Chapter 1st
B.sc 1st year Book
(Page 14)

Moseley Periodic law

Mendeleefs periodic law was modified by Moseley in 1911 and he defined that the properties of elements depends on their atomic number rather than atomic weights that is
“The chemical and physical properties of elements are the periodic functions of their atomic numbers “but the modern periodic law states that “Properties of elements are the periodic functions(*) of their electronic configurations.

Long form of the Bohr’s table or Periodic Table

This is the combination of both the Moseley’s and modem periodic laws. The main features of the Bohr’s table are :

1- (i) Like Mendeleef periodic table it has also 1 to 7 periods.
    (ii) The first period consists only two elements and is called ‘shortest period’ (H to He).
    (iii) There are 8 elements each in 2nd and 3rd periods. These are called ‘short periods’ (Li to Ne and Na to Ar).
   (iv) The 4th and 5th periods consist 18 elements and are called ‘long period” ( K to Kr and Rb to Xe).
   (iv) the longest period’ is 6th period which have 32 elements (Cs to Rn).
   (v) The seventh period is still incomplete and have 6 naturally occurring elements and rests are called ‘transuranium elements’. Which are only discovered during nuclear research. Elements with atomic numbers 90 to 403 constitute actinide series which have been grouped and placed separately below lanthanides. The elements which belonging to this series resemble one another in several respects.

2- It has 18 vertical columns (including VIII and zero groups) called ‘groups’ and denoted by IA to VIIA, IB to VIIB and VIII and zero group. These groups are in accord with the IUPAC system’s of modification in 1984 as shown in the periodic table 1.4.

(a) All the elements of sub group IA(1), HA(2) and IlIA to VIIA 13 to 17 have complete inner shells and incomplete outer shells. These are known as normal elements.

(b) The elements of IA and IIA are known as ‘s-block elements‘ since In these atoms. S-orbitals are being tilled up and the elements of IIIA, IVA, VA, VIA, VIIA, and zero groups are known as ‘p-block elements‘, It is because in these atoms the p-orbitals are being tilled up.

(c) All the elements of sub-group IB(11), IBB(12) and IIIB to VIIB (3 to 7) and VII (8 to 10) have incomplete cuter and penultimate inner shells.These are known as ‘transitional elements’or ‘d-block elements’ because in these atoms d-orbitals are being filled up:

(d) The elements of zero group is 18 th group and have complete outermost shell hence they have inert character and known as “inert’ or ‘noble gases“.

(e) There are also two series of 14 elements from atomic number 58 to 71 are called ‘lanthartides’ and atomic number 90 to 103 are called ‘actinides’ respectively. They are placed in two separate periods at the bottom of the periodic table. The elements of these series are called f. block elements because in these atoms f-orbitals are being filled up.

(f) The valence-shell electronic configurations of s, p and d-block elements are :

s-block, p-block, d-block

Characteristics of a period :

The first elements of all the periods (I to VII) are H, Li, Na, K, Rb, Cs and Fr. With the increase in the number of periods there is addition of a new shell. The last member of each period is an inert gas like He,Ne,Ar,Kr,Xe and Rn. The last column of 7th period is still vacant. From left to right in each period there is a regular trend of variation in physical and chemical properties of elements e.g. when we moving from left to right in a period there is-
(i) a decrease in atomic radii.
(ii) an increase in electro-negativity and ionization energies (with some exception).
(iii) decrease In metallic character.

Typical elements :

Elements of 3rd period Na,Mg,Al,Si,P,S and Cl are known as a “typical elements:. Properties of all the elements present in a specific group (e.g. IA) shows similarities with the properties of the corresponding typical element (e.g. Na).

Group characteristics :

In a group as we move from top to bottom all the elements show similar chemical properties but there is gradation in their physical properties. For example: when we move in a group from top to bottom :
(i) atomic size increases with increase of atomic number.
(ii) there is decreases in ionization energy with increasing size of the atom.
(iii) there is decreases in electro-negativity with increase of metallic character.
(iv) there is increase in metallic character and basicity of their oxides.

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