Quick Revision: Classification of Elements & Periodicity in Properties | Chemistry Class 11 - NEET PDF Download

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PERIODIC TRENDS IN PROPERTIES
A. Atomic Size (or atomic radius)
Atomic radius is the size of the atom of an element. Atomic radius is defined as “the distance from the centre
of the nucleus upto the centre of outermost electron.” It is measured in Angstrom unit (Å). It is not possible
to measure exact atomic radius as an atom is unstable and it cannot be isolated to get its radius. Moreover,
the exact position of the outermost electron is uncertain. The values for radii are obtained from X-ray
measurements. Following points are to be noted in this context :
(a) The size of an atom or ion decreases in a horizontal period as we move from left to right.
(b) The atomic radius increases in a group with the rise in atomic number.
(c) A positive ion (cation) is smaller than the corresponding atom : A positive ion or cation is formed
by the loss of one or more electrons from an atom and the number of protons remains the same in the
nucleus. Thus the ratio of the positive charge in the nucleus to the number of electrons i.e., effective nuclear
charge increases. Hence the force of attraction of nucleus to the outer electrons increases thus decreasing
the size of cation. In case of alkali metals, the removal of an electron removes the entire outermost shell.
(d) A negative ion (anion) is bigger than the corresponding atom : In the formation of negative ion (anion)
one or more electrons(s) are added to the atom. This results in the expansion of the size of the nuclear
charge, which in turn decreases the force of attraction and increases the size of an anion or the pull
exercised by the nucleus on the electron becomes less i.e., they move a little farther resulting in an
increase in the ionic size.
Note : Size of Iso-electronic ions : These are such cations or anions which carry the same number
of electrons. The size of such ions depends upon the effective nuclear charge. Greater the
nuclear charge of an ion, greater will be the force of attraction for same number of electrons.
As a result, the size of the ion decreases. For example :
N
3–
, O
2–
, F
–
, Na
+
, Mg
2+
 and Al
3+
 are isoelectronic ions, among these N
3–
 is largest (1.71 A) and
Al
3+
 is smallest (0.50 A).
B. Ionization Enthalpy
It is the amount of energy required to remove most loosely held electron from the ground state of an Avogadro
number of the isolated atoms, ions or molecules in the gaseous state. The ion formed by loss of first electron
may lose further electrons and thus we may have successive ionization energies for removal of 2nd, 3rd and
4th electrons in the gaseous state.
Ionization is always an endothermic process and ionization energies are therefore always assigned positive
values.