CHEM 1405 CHAPTER -5
THE PERIODIC TABLE
Periodic Table of Elements
The periodic Table is a systematic arrangement of elements in the order of increasing atomic
numbers.
Classification of the Elements
In the modern Periodic table, the classification of elements is based on Moseley’s Periodic law, which
states that the physical and chemical properties of elements are a periodic function of their atomic
numbers. Thus, atomic number of elements is the basis of classification of elements.
In the periodic table, elements are mainly classified in to 4 groups.
1. Representative Elements (Main group Elements) ( s and p block elements)
These are the elements of the groups IA through 7A. They have incompletely filled valence shells.
2. Noble gases
These are the elements of the 8A group. They have completely filled sub shells in the valence shell.
3. Transition Metals ( d - block elements)
They are the elements of the groups IB and 3B through 8B. All of them are metals with
incompletely filled (n-1) d sub shells. Elements of 2B have completely filled (n-1) d sub shells. But
are studied along with transition metals because their properties are similar to that of transition
metals.
4. Lanthanides and Actinides (Rare earths) ( f - block elements)
Lanthanides constitute a group of elements with atomic numbers 58 – 71 and Actinides constitute
another group elements from atomic numbers 90 – 103. They have incompletely filled (n-2) f
orbital. All of them are metals and actinides are radioactive.
In the periodic table all the elements of a particular group have similar chemical properties because they
have same number of valence electrons (Outer shell electrons).
Cation is a positively charged ion formed by the loss of one or more electrons from the valence shell of
the atom. An anion is a negatively charged ion formed by the gain of one or more electrons.
Group number
# of valence electrons
Ion charge
1(IA)
1
1+
2(IIA)
2
2+
13(IIIA)
3
3+
14(IVA)
4
4+/-
15(VA)
5
3-
16(VIA)
6
2-
17(VIIA)
7
1-
Electron configuration of cations and anions
Iso electronic Ions - Ions with same number of electrons are iso electronic ions
Periodic variation in properties of elements
1. Effective nuclear Charge
The shielding effect of the inner electrons and increase in the size of the atom decreases the
electrostatic forces of attraction between the nucleus and the valence electrons. The shielding
effect of electrons decreases in the order s > p > d > f. This will reduce the effective nuclear
charge. Effective nuclear Charge of elements increases across a period and decreases down the
group.
2. Atomic radius
It is half of the distance between the two nuclei of two adjacent atoms in a metal or a diatomic
molecule.
Atomic radius decreases across a period. This is because of the increase in effective nuclear charge
on the valence electrons. However, atomic radius increases down the group as the effective nuclear
charge on the valence electrons decreases due to the addition of more shells.
3. Ionic radius
Radius of a cation is always smaller than that of the corresponding atom because a cation is
formed by the loss of one or more electrons and this causes an increase in effective nuclear charge
on the valence electrons. On the other hand, the radius of an anion is always larger than that of the
corresponding atom because an anion is formed by the gain of one or more electrons and this
causes a decrease in effective nuclear charge on the valence electrons.
4. Ionization energy (KJ / mol)
The minimum amount of energy required to remove an electron from the valence shell of a
neutral gaseous atom in the ground state is called the first ionization energy (I1)
The minimum amount of energy required to remove an electron from the valence shell of a mono
positive gaseous ion in the ground state is called the second ionization energy (I2)
I1 < I2 < I3 <….
In most cases the ionization energy of elements increases from left to right in the period because,
 Increase in effective nuclear charge
 Decrease in atomic radius
Both conditions require more energy to remove the electron from the valence shell
Ionization energy of elements decreases down the group because,
 Decrease in effective nuclear charge
 Increase in atomic radius
4. Electron Affinity
The energy released when an electron is added to the valence shell of a neutral gaseous atom in its
ground state is called the electron affinity.
 Electron affinity of elements increases from left to right in the period, because of the
increase in effective nuclear charge on the valence electrons.
 Electron affinity of elements decreases down the group in the periodic table, because of the
decrease in effective nuclear charge on the valence electrons.
Electron affinity of Fluorine is lesser than that of Chlorine because of its small size.
Lewis Symbols of elements. The nucleus and the inner electrons are represented by
the symbol and the valence electrons by equal number of dots surrounding the
symbol.
.
..
.
.
.
.
..
H
He
Li
. Be . : B.
.C .
.N:
:O :
:F :
.
.
.
.
Q. Find the Lewis symbols of the following elements
Al
Si
S
Cl
Ar
Na+
F-
..
:Ne :
..