Electrons and Reactivity
Atoms contain
 a very small nucleus packed with neutrons and positively
charged protons.
 a large volume of space around the nucleus that contains the
negatively charged electrons.
It is the electrons that determine the physical and
chemical properties of atoms.
Electron Energy Levels
 Electrons surround the nucleus in specific energy levels.
 Each energy level has a principal quantum number (n).
 The lowest energy level, which is closest to the nucleus, is
labeled n = 1.
 The second-lowest energy level is labeled n = 2, the third n =
3, and so on.
Electron Energy Levels
Electron energy levels increase
in energy and number as
electrons get farther away from
the nucleus.
The higher the electron energy
levels,
 the more electrons they hold.
 the more energy the electrons
have.
Sublevels
Within each energy level, we have sublevels that
 contain electrons with identical energy.
 are identified by the letters s, p, d, and f.
The number of sublevels within a given energy level
is equal to the value of the principal quantum number, n.
Energy Levels and Sublevels
Energy of Sublevels
Within any energy level,
 the s sublevel has the lowest energy.
 the p sublevel follows and is slightly higher in energy.
 the d sublevel follows the p and is slightly higher in energy
than the p.
 the f sublevel follows the d and is slightly higher in energy
than the d.
Orbitals
Each electron sublevel consists of orbitals, which
 are regions where there is the highest probability of
finding an electron.
 have their own unique three-dimensional shape.
 can hold up to 2 electrons.
s Orbitals
We know that s orbitals have a spherical shape,
centered around the atom’s nucleus.
 The s orbitals get bigger as the principal quantum
number, n, gets bigger.
 The s orbitals can hold up to 2 electrons that must spin in
opposite directions.
p Orbitals
There are three p orbitals in each energy level,
starting with energy level 2. They
 have a two-lobed
shape, much like tying
a balloon in the middle,
and can hold 2 electrons.
 are labeled x, y, and z.
 increase in size as the
value of n increases.
Sublevels and Orbitals
Each sublevel consists of a specific number of
orbitals.
 An s sublevel contains one s orbital.
 A p sublevel contains three p orbitals.
 A d sublevel contains five d orbitals.
 An f sublevel contains seven f orbitals.
Electron Capacity in Sublevels
Order of Filling
Energy levels are filled with electrons
 in order of increasing energy.
 beginning with quantum number n = 1.
 beginning with s followed by p, d, and f in each energy level.
Energy Diagram for Sublevels
Orbital Diagrams
An orbital diagram shows
 orbitals as boxes in each sublevel.
 electrons in orbitals as vertical arrows.
 electrons in the same orbital with opposite spins (up and
down vertical arrows).
Example:
Orbital diagram for Li
1s2
2s1
filled half-filled
2p
empty
Order of Filling
Electrons in an atom
 fill the lowest energy level and orbitals first,
 fill orbitals in a particular sublevel with one electron each until
all orbitals are half full, and then
 fill each orbital using electrons with opposite spins.
Writing Orbital Diagrams
The orbital diagram for
carbon has 6 electrons:
 2 electrons are used to fill
the 1s orbital.
 2 more electrons are used
to fill the 2s orbital.
 1 electron is used in two of
the 2p orbitals so they are
half-filled, leaving one p
orbital empty.
Electron
arrangements in
orbitals in
energy levels 1
and 2.
Learning Check
Write the orbital diagrams for each of the following:
1. nitrogen
2. oxygen
3. magnesium
Solution
Write the orbital diagrams for each of the following:
1. nitrogen
1s
2s
2p
1s
2s
2p
1s
2s
2p
2. oxygen
3. magnesium
3s
Electron Configuration
An electron configuration
 lists the filled and partially filled energy levels in order of
increasing energy.
 lists the sublevels filling with electrons in order of increasing
energy.
 uses superscripts to show the number of electrons in each
sublevel.
 for neon is as follows: number of electrons = 10
1s22s22p6
Period 1 Configurations
In Period 1, the first two electrons enter the 1s orbital.
Period 2 Configurations
In Period 2,
 lithium has 3 electrons –2 in the 1s and 1 in the 2s.
 beryllium has 4 electrons –2 in the 1s and 2 in the 2s.
 boron has 5 electrons –2 in the 1s, 2 in the 2s, and
1 in the 2p.
 carbon has 6 electrons –2 in the 1s, 2 in the 2s, and
2 in the 2p.
Abbreviated Configurations
In an abbreviated configuration,
 the symbol of the noble gas is in brackets, representing
completed sublevels.
 the remaining electrons are listed in order of their sublevels.
Example: Chlorine has the following configuration:
1s22s22p63s23p5
[Ne]
The abbreviated configuration for chlorine is
[Ne]3s23p5.
Period 2 Configurations
Period 3 Configurations
Learning Check
1. The correct electron configuration for nitrogen is
A. 1s22p5
B. 1s22s22p6
C. 1s22s22p3
2. The correct electron configuration for oxygen is
A. 1s22p6
B. 1s22s22p4
C. 1s22s22p6
3. The correct electron configuration for calcium is
A. 1s22s22p63s23p63d2
B. 1s22s22p63s23p64s2
C. 1s22s22p63s23p8
Learning Check
Write the electron configuration and abbreviated
configuration for each of the following elements:
1. Cl
2. S
3. K
Electron Configurations and the
Periodic Table
The periodic table consists of sublevel blocks
arranged in order of increasing energy.
 Groups 1A and 2A
= s block
 Groups 3A to 8A
= p block
 Transition Elements
(This sublevel is (n-1), 1 less
than the period number.) = d block
 Lanthanides/Actinides
(This sublevel is (n-2), 2 less
than the period number.) = f block
Sublevel Blocks
Guide to Using Sublevel Blocks
Writing Electron Configurations
Using the periodic table, write the electron configuration
for silicon.
Solution:
Period 1
Period 2
Period 3
1s block
2s → 2p blocks
3s → 3p blocks
1s2
2s2 2p6
3s23p2 (at Si)
Writing all the sublevel blocks in order gives the
following:
1s22s22p63s23p2
Electron Configurations of the d Level
The 4s orbital has a lower energy than the 3d orbitals.
Writing Electron Configurations
Using the periodic table, write the electron configuration
for manganese.
Solution:
Period 1
1s block
1s2
Period 2
2s → 2p block
2s2 2p6
Period 3
3s → 3p block
3s2 3p6
Period 4
4s → 3d block 4s2 3d5 (at Mn)
Writing all the sublevel blocks in order gives the
following:
1s22s22p63s23p64s23d5
Valence Electrons
The valence electrons
 determine the chemical properties of the elements.
 are the electrons in the outermost, highest energy level.
 are related to the group number of the element.
Example: Phosphorus has 5 valence electrons.
5 valence
electrons
P Group 5A(15)
1s22s22p63s23p3
Groups and Valence Electrons
All the elements in a group have the same number of
valence electrons.
Example: Elements in Group 2A (2) have two (2)
valence electrons.
Be 1s22s2
Mg 1s22s22p63s2
Ca [Ar]4s2
Sr [Kr]5s2
Periodic Table and
Valence Electrons
Learning Check
Identify the number of valence electrons for each of the
following:
1. O
A. 4
B. 6
C. 8
2. Al
A. 13
B. 3
C. 1
3. Cl
A. 2
B. 5
C. 7
Learning Check
Identify the number of valence electrons for each of the
following:
1. Calcium
A. 1
B. 2
C. 3
2. Group 6A (16)
A. 2
B. 4
C. 6
B. 4
C. 14
3. Tin
A. 2
Learning Check
Identify the number of valence electrons for each of the
following:
1. 1s22s22p63s23p1
2. 1s22s22p63s2
3. 1s22s22p5
Electron-Dot Symbols
An electron-dot symbol
 indicates valence electrons
as dots around the symbol of the
element.
 of Mg shows two valence electrons
as single dots on the sides of the
symbol Mg.
Mg
Mg
Mg
Mg
Mg
Writing Electron-Dot Symbols
The electron-dot symbols for
 Groups 1A (1) to 4A (14) use single dots:
Na
Mg
Al
C
 Groups 5A (15) to 7A (17) use pairs and single dots:
P
O
Cl
Groups and Electron-Dot Symbols
In a group, all the electron-dot symbols have the same
number of valence electrons (dots).
Example: Atoms of elements in Group 2A (2) each have
2 valence electrons.
Group
2A (2)
· Be ·
· Mg ·
· Ca ·
· Sr ·
· Ba ·
Learning Check

1. X is the electron-dot symbol for
A. Na
B. K
C. Al

2.

X

is the electron-dot symbol for
A. B
B. N
C. P
Atomic Size
Atomic size
 is described using the atomic radius.
 is the distance from the nucleus to the valence electrons.
 increases going down a group.
 decreases going across a period from left to right.
Atomic Radius
Learning Check
Select the element in each pair with the larger atomic
radius.
1. Li or K
2. K or Br
3. P or Cl
Ionization Energy
Ionization energy
 is the energy it takes to remove a valence electron from an
atom in the gaseous state.
Na(g) + Energy (ionization)
Na+(g) + e–
 decreases down a group, increasing across the periodic table
from left to right.
Ionization Energy and Valence Electrons
Ionization Energy
The ionization
energies of
 metals are low.
 nonmetals are high.
Metallic Character
The metallic character increases when an element can
lose its valence electrons more easily, it
 increases down a group where electrons are easier to remove.
 decreases across the period because electrons are harder to
remove.
Metallic Character
Periodic Table Trend Summary
Learning Check
Select the element in each pair with the higher
ionization energy.
1. Li or K
2. K or Br
3. P or Cl
Next Week
• Clicker Quiz
– Finish Chapter 3
• Lecture on Chapter 4
• Lab #3