REVIEW OF PERIODIC PROPERTIES
Repetition of properties of elements at regular intervals in the periodic table is called periodicity in properties. The periodicity is mostly due to similar outermost electronic configuration of the elements. Some of the properties are briefly reviewed.
1 Atomic and Ionic radii
Atomic or Ionic radius is generally defined as the
distance between the centre of the
nucleus and the outermost shell of electrons in an atom or ion.
As we move from left to right across a period, there is regular
decrease in atomic and ionic radii of the elements. This is due to the increase
in the nuclear charge and the
additive electrons are addedto the same electronic level. On moving down a group both atomic and ionic radii increase
with increasing atomic number. The
increase in size is due to introduction of extra energy shells which outweigh the effect of increased nuclear charge.
2 Ionisation Energy (Ionisation Potential)
The energy required to remove the most loosely bound
electron from an isolated atom in
the gaseous state in known as Ionisation Energy.
Atom(g) ---- Energy --- > PositiveIon(g) + Electron
The ionisation energy of an atom depends on the following
factors (i) size of the atom (ii)
charge on the nucleus (iii) screening effect of inner electrons (iv) penetration effect of electrons (v) effect of half-filled
and completely filled sublevels.
In a period, the value of ionisation potential increases
from left to right with breaks where the
atoms have somewhat stable configurations. In a group, the ionisation potential decreases from top to bottom.
3 Electron affinity
Electron affinity or electron gain enthalpy is the amount
of energy released when an isolated
gaseous atom accepts an electron to form a monovalent gaseous anion.
Atom(g) +
Electron -- -- >
Anion(g) + Energy
Electron gain enthalpies generally increase on moving
from left to right in a period. Electron
gain enthalpies generally decrease on moving down the group.
4
Electronegativity
Electronegativity may be defined as the relative
tendency of an atom in a molecule to attract
the shared pair of electrons towards itself.
In a period, electronegativity increases on moving from
left to right. This is due to the reason
that the nuclear charge increases whereas atomic radius decreases. In a group, electronegativity decreases on
moving down the group. This is due to the
effect of the increased atomic radius.
5 Anomalous periodic properties
A few irregularities that are seen in the increasing
values of ionisation potential along a period
can be explained on the basis of the concept of half-filled and completely filled orbitals.
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