Mendeléev's Periodic Table

Mendeléev's Periodic Table

• This classification was done by Dmitri Ivanovich Mendeléev, a Russian chemist.
• As per this classification
  • Elements were arranged in increasing order of atomic masses
  • They were grouped according to their chemical properties in different columns

1 → General formula of the Oxides and Hydrides of Elements in the particular group

2 → Groups I to VIII

3 → Periods 1 to 6

4 → Subgroups A and B with elements having same physical and chemical properties

5 → Symbol of element with atomic mass

What is increasing order of atomic mass?

• Atomic Mass of an atom is the sum of the number of protons and neutrons in the atom.
• Atomic mass of different elements is different.
• Elements having lowest atomic mass were placed first, then those having higher atomic mass.

Example: Atomic Mass of Sodium is 23 and Magnesium is 23.5
So, Sodium was placed before Magnesium in the Periodic Table

Details of Mendeléev's Periodic Table

• Different elements were arranged in order of increasing atomic mass in horizontal rows.
• He started working with 63 known elements at that point.
• His aim was to study the elements on the basis of atomic masses as well their physical and chemical properties.
• For this he considered the formulae of the Hydrides and Oxides of elements as the criteria as most elements react with Hydrogen and Oxygen.

• He took 63 cards with the properties of each element written and sorted them on the basis of their similarities and arranged them on the basis of increasing atomic masses.
• Due to the periodic recurrence of properties, he formulated the Periodic Law as;

‘The properties of elements are the periodic function of their atomic masses’

• There were 7 periods (horizontal rows) and 8 groups (vertical columns) in the table.

Merits /Advantages of Mendeléev's Periodic Table

• Elements with similar properties were grouped together.
  In that process, elements of greater atomic masses were placed before those of lesser atomic mass.
  Example: Cobalt (atomic mass 58.9) appeared before Nickel (atomic mass 58.7); so that Cobalt could be in the same group as Rhodium (Rh) as they had similar properties.

Similarly; 

Aluminium (Al) and Silicon (Si)

Vanadium (V) and Chromium (Cr)

Tellurium (Te) and Iodine (I)

• The table predicted the presence of unknown elements and gaps were left to incorporate those later.

• He also predicted the properties of unknown elements.  

He named such elements as Eka aluminum, Eka Silicon and Eka Boron

(The word Eka in Sanskrit means first ) 

Example: Eka Aluminium means First Aluminium

• Noble Gases could be incorporated later on (which were not discovered at that time)
  When these gases were discovered, they could be put in new group without disturbing the existing order
  [The Noble gases are Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn)]

Demerits/Limitations of Mendeléev’s Periodic Table

• Atomic Mass of different elements did not increase in a regular manner.
  Though it was done to place elements of similar properties in one group, the wrong order of atomic masses of some elements could not be explained.
  It was difficult to predict how many unknown elements could be present between two known elements.
• Isotopes could not be placed in Periodic Table
  Isotopes are atoms having the same atomic number but different atomic mass and so similar chemical properties.
  Example: Chlorine has 2 isotopes  Cl-35 and CL-37. Both have different atomic masses.
  It is difficult to put isotopes In Mendeléev periodic table which classified elements on the basis of atomic mass.
• No fixed position can be given to Hydrogen
  Properties of Hydrogen are similar to both Alkali Metals* as well as Halogens**.
  *Alkali Metals consist of chemical elements Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs) and Francium (Fr). These belong to Group 1 of the periodic table.
  **Halogens are any of the elements Fluorine, Chlorine, Bromine, Iodine and Astatine. These belong to Group 17 of the periodic table.

• Hydrogen is similar to Alkali metals as both;

Have similar electronic configuration ns1 

(Have one valence electron) 

Example: Hydrogen - 1 (1s1)

                Lithium - 2,1 (1s2 2s1)

• Combine with Halogen, Oxygen and Sulphur in the same manner

• However, Hydrogen is not an alkali metal (it is not even a metal)

• Hydrogen is similar to Halogens as both;
  • Are diatomic (exist as a molecule of 2 atoms)
  • Combine with metals and non-metals to form covalent compounds