Organizing the Elements
1. Dmitri Mendeleev organized the elements by
increasing atomic mass.
2. Mendeleev left spaces in his periodic table and he
was able to predict new elements based on chemical
properties.
Is the periodic table still organized by increasing atomic
mass?
Organizing the Elements
1. Periodic table - elements are
arranged by increasing atomic number
and by changes in physical and
chemical properties.
2. Henry G.J. Moseley led to the
arrangement of elements based on
their increasing atomic numbers.
Parts of the Periodic Table
1. Groups (or families) - vertical
columns in the periodic table numbered
1-18.
– Elements in each group have similar
properties.
2. Periods - horizontal rows of elements
on the periodic table.
– The elements increase by one proton and
one electron as you go from left to right in
a period.
Electron Cloud Structure
Electrons within the electron cloud
have different amounts of energy.
– The closer the energy levels are to the
nucleus the less energy that they have.
Energy Levels: # of Electrons
Energy level 1  2 electrons
Energy level 2  8 electrons
Energy level 3  18 electrons
Energy level 4  32 electrons
When an energy level is full of electrons, the next
energy level begins to fill up with electrons until it has
reached its limit.
Valence Electrons
1. Elements in the same group have the same number of
valence electrons (electrons in the outer energy level).
2. Valence electrons determine chemical properties.
3. Maximum number of valence electrons is in the noble
gases (group 18)… they have 8 and are very stable.
1
2
3
4
5
6
7
8
Draw the electron cloud structure
(showing energy levels) for:
1. Boron
2. Oxygen
3. Lithium
4. Chlorine
5. Nitrogen
6. Neon
7. Magnesium
8. Silicon
P=5
N=6
Boron:
Atomic # = 5
Atomic mass = 11
Boron
P=8
N=
Oxygen
Boron
Oxygen
Boron
Oxygen
Boron
4. Chlorine
5. Nitrogen
6. Neon
7. Magnesium
8. Silicon
Boron
4. Chlorine
5. Nitrogen
6. Neon
7. Magnesium
8. Silicon
Boron
2. Oxygen
3. Lithium
4. Chlorine
5. Nitrogen
6. Neon
7. Magnesium
8. Silicon
Rows on the Periodic Table
An element’s period number (row
number) indicates the number of
occupied energy levels.
Example: All elements in period 3 have
three occupied energy levels.
1
2
3
Period or Row 1 on the Periodic
Table: How many energy levels?
Period or Row 2 on the Periodic
Table: How many energy levels?
Last Vertical Row on the Periodic
Table: Stable
The last element in each period ends
with a full outer energy level and is
therefore VERY stable!
Valence Electrons
Group 1  1 valence electron
Group 2  2 valence electrons
Group 13  3 valence electrons
Group 14  4 valence electrons
Group 15  5 valence electrons
Group 16  6 valence electrons
Group 17  7 valence electrons
Group 18  8 valence electrons (except He)
• Skip groups 3-12 (transition elements)
because they can vary.
Electron (Lewis) Dot
Diagrams
Outer electrons are so important in determining
the chemical properties of an element… this is
a special way to represent them.
It uses:
1. the chemical symbol
2. dots to represent the valence electrons
(electrons in outer energy level)
Examples:
1. Fluorine
2. Carbon
3. Sulfur
4. Magnesium
Same Group = Similar Properties
Example 1: Group 1:Alkali metals
shiny, malleable (can be hammered
into thin sheets) and ductile (can be pulled
into wires)
Example 2: All group 17 elements have
seven electrons in their outer energy
levels
Same Group = Similar Properties
Example 3: Group 18 elements already
have a full stable outer energy level of 8
valence electrons; that is why they do not
react readily with other elements.
Review Questions
1. How are the elements arranged in the
periodic table?
Review Questions
1. How are the elements arranged in the
periodic table?
Increasing atomic number and by changes
in physical and chemical properties.
Review Questions
2. What do the elements in a vertical
column of the periodic table have in
common?
Review Questions
2. What do the elements in a vertical
column of the periodic table have in
common?
elements in the same group have similar
properties because they have the same
number of valence electrons
Review Questions
3. What do the dots in this electron dot
diagram represent?
Review Questions
3. What do the dots in this electron dot
diagram represent?
valence electrons
Review Questions
4. What is the maximum number of
electrons that can fill the first and second
energy levels?
Review Questions
4. What is the maximum number of
electrons that can fill the first and second
energy levels?
first energy level  2
second energy level  8
Review Questions
5. The horizontal rows of elements are
called (groups, periods).
Review Questions
5. The horizontal rows of elements are
called periods.