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.
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.