Periodic Properties of
the Elements
The Periodic Table
The modern periodic table was developed in
1872 by Dmitri Mendeleev (1834-1907). A
similar table was also developed independently
by Julius Meyer (1830-1895).
The table groups elements with similar
properties (both physical and chemical) in
vertical columns. As a result, certain properties
recur periodically.
The Periodic Table
Mendeleev left empty spaces in his table for
elements that hadn’t yet been discovered. Based
on the principle of recurring properties, he was
able to predict the density, atomic mass, melting
or boiling points and formulas of compounds
for several “missing” elements.
The Periodic Table
The Periodic Table
metal/non-metal
line
The Periodic Table
The periodic table is based on observations
of chemical and physical behavior of the
elements. It was developed before the discovery
of subatomic particles or knowledge of the
structure of atoms.
The basis of the periodic table can be
explained by quantum theory and the electronic
structure of atoms.
Periodic Trends
Many of the properties of atoms show clear
trends in going across a period (from left to
right) or down a group.
In going across a period, each atom gains a
proton in the nucleus as well as a valence
electron.
Periodic Trends
The increase of positive charge in the
nucleus isn’t completely cancelled out by the
addition of the electron.
Electrons added to the valence shell don’t
shield each other very much. As a result, in
going across a period, the effective nuclear charge
(Zeff) increases.
Effective Nuclear Charge
The effective nuclear charge (Zeff) equals the
atomic number (Z) minus the shielding factor
(σ).
Zeff= Z-σ
Effective Nuclear Charge
The effective nuclear charge (Zeff) equals the
atomic number (Z) minus the shielding factor
(σ).
Zeff= Z-σ
Within the valence shell, the shielding factor
is approximately 0.35, so going across a period
results in an increase in Zeff of roughly .65 for
each element.
Effective Nuclear Charge
Zeff= Z-σ
Effective Nuclear Charge
Electrons in the
valence shell are
partially shielded
from the nucleus by
core electrons.
Effective Nuclear Charge
Electrons in p or d
orbitals don’t get too
close to the nucleus, so
they are less shielding
than electrons in s
orbitals. As a result,
effective nuclear charge
increases across a period.
Periodic Trends
Periodic Trends
In going down a group or family, a full
quantum level of electrons, along with an equal
number of protons, is added.
As n increases, the valence electrons are, on
average, farther from the nucleus, and
experience less nuclear pull due to the shielding
by the “core” electrons. As a result, Zeff
decreases slightly going down a group.
Trends- Atomic Radii
Atomic radii are obtained in a variety of ways:
1. For metallic elements, the radius is half the
internuclear distance in the crystal, which is
obtained from X-ray data.
2. For diatomic molecules, the radius is half the
bond length.
3. For other elements, estimates of the radii are
made.
Trends- Atomic Radii
Atomic radii follow trends directly related to
the effective nuclear charge. As Zeff increases
across a period, the electrons are pulled closer to
the nucleus, and atomic radii decrease.
As Zeff decreases down a group, the valence
electrons experience less nuclear attraction, and
the radius increases.
TrendsAtomic Radii
Atomic size roughly
halves across a
period, and doubles
going down a group.
Trends – Ionization Energy
Ionization energy is the energy required to remove
an electron from a mole of gaseous atoms or
ions.
X(g) + energy  X+(g) + eElements can lose more than one electron,
so there are 1st, 2nd, 3rd, etc., ionization energies.
Ionization Energy
It always requires energy to remove an
electron from a neutral atom.
As more electrons are removed and the ion
becomes positively charged, it requires
increasingly greater energy to remove electrons.
Trends – Ionization Energy
Ionization energy is a measure of how tightly
the electrons in the highest occupied orbitals are
held by the nucleus. As a result, it is directly
related to the effective nuclear charge.
Ionization energy increases going across a
period, and decreases going down a group.
Trends – Ionization Energy
Trends – Ionization Energy
Ionization Energy
Trends – Electron Affinity

Electron Affinity involves the addition of an
electron to a mole of gaseous atoms. There are
different conventions to defining electron
affinity. Your text defines the EA as the energy
released during the following process:
X(g) + e-  X-(g)
Electron Affinity
Your text defines the EA as the energy
released during the following process:
X(g) + e-  X-(g)
A positive value for EA indicates that the
process releases significant energy. Thus, the
halogens tend to have high electron affinities.
Trends – Electron Affinity
There is less of a predictable trend in
electron affinities. In going across a period
(ignoring the noble gases), the electron affinity
should become more negative. Although this is
observed, there are many inconsistencies.
Trends – Electron Affinity
Trends - Electron Affinity
Trends- Electron Affinity
In going down a group, the electron affinity
should become become smaller. Although this
trend is observed, there is only a slight change in
electron affinities within a group. There may
also be inconsistencies in the general trend.
Electron Affinity
It should be noted that the addition of a
second electron to an anion is always highly
unfavorable. The electron affinity of oxygen is
141 kJ/mol to form O–. Addition of the second
electron to form the oxide ion (O2–) requires
744 kJ/mol.
Metallic Character
Metals are shiny, malleable and ductile. They
are generally good conductors of heat and
electricity, and low ionization energies.
In reaction with non-metals, metals tend to
lose electrons and form cations.
Metallic Character
Metallic Character
Across a period, metallic behavior decreases.
Non-metals are often crumbly solids, liquids or
gases at room temperature.
Metallic Character
Metallic behavior
increases going down a
group.
Electron Configurations of Ions
The atoms of the main group elements
(groups IA-VIIA) will form ions by losing or
gaining electrons. The resulting ion will have
the same electron configuration as a noble gas
(group VIIIA). These configurations are usually
very stable.
Electron Configurations of Ions

Atoms or ions with the same electron
configuration (or number of electrons) are called
isoelectronic.
For example, Na+, Mg2+, Ne, F-, and O2- are
isoelectronic. The size will decrease with
increasing positive charge.
O2- > F- >Ne> Na+> Mg2+
Electron Configurations of Ions
When atoms lose electrons, the electrons are
always removed from the highest quantum level
first.
For the first row of transition metals, this
means that the electrons in 4s subshell are lost
before the 3d subshell.
Fe: [Ar]4s23d6
Fe2+: [Ar] 3d6 or [Ar]4s03d6
Common Ionic Charges
The charges of ions of elements in groups
1A-7A (the main groups) are usually predictable.
Group 1A metals form +1 ions, group 2A
metals form +2 ions, etc.
The non-metals of group 5A have a -3
charge, those of group 6A have a -2 charge, and
the halogens form ions with a -1 charge.
Typical Ionic Charges
Trends – Ionic Size
Cations are always smaller than the neutral
atom. The loss of one or more electrons
significantly increases Zeff, resulting in the
valence electrons being pulled closer to the
nucleus.
Ionic Size - Cations
Within a group, assuming the same ionic charge,
the size of the ion increases going down the
group, due to more core electrons shielding the
nucleus as n increases.
Trends – Ionic Size
Across period,
the cations get
more positive,
and as a result,
considerably
smaller.
Trends – Ionic Size
Anions are always larger in size than the
neutral atom. The addition of one or more
electrons results in greater electronelectron repulsion, which causes the
valence electrons to “spread out” a bit.
Size of Anions
Anions are always
larger than the neutral
atom.
Size of Anions
Within a group, assuming the same
ionic charge, the size of the ion increases
going down the group, due to more core
electrons shielding the nucleus as n
increases.
Trends – Ionic Size
Group IA – the Alkali Metals
In discussing the chemistry, preparation and
properties of the group IA elements, it is
important to remember that hydrogen is not a
group IA metal. It’s properties and reactivity
would place it within group 7A (diatomic nonmetals), rather than group IA.
Group 1A Metals
The group 1A metals are soft shiny metals
with fairly low densities (Li, Na and K are less
dense than water) and low melting points.
Sodium melts at 98oC, and cesium melts at 29oC.
The softness, low density and low melting
points are the result of weaker metallic bonding
due to only one valence electron in this group.
Group 1A Metals - Production
Due to the high reactivity with oxygen and
water, all of the metals are found in nature in
ionic form (M1+).
The pure metal must be produced in an
oxygen and water-free environment. Typically,
an electrical current is passed through the
melted chloride salt. The metal and the chlorine
gas are collected separately.
Reactivity Trends
The chemical behavior of the group IA
metals illustrates periodic trends. As the valence
electron occupies a higher quantum level, it
experiences less nuclear attraction, and is more
easily removed.
Group 1A Metals + Water
The reaction with water forms hydrogen gas
and the aqueous metal hydroxide. The reaction
is so vigorous, that the hydrogen may ignite.
2 M(s) + 2 H2O(l)  H2(g) + 2 MOH(aq)
Metallic Character
The group IA metals react with water to
produce hydrogen and the metal hydroxide.
Metallic behavior increases going down a group.