B.sc 1st year Book
(Page 14)

Moseley Periodic law

Mendeleev’s periodic law was modified by Moseley in 1911 and he defined that the properties of elements depend 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 Bohr’s table or Periodic Table

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

1- (i) Like Mendeleev’s periodic table it has also 1 to 7 periods.
    (ii) The first period consists of only two elements and is called the ‘shortest period‘ (H to He).
    (iii) There are 8 elements each in the 2nd and 3rd periods. These are called ‘short periods’ (Li to Ne and Na to Ar).
   (iv) The 4th and 5th periods consist of 18 elements and are called ‘long periods” ( K to Kr and Rb to Xe).
   (iv) the longest period is the 6th period which has 32 elements (Cs to Rn).
   (v) The seventh period is still incomplete and has 6 naturally occurring elements and the rest 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 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 groups. These groups are in accord with the IUPAC system’s modification in 1984 as shown in 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 the zero group are 18 the group and have a complete outermost shell hence they have inert character and are known as “inert’ or ‘noble gases“.

(e) There are also two series of 14 elements from atomic numbers 58 to 71 called ‘lanthanides’ and atomic numbers 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

Related Topic | Atomic Structure and Periodic Table

Bohr’s Atomic Model De Broglie Hypothesis
De Broglie Equation Heisenberg uncertainty Principle
Schrodinger Wave Equation Shapes of orbitals
Quantum Numbers Hunds Rule
Pauli Exclusion Principle Energy level diagram of Atom
Aufbau Principle Screening effect or shielding effect
Periodic Table Moseley Periodic law
Classification of elements

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 the 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 the 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 move from left to right in a period there is-
(i) a decrease in atomic radii.
(ii) an increase in electronegativity and ionization energies (with some exceptions).
(iii) decrease In metallic character.

Typical elements :

Elements of 3rd period Na, Mg, Al, Si, P, S, and Cl are known as “typical elements: Properties of all the elements present in a specific group (e.g. IA) show 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 the increase in atomic number.
(ii) there is a decrease in ionization energy with the increasing size of the atom.
(iii) there are decreases in electro-negativity with an increase in metallic character.
(iv) there is an increase in the metallic character and basicity of their oxides.

Spread The Love