining Electron Arrangement
Practice
List the total number of electrons, total
number of valence electrons, and energy
level of the valence electrons for:
•
•
Na total: 11, valence 1
Ar
Determining Electron Arrangement
Practice
List the total number of electrons, total
number of valence electrons, and energy
level of the valence electrons for:
•
•
Na total: 11, valence 1, energy level
n=3
Ar
6
Determining Electron Arrangement
Practice
List the total number of electrons, total
number of valence electrons, and energy
level of the valence electrons for:
•
•
Na total: 11, valence 1, energy level
n=3
Ar total 18,
Determining Electron Arrangement
Practice
List the total number of electrons, total
number of valence electrons, and energy
level of the valence electrons for:
•
•
Na total: 11, valence 1, energy level
n=3
Ar total 18, valence 8,
7
Determining Electron Arrangement
Practice
List the total number of electrons, total
number of valence electrons, and energy
level of the valence electrons for:
•
•
Na total: 11, valence 1, energy level
n=3
Ar total 18, valence 8, energy level n=3
The Quantum Mechanical Atom
• Bohr’s model of the hydrogen atom didn’t
clearly explain the electron structure of other
atoms
– Electrons in very specific locations, principal energy
levels
– Wave properties of electrons conflict with specific
location
• Schröedinger developed equations that took into
account the particle nature and the wave nature
of the electrons
8
Schröedinger’s equations
• Equations that determine the probability of
finding an electron in specific region in space,
quantum mechanics
– Principal energy levels (n = 1,2,3…)
– Each energy level has one or more sublevels or
subshells (s, p, d, f)
– Each sublevel contains one or more atomic
orbitals
Energy Levels and Sublevels
PRINCIPAL ENERGY LEVELS
• n = 1, 2, 3, …
• The larger the value of n, the higher the energy level
and the farther away from the nucleus the electrons
are
• The number of sublevels in a principal energy level
is equal to n
– in n = 1, there is one sublevel
– in n = 2, there are two sublevels
9
Principal Energy Levels
• The electron capacity of a principal energy
level (or total electrons it can hold) is
2(n)2
– n = 1 can hold 2(1)2 = 2 electrons
– n = 2 can hold 2(2)2 = 8 electrons
• How many electrons can be in the n = 3 level?
– 2(3)2 = 18
• Compare the formula with periodic table…..
n=1, 2(1)2=2
n=2, 2(2)2=8
n=3, 2(3)2=18
n=4, 2(4)2=32
10
Sublevels
• Sublevel: a set of energy-equal orbitals within
a principal energy level
• Subshells increase in energy:
s<p<d<f
[sharp, principal, diffuse, and fundamental]
• Electrons in 3d subshell have more energy than
electrons in the 3p subshell
• Specify both the principal energy level and a subshell
when describing the location of an electron
Sublevels in Each Energy Level
Principal energy
level (n)
Possible
subshells
1
1s
2
2s, 2p
3
3s, 3p, 3d
4
4s, 4p, 4d, 4f
11
Orbitals
• Orbital - a specific region of a sublevel
containing a maximum of two electrons
• Orbitals are named by their sublevel and
principal energy level
– 1s, 2s, 3s, 2p, etc.
• Each type of orbital has a characteristic shape
– s is spherically symmetrical
– p has a shape much like a dumbbell
Orbital Shapes
• s is spherically
symmetrical
12
Orbital Shapes
• Each p has a shape much like a dumbbell,
differing in the direction extending into space
Orbital Shapes
• There are five different d shapes.
• The f orbitals have seven different shapes, too
complicated and therefore seldom shown.
13
Electron Spin
• Electron Configuration - the
arrangement of electrons in atomic orbitals
• Aufbau Principle - or building up
principle helps determine the electron
configuration
– Electrons fill the lowest-energy orbital that is
available first
– Remember s<p<d<f in energy
– When the orbital contains two electrons, the
electrons are said to be paired
Subshell
Number of
orbitals
s
1
p
3
d
5
f
7
• How many electrons can be in the
4d subshell?
•10
14
Rules for Writing Electron
Configurations
• Obtain the total number of electrons in the atom from
the atomic number
• Every electron has a place to stay
• Electrons in atoms occupy the lowest energy orbitals
that are available – 1s first
• Each principal energy level, n contains only n
sublevels
• Each sublevel is composed of orbitals
• No more than 2 electrons in any orbital
• Maximum number of electrons in any principal
energy level is 2(n)2
Rules for Writing Electron
Configurations
15
Rules for Writing Electron
Configurations
• Remember:
– The s sublevel has one orbital and can hold two
electrons.
– The p sublevel has three orbitals. The electrons will
half-fill before completely filling the orbitals for a
maximum of six electrons.
– The d sublevel has five orbitals. The electrons will
half-fill before completely filling the orbitals for a
maximum of ten electrons.
Electron Distribution
• This table lists the number of electrons in each shell for
the first 20 elements
• Note that 3rd shell stops filling at 8 electrons even though if could
hold more
16
Electron Distribution
Writing Electron Configurations
• H
– Hydrogen has
only 1 electron
– It is in the
lowest energy
level & lowest
orbital
– Indicate
number of
electrons with a
superscript
– 1s1
• Li
– Lithium has 3
electrons
– First two have
configuration
of helium – 1s2
– 3rd is in the
orbital of
lowest energy
in n=2
– 1s2 2s1
17
Shorthand Electron
Configurations
• Uses noble gas symbols to represent the
inner shell and the outer shell or valance
shell is written after
• Aluminum- full electron configuration is:
1s22s22p63s23p1
What noble gas configuration is this?
•Neon
•Configuration is written: [Ne]3s23p1
• Remember:
– How many subshells are in each principal energy
level?
– There are n subshells in the n principal energy level.
– How many orbitals are in each subshell?
– s has 1, p has 3, d has 5, and f has 7
– How many electrons fit in each orbital?
– 2
– Hence: s can have 2, p 6, d 10 and f 14 electrons
18
How many orbitals in 3rd principal energy level?
It contains s, p, and d
s=1
p=3
d=5
Hence, the answer is 9
(and it can hold 18 electrons)
• Remember:
– How many electrons is subshells?
– There are n subshells in the n principal energy level.
s up to 2, p up to 6, d up to 10, f up to 14 electrons
Therefore:
for n=1: s subshell - up to 2 electrons
for n=2: s and p subshells - up to 8 electrons
for n=3: s, p, and d subshells - up to 18 electrons
for n=4: s, p, d and f subshell, up to 32 electrons
19
Write the electron
configuration for phosphorus
Phosphorus has 15 electrons
start with the sequence 1s2 2s2
1s2 2s2 2p6 3s2 3p3
Write the electron
configuration for strontium
Strontium has 38 electrons
start with the sequence 1s2 2s2
simplified notation: [Kr] 5s2
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2
20
Classification of Elements
According to the Type of
Subshells Being Filled
The Octet Rule
[eight in Latin of Greek - octo, οκτώ …ocho]
• The noble gases are extremely stable
– Called inert as they don’t readily bond to other
elements
• The stability is due to a full complement of
valence electrons in the outermost s and p
sublevels:
– 2 electrons in the 1s of helium
– the s and p subshells full in the outermost shell of
the other noble gases (eight electrons)
21
Octet of Electrons
• Elements in families other than the noble
gases are more reactive
– Strive to achieve a more stable electron
configuration
– Change the number of electrons in the atom to
result in full s and p sublevels
• Stable electron configuration is called the
“noble gas” configuration
The Octet Rule
• Octet Rule - elements usually react in such a way
as to attain the electron configuration of the noble
gas closest to them in the periodic table
– Elements on the right side of the table move right to the
next noble gas
– Elements on the left side move “backwards” to the
noble gas of the previous row
• Atoms will gain, lose or share electrons in
chemical reactions to attain this more stable
energy state
22
Ion Formation and the Octet Rule
• Metallic elements tend to form positively
charged ions called cations
• Metals tend to lose all their valence
electrons to obtain a configuration of the
noble gas
Na
Na+ + e-
Sodium atom
11e-, 1 valence e[Ne]3s1
Sodium ion
10e[Ne]
23