Electrons and Periodicity
Wow, that periodic table is useful.
Arrangement of the Modern Periodic
Table
 In the modern periodic table, the elements are organized into
groups (vertical columns) and periods (horizontal rows) in
order of increasing atomic number.
 Groups are also called FAMILIES since they are similar in
chemical properties.
Numbering Groups
 Groups are numbered 1–18 or 1–8 with A/B notation
 Tall columns are notated as “A”
 Main group elements
 Short columns are notated as “B”
1A
2A
3B
4B
5B
6B
7B
8B
1B
Main Group Elements
2B 3A
4A
5A
6A
7A
8A
Metals, Nonmetals, and Metalloids
 Step-wise line that begins at the element boron and moves
downward in a zigzag pattern divides the table
 Metals are to the left of the line
 Nonmetals are to the right of the line
 Along the line are the metalloids
 Possess properties of both metals and nonmetals
Metalloids
Properties of Metals
They are malleable, ductile, and have luster.
2. They are good conductors of heat and electricity.
3. They have relatively high densities.
4. They are solids at standard temperature and pressure
(STP), except for mercury, which is a liquid.
1.
Properties of Nonmetals
 They are dull and brittle.
 They don’t conduct heat and electricity well.
 They have relatively low boiling and freezing points.
 They exist in all phases at STP, but most are gases.
Properties of Metalloids
 Metalloids have properties of both metals and nonmetals.
 They are semiconductors at temperatures higher than room
temperature. This means that they have a conductivity
between that of a metal and that of a nonmetal, and can even
change energy input and output by using electrical forces.
 They are all solids at STP.
Alkali Metals
The alkali metals are found in group 1 (1A) of the periodic table.
Valence electrons: 1
Oxidation Number: +1
 They do not occur in nature as elements (in other words, they’re
always in a compound).
 They form ionic compounds such as salts and oxides. They are
also in bases.
 They are good conductors of heat and electricity, ductile,
malleable, and soft enough to be cut with a knife.
 They have a silvery luster, low density, and low melting point.
 They are the most reactive metals.
Alkaline Earth Metals
The alkaline earth metals are found in group 2 (2A) of the
periodic table.
Valence electrons: 2
Oxidation number: +2
Alkaline earth metals have similar characteristics to alkali
metals, except that they are less reactive. They form salts,
oxides, and bases.
Transition Metals
The transition metals are found in groups 3 through 12. They
have similar characteristics to the other metals except that:
 They are usually harder and more brittle than the metals in
group 1 and 2.
 They often form colored compounds. (Think of the blue
copper sulfate we’ve worked with.)
 Oxidation number: varies
3B
2B
4B
5B
6B
7B
3
4
5
6
7
12
8B
8
9
1B
10
11
Halogens
The halogens are found in group 17 (7A) of the periodic table.
Valence electrons: 7
Oxidation number: -1
 They are all nonmetals and occur in combined form in nature.
 They exist in room temperature as gases (F2 and Cl2), a liquid
(Br2) and solids (I2 and At).
 They are the most reactive nonmetals. Fluorine is the most
reactive of all nonmetals (as anyone who has seen Breaking Bad
discovered).
Noble Gases
The noble gases are found in group 18 (8A) of the periodic table.
Valence electrons: 8
Oxidation number: 0
 They are colorless and odorless.
 They have very low boiling and freezing points.
 They rarely combine with other elements and are considered to be
inert (nonreactive chemically).
By Felix Burton (Flickr) [CC-BY-2.0]
Periodicity
 When the elements are arranged in order of increasing
atomic number, there is a periodic reoccurrence of
properties that leads to the group of elements in the periodic
table. This periodic recurrence is known as periodicity.
 The position of the element in the periodic table can also be
used to compare periodic trends in atomic radii,
electronegativity, ionization energy, and ionic radii.
Nuclear Charge and the Shielding
Effect
All the periodic trends can be understood in terms of three basic rules.
1. SHIELDING EFFECT: Electrons tend to repel each other.
2. NUCLEAR CHARGE: Electrons want to get as close to the positively-charged
nucleus as possible. (Opposites attract.)
3. OCTET RULE: Atoms want to have eight electrons in their outer shell,
resembling a noble gas.
Nuclear charge plays an important role in determining periodic trends. The
shielding effect plays an important role in determining group trends.
Atomic Radii
The atomic radius is half the distance between nuclei in two
adjacent atoms. (Radius of an atom)
Atomic radius increases moving down a group and decreases
moving to the right across a period
Increases
Decreases
Atomic Radii
 Why?
 Shielding Effect: When you go down a group, more electrons
are added, and these electrons repel each other. The atom gets
bigger.
 Nuclear charge: When you go across a period, the electrons are
more strongly attracted to the nucleus, and get smaller.
Ionization Energy
Electrons are attracted to the nucleus of an atom, so it takes energy to remove an
electron. The energy required to remove an electron from an atom is
called the first ionization energy.
Ionization energy decreases moving down a group and increases moving to
the right across a period.
In other words, the bigger the atom, the lower the ionization energy.
Decreases
Increases
1st Ionization Energy
 Why?
 Shielding effect: As you go down a group, the electrons
increase and repel each other. The electrons are further away
and easier to pluck out of an atom. This results in low
ionization energy.
 Nuclear charge: As you go across a period, the electrons are
closer together. It is thus harder to separate them. This results
in high ionization energy.
Ionic Radii
 The ionic radius is the radius of a cation or anion. When the
atom loses or gains electrons, the resulting ion
changes in size from the original atom.
 Metals tend to lose electrons and form cations (positive
ions).
 Nonmetals tend to gain electrons and form anions (negative
ions).
Notice that
since the
metals are
LOSING
electrons,
the other
electrons can
get closer to
the nucleus,
and
SHRINK.
(NUCLEAR
CHARGE!)
Increases
Atomic Radii & Ionic Radii
Since the
nonmetals are
GAINING
electrons, the
other electrons are
now being
repelled, and the
ion will grow in
size. (SHIELDING
EFFECT!)
Electronegativity
 Electronegativity refers to the tendency for an atom to
attract electrons to itself when it is chemically
combined with another element.
 Electronegativity decreases moving down a group and
increases moving to the right across a period
Decreases
Increases
Electronegativity
 Why?
 Shielding effect: As the electrons repel and spread out, they
don’t want any more electrons. Thus the electronegativity is
low. (ESPECIALLY IN METALS WHERE THEY ARE TRYING
TO LOSE ELECTRONS!)
 Nuclear charge: As the electrons get closer, more can be added
in. Thus, electronegativity is high. (ESPECIALLY IN
NONMETALS WHEN THEY ARE TRYING TO GAIN
ELECTRONS!)
Summary of Periodic Trends
Summary of Trends
Ionic Radii Increase
Cation Radii Increase
Anion Radii
Increase