Structure of the Periodic Table
Aim PT2 – what are some of
the characteristics of
elements we study using the
Periodic Table?
Groups of the Periodic Table
• Elements in the same group have
similar chemical and physical properties
• They tend to react the same way in
chemical reactions
• Examples
– Group 1 elements all react violently in water
– Group 17 elements all occur in nature only
combined with other elements
– Group 18 elements are all unreactive gases
• Mendeleev did that on purpose.
Groups of the Periodic Table
• Mendeleev put elements in the same
group that had the same number of
valence electrons
– These valence electrons are lost
or gained during chemical reactions
– Elements in Period 1 and the first half of
Period 2 gain or lose electrons in order to
become like HELIUM with 2 electrons in
the valence shell (Duet Rule)
– Everyone else gains or loses electrons to
become like Group 18 Noble Gases and
have 8 electrons in the valence shell
(Octet Rule)
Periods in the
Periodic Table
• Rows in the Periodic
Table are called
PERIODS
– Each period has elements in it with the
same number of energy levels or shells
– The characteristics of the elements change
as you move from left to right across each
period, forming trends
– This trend repeats itself in the next row
– This is called PERIODICITY
Periodicity = Trends
• There are many properties we look at in chemistry,
each having specific patterns or trends in the PT
• Most of the values for these properties are given,
along with other elemental properties, in
Table S of your Chem Reference Tables
Atomic Radius
Radius of
bonded atoms
• The distance from the
nucleus to the edge of
the electron cloud
• Measured as ½ the
internuclear distance between two bonded atoms
• Table S gives atomic radii values in picometers
(pm), or trillionths of a meter
• Trends:
– As you move from top to bottom in a group the
atomic radius increases
– as you move from left to right across a period, the
atomic radius decreases
Group Atomic Radius
Trends
• As you move down a
group
• You increase the
number of shells or
energy levels
• Making the atoms
bigger in size
• Example – Period 1 of
the table
Li
Na
K
Rb
Li - Two
energy
levels
Na - Three
energy
levels
K - Four
Energy
levels
Rb – Five
Energy
levels
Period Atomic Radius Trends
• As you move across a period
• You don’t increase the number of shells, but the
number of electrons and protons increase
• Therefore the attractive forces between them
increases, pulling the electrons closer to the
nucleus and decreasing the atomic radius
Li
Be
B
N
F
• Ionic Radius
– The radius of the ion formed from
an atom’s gain or loss of electrons
– Based on whether an atom
becomes a cation or anion
• Cations - positive ions
– Lose their valence electrons,
losing a whole shell
– Become smaller than their atoms
• Anions - negative ions
– Gain valence electrons, whose
negative charges push against
each other, making them larger
than their atoms
Na
Atom
Cation
Na +
Cl
Atom
Anion
Cl-
• First Ionization Energy
– Amount of energy needed to remove the first
valence electron from a given element
– Measured in kilojoules / mole
– A mole – represents 6.02 x 1023 particles in a
sample
– More later
– Example of first ionization energy from Table S:
K + First ionization energy  K+ + eK + 418 kilojoules/mole
 K + + e-
• Electronegativity
– A relative measure of an atom’s ability to
attract electrons
– Measured on a scale of 0 – 4 Pauling Units
(named after chemist Linus Pauling)
– Values given on Table S
– HIGH electronegativity
• higher attraction for electrons
• usually nonmetals – flourine has the highest
– LOW electronegativity
• lower attraction for electrons
• usually metals – francium has the lowest
How do I quickly find the trends in the Periodic
Table?
• REMEMBER THIS
– With three elements only
– All trends in the periodic table
can be determined
– Lithium vs Francium – the trends in groups can
be determined
– Lithium vs Flourine – the trends in periods can
be determined
– FLOURINE – the most active nonmetal
– FRANCIUM – the most active metal