Modern Periodic Table

This law was given by Henry Moseley in 1913 . it states properties of the elements are the periodic function of their atomic numbers.

Periodicity may be defined as the repetition of the similar properties the elements placed in a group and separated placed in a group and seprated by certain definite gap of atomic numbers the cause of periodicity is the resemblance in properties of the elements is the repetition of the same valence shell electronic configuration

• Mosely proposed this modern periodic table according to which “ the physical and chemical properties of elements are periodic function of their atomic number and the 7 horizontal rows are called periods and 18 vertical columns known as groups.
• The elements belonging  to a particular group make a family and usually named after the first member .
• In a group all elements contain the same number if valence electrons.
• In a period , all elements contain the same number of shells , but as we move from left to right , the number of valence shell electrons increases by one unit.
• The maximum number of electrons that can be accommodated in a shell can be calculated by the formula 2n² , where n is the formula of the given shell from the nucleus.

For example

K shell        → 2(1)² =  2, hence the first periodic has 2 elements .  

L shell        → 2(2)²  = 8, hence the second period has 8 elements.     

The third, fourth , fifth, sixth, and seventh periods have 8,18, 18, 32, and 32 respectively.

(i)    Trends in modern periodic table  

Some trends were observed of the elements in moving down the group ( from top to bottom of the table ) and across a period ( from left to right ) are as follows:-

Valency :-  the valancy of an elements is determined by the number if valence electrons present in the outermost shell of its atom (i.e 8 electrons in valence shell, in some special cases it is 2 electrons.

(ii) Atomic size :- the term atomic size refers to the radius of an atom the atomic radius of hydron atom is 37 pm ( picometre , 1=10^-12m)

• The atomic radius decreases on moving from left to right along a period. This is due to an increase  nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
•  In a group atomic decreases  from top  to buttom due to increase in number of shells.

(iii)   Metallic and non- metallic properties

• The metals like Na and Mg are towards the left hand side of the periodic table. The non- metals like sulphur and chlorine are formed on the right hand side. In middle , silicon, is classified as a semi- metal or metalloid ( which exhibits some properties of both metals and non-  metals )
• In a eriod from left to right metallic Character  increases.
•  In a group , metallic character increases from top to bottom while non- metallic character decrease.

(iV)  Electronegativity :-

The relative tendency of an atom to attract the shared electrons pair of electrons towards itself is called electronegativity.

In a period left to right the value to right the value of electro negativity increases while in a group from top to buttom, the value of electronegativity decreases.