SCIENCE BLOG – PERIODIC CLASSIFICATION OF ELEMENTS

INTRODUCTION

There are 117 elements which are having diversified chemical properties. From long before many scientists had worked on these elements to arrange them sequentially to form Periodic classification of elements. Out of them Dobereiner, Newland, Mendeleev and their works are very famous. According to their experiments, Modern Periodic table has been formed where they tried to arrange all elements in sequence to form periodic table. Here are some concepts and questioner to clear all doubts.

PROBLEMS RELATED TO PERIODIC CLASSIFICATION OF ELEMENTS 

ANSWER IN BRIEF

Qs1: How do you show the triad of Dobereiner in Alkali metal group in periodic classification of elements?

Ans1: Lithium, Sodium and Potassium are alkali metals having similar chemical properties and form a triad.  Lithium and Potassium have one intermediate element  with same property Sodium in periodic table.

Element	Lithium	Sodium	Potassium
Atomic mass	7.0	23.0	39.0

Atomic mass of the middle element Na = Average atomic masses of Li and K

• (7+ 39)/23
• 23
• Atomic mass of Na Proved.

Qs2: What are the similarities in chemical properties in Alkali metal group in Periodic classification of elements?

Ans2: Similarities are:

• They are all metals
• These metals easily react with water to produce alkali and a gas (hydrogen)
•  They all have a valency of ¹⁺ (LiCl, NaCl, KCl)
• Their carbonates are resistant to thermal decomposition.

Qs3: Show the triad character in  Alkaline Earth Metal Group.

Element	Calcium	Strontium	Barium
Atomic mass	40.0	88.0	137.0

 Atomic mass of the middle element Sr = Average atomic masses of calcium and barium

• (40+ 137)/2
• 88.5
• Atomic mass of Sr proved

BRIEF QS/ANS IN PERIODIC CLASSIFICATION OF ELEMENTS

Qs4: what are the similarities in  Alkaline Earth metal elements?

Ans4: Similarities in chemical properties :

• They are all metals
• The oxides of all these metals react with water to form alkaline solutions.
• They have similar valency 2⁺ (CaCl₂, BaCl₂, SrCl₂)
• Their hydroxides are resistant to thermal decomposition.

Qs5 : How do you show the characteristics of Dobereiner’s Triad of periodic classification?

Ans5: Halogen group has three elements which exhibit same  chemical properties and forms a triads

ELEMENT	Chlorine	Bromine	Iodine
ATOMIC MASS	35.5	80.0	127.0

 Atomic mass of the middle element Br= Average of atomic mass of Cl and that of I => (35.5 + 127.0)/2

• 81.25 = almost same atomic mass of Bromine

Qs6: What are the similarities in Alkaline earth metals and Halogen?

Ans6:  Similarities in chemical properties are:

• They are all non- metals
•  These metals easily combine to produce various  mineral acids e.g. HCl, HBr, HF etc.
• They all have valency one (1^–)
• All metals react with alkali metals to form neutral salts (NaCl, NaBr, NaI)

STATE THE REASON IN Periodic classification of elements

Qs7: Why are the noble gases missing from Newland’s octave?

Ans7: The noble gases are missing from Newland’s octaves because those were not known at that time. In Newland octaves, the arrangement of elements will not similar to the musical scale in which the eighth musical note equalizing the first one.

Qs8:  An element of Group 14 has the atomic number 14. Examine if this element will have metallic property or not.

Ans8: Since the element is in Group 14, it has four valence electrons and it lies in the middle of the 3^rd period. The element does not have any tendency to lose its valence electrons. Hence, it does not exhibit any metallic character.

Qs9: What is your opinion  changing in a group will be effected in tendency to loss electron in Periodic table  ?

 Ans9: The tendency to lose electrons increases on moving down a group from top to bottom because the effective nuclear charge acting on the valence electrons decreases on moving down a group.

Qs10: Out of the elements S, P, Cl and P which has the highest electron affinity? Which has the lowest electron affinity? Give reason.

Ans10: Electron affinity decreases on moving down a group and increases in a period from left to right of the Periodic table.  In the second period the elements are having smallest size as they have decreased atomic radius. As there is smaller atomic radius there is a repulsive force between the electron of an atom. Hence the added electron is not accepted with the same case like the other elements of the group. For this reason chlorine (Cl) has maximum electron affinity and the element  Phosphorus has the lowest electron affinity.

 KNOW THE PERIODIC TABLE in Periodic classification of elements

Qs11: Except Gallium,  in the periodic  table, Mendeleev had discovered which element to cover the gap?

Ans11: The other element beside Gallium , which have since been discovered to fill the gaps left by Mendeleev in his Periodic classification of elements

• Scandium
• Germanium

Qs12: What are defects of Mendeleev’s Periodic table in Periodic classification of elements?

Ans12: The defects are

• The arrangement of elements in the order of increasing atomic masses is not maintained in the following cases:  Pair 1 Co(58.93) and Ni(58.69) : Pair 2 Te(127.6) and I(126.9) [ Discrepancy in periodicity]
• Position of Hydrogen: It was placed in Group 1A. but hydrogen exhibits close similarities in properties wit Group 1A and VIIA elements.
• Position of Isotopes:  Isotopes of the same element have similar chemical properties and different atomic masses.  Hence they are placed in different groups according to the Mendeleev’s Periodic law. He did not provide separate places to the isotopes of an element in Periodic classification of elements
• Position of noble gases: Mendeleev did not provide any place for noble gases in his Periodic table; that time those gases were not discovered.
•  Elements with similar properties of separate groups are placed in Periodic classification of elements.
•   Keeping various elements of the same groups but with different properties .
•  Relation of chemical properties of elements and electronic configuration of elements:  In Mendeleev’s periodic table there is no relation between chemical properties and electronic configuration of elements.

 Ans13: No, in the modern Periodic table the position  of the elements is based on atomic numbers, which are integral numbers and not fractional number.

Qs13:  Can you place an element having 1.5 atomic number after hydrogen and before helium in Periodic classification of elements?

Qs14: State which has a higher ionization energy in the following pairs of element: a Cl, Br:  b Br, Se c Rb, Kr:  d Ar, Kr?

Ans14: Ionization energy  decreases on moving down a group, increases in a period from left to right.

• Br is below  Cl in group 17, therefore, Br has lower ionization energy than Cl.
• Br is in group 17 and Se is in group 16 of same third period. Hence Se has lower ionization energy than Br.
• Mendeleeve placed Rb in the periodic table just next to Kr. Rb is in group 1 and it has lower ionization energy than Kr.
• Kr occurs below Ar in group 18 of the periodic table and hence Kr has lower ionization energy than Ar.

Qs15: How do you think the tendency to lose electrons will change in a group?

Ans15: The tendency to lose electrons increases on moving down a group from top to bottom because the effective nuclear charge acting on the valence electrons decreases on moving down a group.