Saturday, 13 June 2015

Mendeleev’s Periodic Table

Mendeleev’s Periodic Table

                                                In Mendeleev’s Periodic Table, Mendeleev classify elements according to their atomic masses and arranged these elements in table according to their increasing order of atomic masses.

Mendeleev’s Periodic Law

                                                According to Mendeleev’s Periodic Law – “Physical and chemical properties of elements are periodic function of their atomic masses”.
Mendeleev’s Periodic Table contains seven horizontal rows known as periods and nine vertical columns known as groups.

Groups of Mendeleev’s Periodic Table

1.     In original Mendeleev’s Periodic Table only 8 groups present because Zero group contains noble gases is added later after discovery of noble gases.
2.     So, total vertical column is 9 including group 1 to 8 and Zero group.
3.     Group 1 to 7 are divided into two subgroups (A and B) each.
4.     In eighth group, three elements found together in each period (from fourth period onward) known as transition triplet.

Periods of Mendeleev’s Periodic Table

1.     Mendeleev’s Periodic Table has seven periods.
2.     First period is known as very short period because contain only two elements.
3.     Second and Third period is known as short period because contain only 8 elements each.
4.     Fourth and Fifth period is known as long period because contain 18 elements each.
5.     Sixth period is known as very long period because contain 32 elements each, 18 elements in table and remaining 14 elements called Lanthanides (from atomic no. 58 to 71)
6.     Seventh period is known as incomplete period because it contains only 24 elements and some places are left vacant for elements as they are not known yet. 10 elements in the table and remaining 14 elements called Actinides (from atomic no. 90 to 103) placed outside at bottom of periodic table.

Demerits of Mendeleev’s Periodic Table

These below points are some Demerits of Mendeleev’s Periodic Table
1.     Position of Hydrogen: Hydrogen is placed in 1st as well as in 7th group because its properties are similar to both groups, which is not explained well.
2.     Elements which have similar properties are placed apart in different groups.
Example: Copper and Mercury with similar properties placed in group first and second.
3.     Elements with different properties are placed in one group.
Example: Coin metals like Li, Na, K, Rb, Cs.
4.     Atomic weights are not kept in serial order.
Example: Argon with higher atomic weight 39.948 is placed before potassium, which have lower atomic weight of 39.102
5.     Position of isotope is not proper because all isotopes of elements are placed in one group though the atomic weights of isotopes are different.
6.      Eighth group elements position is not appropriate because elements put in triplet form.
7.     Rare earth elements (Lanthanides and Actinides) position is not proper so they placed outside at bottom of periodic table.

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