Unit 3
Periodic Table
and Valence electrons
Modern Periodic Table

Elements are arranged by increasing
atomic number.
– Recall that atomic number gives the number
of protons in the nucleus of an atom.
Periods

Horizontal rows on the periodic table are
called periods.

Elements in the same period have the
same number of energy levels.

Elements in the same period share NO
similar chemical properties.
Groups / Families

Vertical columns on the periodic Table are called
groups.

Groups are numbered 1-18
Elements in the same group have the same
chemical properties because they have the
same number of valence electrons.
 Valence Electrons – electrons in the highest
energy level s and p orbitals only.

– They participate in bonding and ion formation

Some groups have special names.
Groups/Families
Group 1 Metals
Known as Alkali Metals
 Most active metals on periodic table
 React violently with water, and become
more reactive down the group
 Only have 1 valence electron (because
they all end in s1)
 They all form ions with a +1 charge.

Group 2 Metals
Known as Alkaline Earth Metals
 Also extremely reactive, but not as much as
Alkali Metals
 Contain 2 valence electrons (they end in s2)
 They all form ions with a +2 charge.

Group 1 are the alkali metals
(but NOT H)
Group 2 are the alkaline earth metals
H
Groups 3-12
These groups are known as Transition
Metals
 Not very reactive metals
 Generally have 2 valence electrons
 Some of their “d” orbital electrons jump
around into “p” orbitals so many of these
elements have more than one possible
positive charge.
 Many form colorful ion solutions (bright
vibrant colors)

The group B are called the
transition elements
 These
are called the inner
transition elements, and they
belong here
Group 17






Known as Halogens
Extremely reactive nonmetals; reactivity
increases up a group (Fluorine the most active)
Have 7 valence electrons (e- configuration ends
in s2p5)
Most of these elements are toxic in their natural
state
Most are also diatomic in their natural state
(F2, Br2, Cl2, I2)
They all form ions with a (-1) charge.
Group 18
Known as Noble gases
 Unreactive nonmetals
 All have 8 valence electrons
 (e- configuration ends in s2p6), except
Helium (which has 2 electrons)
 They do not form ions because they have
a filled energy level!

Group 18 are the noble gases
 Group 17 is called the halogens

Valence electrons

Valence electrons are electrons located in
the outermost energy level.

They are located only in the s and p
sublevels

These electrons are involved in chemical
bonding.
Electron Configurations

An element’s electron configuration shows
how many valence electrons it has.
Example:
1s22s22p63s23p64s23d104p4

Valence electrons = 6
Practice
How many valence electrons do the
following elements have?
 1s22s22p63s23p64s23d104p6 8 valence electrons


1s22s22p63s23p2

1s22s2
4 Valence Electrons
2 valence electrons
Octet Rule

Most elements want to have 8 valence
electrons (exception He)
Elements will gain or lose electrons in their
valence shell in order to achieve an octet.
– This process forms Ions.
 Ions have the same electron configuration
as their nearest noble gas.

Positive Ions
Elements with less than 4 valence electrons
lose their outermost “s” and “p” sublevel
electrons .
 Loss of electrons always results in positive
ions.

Example
Be: 1s22s2 has 2 valence electrons
Be2+: 1s2 notice the ion has the same
configuration as He


Metals tend to form positive ions
Negative Ions
Elements with more than 4 valence electrons
gain electrons in their outermost “p”
sublevel
 Gain of electrons always results in negative
ions

Example
O: 1s22s22p4
O2-: 1s22s22p6

has 6 valence electrons
now has 8 valence
electrons just like Ne
Non-Metals usually tend to form negative
ions
Ion Formation Summary

Atoms gain or lose electrons to become
more stable.
– They achieve the same electron configuration
as the Noble Gases.
1
2
3
4
5
6
7
Noble Gas Shorthand Electron
Configurations

Longhand Configuration
S 16e- 1s2 2s2 2p6 3s2 3p4
Core Electrons

Valence Electrons
Shorthand Configuration
S
16e
2
4
[Ne] 3s 3p
Noble Gas Shorthand Configuration

Shorthand Configuration
– Core e-: Go up one row and over to the
Noble Gas.
– Valence e-: On the next row, fill in the # of
e- in each sublevel.
Noble Gas Shorthand Notation

Example - Germanium
1
2
3
4
5
6
7
[Ar]
2
4s
10
3d
2
4p