Periodicity of valence or Oxidation State
The electrons present in the outermost shell of an atom are called valence electrons and the number of these electrons determine the valence or the valency of the atom.
The orbitals present in the valence shell are called valence orbitals.
The valence of an atom equal to either the number of valence electrons are equal to 8 minus the number of valence electron.
Transition and inner transition elements, exhibit variable valency due to the involvement of not only the valence electrons but the d or f electrons as well.
Variation along a period
As we move across a period from left to right, the number of valence electron increases from 1 to 8 .
The oxidation state of an element in a given compound may be defined as the charge acquired by its atom on the basis of electronegativity of the other atoms in the molecule.
Variations within a group
When we move down the group, the number of valence electrons remain the same, therefore all the elements in a group exhibit the same valence.
For Ex: All the elements of group 1 have valence one while all the elements of group 2 have valence of 2.
Noble gases present in group 18 are zerovalent i.e. their valance is zero since these elements are chemically inert.
Anomalous properties of second period elements
The first element of group 1 and 2 and group 13 to 17 differ in many respect from the other members of their respective group.
Some elements on the 2nd period shows similarities with the elements of the 3rd period present diagonally to each other, though belonging to different group.
Similarities in properties of the elements placed diagonally to each other is called diagonal relationship.
The reason for the different chemical behaviour of the first member of a particular group of elements in the s and p block compared to the other members of the same group are
1) small size
2) large charge/ radius ratio
3) high electronegativity
4) absence of d orbital
5) the maximum covalency of first member of each group is 4 whereas other members of the groups can have a maximum covalency of 6.
6) because of smaller size and high electronegativity ,first member of each group show greater ability to form pπ-pπ multiple bonds either with itself or with other members of the second period.
The chemical and physical properties of the elements depend mainly upon their electronic configuration.
The atomic and ionic radii decreases as we move across the period from left to right .As a result ionization enthalpy generally increases and electron gain enthalpy become more negative across a period.
Ionization enthalpy of the extreme left element in a period is the lowest while electron gain enthalpy of the element at the extreme right is the most negative.
There is high chemical reactivity at the two extremes and lowest in the centre.
The chemical reactivity of alkali metals on the extreme left is due to their ability to lose an electron to form corresponding cation.
Chemical reactivity of halogens on the extreme right is due to their ability to gain an extra electron leading to the formation of the anion.
Alkali metals are good reducing agents while halogens are good oxidizing agents.
Metallic character of an element which is highest at the extreme left decreases while the non metallic character increases on moving across the period from left to right.
The elements at the extreme left of the periodic table readily combine with oxygen to form oxides which are most basic.
Elements on extreme right from oxides which are most acidic.
Oxides of the elements in the centre are either amphoteric or neutral.
The amphoteric oxides show both acidic and basic properties .
Neutral oxides have neither acidic not basic properties.