Chapter 5

Periodic Classification of Elements

Early classification of elements.

• Classification means identifying similar species and grouping them together.
• Lavoisier divided lements into two main types known as metals and non- metals.

Doberiner’s law of triads :- 

Doberiner tried to arrange the elements with similar properties. He showed that when the three elements in a triad were written in the order of increasing atomic masses ; the atomic masses ; the atomic mass of the middle element was roughly the average of the atomic masses of other two elements.

i.e atomic masses Li, Na and K are 7,23, and 39 respectively , tus the mean of 1^st and 3^rd elements is 23 and the atomic mass of middle element is 23.

Limitation :-  He could identify only a few such triads and so the law could not gain importance.

For example, Fe, Co, Ni, all the three elements have nearly equal atomic mass and thus does not follow this law.

Newland’s law of octaves :-

 He found that every eighth element had properties similar to that of the first. HP compared this to the octaves found in music . he called it ‘law of octaves’.

For Example. The properties of lithium and sodium were found to be the same sodium is the eighth  element after lithium.

 Limitation :-

(i) law of octaves was applicable only upto calcium as after calcium every eigth element did not possess properties similar to that of first.

(II)    According  to him , only 56 elements exist in nature and no more elements would be discovered in the future. But later on several new element  were discovered wose properties did not fit into law of octaves. 

(III)    In order to fit new elements into his table newland  adjust two elements in the same column, but put some unlike elements under the same column. 

(IV)    Thus, newland’s  classification was not accepted.

 Mendeleev’s Periodic Table

Mendeleev arranged 63 elements known at that time in the periodic table. According to Mendeleev “ the properties of the elements are a periodic function of their atomic masses”  the table consists of eight vertical coloumn called groups and horizontal rows called periods. 

Achievements :- 

(I) The arrangement of elements in group and periods made the study of elements quite systematic in the sence that if properties of one element in a particular group are known, those of the other can be easily .

(II) many gaps were left in  this table for undiscovered elements however, properties of these elements could be predicted in advance from their expected position. This helped in the discovery of these elements the lements silicon , gallium and germanium were ddiscovered in this manner.

(III) Mendeleev   corrected the atomic masses of certain elements with the help of their excepted position and properties. 

(Iv) When  inert  gasses were discovered they could be placed in a new group without disturbing the existing order.

 Limitations :-     

1. He could not assign a correct position of hydrogen in his periodic table 1 as the properties of hydrogen resembles both with alkali metals as well as with halogens.
2. The atomic masses do not increases a regular manner in going from one elements to the next , so it was not possible to predict how many elements could be discovered between two elements.
3. The isotopes of some element will be given different position if atomic number is taken as basis which will disturb the symmetry of the periodic table.

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.