PERIODIC PROPERTIES : atomic size, ionisation enthalpy

Periodic Properties:

Atomic Radius
It is the distance from the centre of the nucleus to the outermost shell of electrons. Covalent
radius for an atom A in a
 molecule.

In general, the atomic size decreases on moving from left to right in a period due to increase in
effective nuclear charge and increases on moving from top to bottom in a group due to addition
of new shells.

van der Waals’ Radius
It is defined as one-half of the distance between the nuclei of two non – bonded isolated atoms
or two adjacent atoms belonging to two neighbOuring molecules of an element in the solid
state.
Metallic Radius
It is define as one-half of the distance between the centres of nuclei of the two adjacent atoms
in the metallic crystal.
onic Radius
An atom can be changed to a cation by losing of electrons and to an anion by gain of electrons.
A cation is always smaller than the parent atom because during its formation effective nuclear
charge increases and sometimes a shell may also decrease. On the other hand, the size of an
anton is always larger than the parent atom because during its formation effective nuclear
charge decreases.
In case of isoelectronic ions, r he higher the nuclear charge. smaller is the size. e.g., AI
3
<
Mg
2+
< Na
+
< f
-< O
2-< N
3-
covalent radius < metallic radius < van der Waals‟ radius

Ionisation Enthalpy (IE)
It is the amount of energy required to remove the loosely bound electron from the isol~ted
gaseous atom.
A(g) + IE → A
+
(g) + e
-Various factors with which IE depends

(i) Atomic size. varies inversely
(ii) Screening effect: varies inversely
(iii) Nuclear charge: varies directly
Generally left to right in periods ionisation enthalpy increases; down the group, it decreases.
IE values of inert gases are exceptionally higher due to stable configuration.
Successive ionisation enthalpies
IE
3 > IE
2 > IE
1
IE
1
of N is greater than that of oxygen due to stable half – filled 2p-orbitals.

DEFECTS IN MENDELEEFS TABLE

Defects in the Mendeleef’s Periodic Table
(i) Position of hydrogen Hydrogen has been placed in group IA (alkali metals). but it also
resembles with halogens of group VIlA. Thus. its position in the Mendeleef‟s Periodic Table is
controversial.
(ii) Position of isotopes As Mendeleef‟s classification is based on atomic weight, Isotopes
would have to be placed in different positions due to therr different atomic weights,
e.g.,
1
H1
2
H1
3
H1 would occupy different positions.
(iii) Anomalous positions of some elements Without any proper justification. in some cases
the element with higher atomic mass precedes the element with lower atomic mass. For
example, AI (atomic weight = 39.9) precedes K (atomic weight = 39.1) and similarly Co
(atomic weight. = 58.9) has been placed ahead of Ni (atomic weight = 58.7).
(iv) Position of Lanthanoids and actinoids Lanthanoids and actinoids were not placed in the
main Periodic Table.

Anomalous behaviour of the first element of a group. The first element of a group differs
considerably from its congeners (i.e., the rest of the elements of its group).
This is due to (i) small size (ii) high electronegativity and (iii) non availability of d·orbitals for
bonding. Anomalous behaviour is observed among the second row elements (i.e., Li to F)

Limitations of Long Form of Periodic Table
In the long form of the Periodic Table :
1. The position of hydrogen still remains uncertain.
2. The inner-transition elements do not find a place in the main body of the table. They are
placed separately.


Predicting the Position of an Element in the Periodic Table:
First of all write the complete electronic configuration. The principle quantum number of the
valence shell represents the period of the element.
Group of the element is predicted from the electrons present in the outermost (n) or penultimate
(n -1) shell as follows:

For s-block elements;
group number = number of ns-electrons
For p-block elements;
group number = 10 + number of ns and np electrons
For d-block elements;
group number = the sum of the number of (n -1) d
and ns electrons.
For f-block elements; group number is 3.

PERIODIC CLASSIFICATION OF ELEMENTS

Newland’s Octaves :(Law of Octaves)

Newland states that when elements are arranged in order of increasing atomic masses, every
eighth element has properties similar to the first just like in the musical note.

This can be illustrated as given below
sa  re  ga  ma pa dha ni
Li  Be  B  C  N  O  F
Na Mg AI Si  P  S  CI

Mendeleef’s Periodic Table:

Mendeleefs Periodic Table is based upon Mendeleefs periodic law which states „The physical
and chemical properties of the elements are a periodic function of their atomic masses.”
Importance of Mendeleers Periodic Table
Few important achievements of Periodic Table are
1 Systematic study of the elements.
2. Prediction of new elements and their properties. he left space for the elements yet to be
discovered. e.g., he left spaces for Ga and Ge and named these elements as ERa-aluminium
(Ga) and EKa-silicon (Ge) respectively
3. Atomic mass correction of doubtful elements on the basis of their expected positions and
properties.