Periodic Classification of Elements | Olympiad Preparation for Class 10 PDF Download

Introduction

It is always easy to distinguish the items from one another by sorting them on some basis. In malls or in shops we always get things sorted at their respective places. In shops, kids' wear can be bought from the kid section which cannot be found in any other section. Grocery shops also align the items in a systematic way.
There are a total of 92 well-known naturally occurring elements of which 70 are metals and the remaining 20 are non-metals. There are certain elements possessing the characteristics of both metals as well as non-metals. They are termed metalloids. For instance, boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.

The naturally occurring elements

Döbereiner’s triads

Sir John Wolfgang Döbereiner

• In 1817 German chemist Johann Wolfgang Döbereiner arranged the elements with similar properties into groups.
• Some groups were identified as having three elements each. So he called these groups ‘triads’. Only three triads could be identified from the elements discovered at that time.
• On writing the three elements of the triad in the order of increasing atomic masses it was found that the atomic mass of the middle element was approximately the average of the atomic masses of the other two elements.

But it could not exist for long because of the following limitations:

• Döbereiner’s triads could find only three triads; .i.e total of 9 elements only but actually there were many more elements known at that time.
• Therefore Dobereiner could not classify most of the elements known at that time.

Newlands’ law of octaves

• In 1866 English scientist John Newlands arranged the elements in the order of
• increasing atomic masses starting from hydrogen having the lowest atomic mass to the 56th element named thorium having the highest atomic mass among the elements discovered at that time.
• It was found that every eighth element possessed properties similar to that of the first. Sodium is the eighth element after Lithium possessing properties similar to one another. Similarly, beryllium and magnesium resemble each other.
• This was comparable to octaves found in music and hence was called the ‘Law of Octaves’ or ‘Newlands’ Law of Octaves’. 
• But the Law of Octaves was applicable only up to calcium, after which every eighth element did not resemble the first element.

Limitations of Newlands’ law of octaves

• Newlands assumed that only 56 elements existed in nature and no more elements would be further discovered in the nearer future. But later on, several new elements were discovered, whose properties couldn’t be defined as per the Law of Octaves.
• In order to fit elements into the law of octaves, Newlands not only adjusted two elements
• in the same slot but also adjusted some unlike elements under the same note.
• Cobalt and nickel are in the same slot and are positioned in the same column with fluorine, chlorine, and bromine possessing different properties than these elements.
• Iron possessing similar properties as cobalt and nickel is placed far away from these elements.

• Thus, Newlands’ Law of Octaves can be obeyed well by the lighter elements only.

Mendeléev’s periodic table

• In Mendeléev’s periodic table only 63 elements were arranged that were examined on the basis of the relationship between the atomic masses of elements
• and their physical and chemical properties.
• Hydrogen and oxygen were selected due to their high reactivity and formation of compounds with most elements giving rise to hydrides and oxides that were treated as one of the basic properties of an element.
• Properties of 63 elements were written on 63 cards and then the elements with similar properties were sorted.
• Most of the elements were arranged in the order of their increasing atomic masses in the Periodic Table with the occurrence of a periodic recurrence of elements with similar physical and chemical properties.
• As per the arrangement of elements, Mendeléev formulated a Periodic Law stating as ‘the properties of elements are the periodic function of their atomic masses.
• The Periodic Table consists of vertical columns termed ‘groups’ and horizontal rows termed ‘periods’.

Achievements of Mendeléev’s periodic table

• Elements with a slightly greater atomic mass was placed before an element with a slightly lower atomic mass with an inverted sequence so as to group the elements with similar properties. For example, cobalt with atomic mass 58.9 was placed before nickel with an atomic mass of 58.7.
• He left some gaps in the Periodic Table with the prediction of
  existence of some elements that were not discovered at that time.
• He named the future elements by prefixing a Sanskrit numeral, Eka (one) to the name of preceding element in the same group.
• For instance, scandium, gallium and germanium, discovered later, have properties
  similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.
• Noble gases like helium (He), neon (Ne) and argon (Ar) were discovered
  later as they are present in exceptionally low concentrations in the atmosphere due to their inertness.

Limitations of Mendeléev’s classification

• No fixed position could be assigned to hydrogen in the Periodic Table.
• Isotopes discovered long after Mendeléev's periodic classification of
• elements imparted a challenge to Mendeleev’s Periodic Law.
• The atomic masses of elements did not increase in a steady manner while going from one element to another which made it difficult to predict the number of elements that could be discovered between two elements.

The Modern Periodic Table

• In 1913, Henry Moseley proved the atomic number of an element to be more vital
• property than its atomic mass.
• An atomic number can be defined as the number of protons in the nucleus of an atom which increases by one in going from one element to next element.
• Mendeléev’s Periodic Law was revised taking atomic number as the basis of Modern Periodic Table.
• Modern Periodic Law can be stated as ‘Properties of elements are a periodic function of their atomic number.’

Position of elements in the modern periodic table

• The Modern Periodic Table consists of 18 vertical columns termed ‘groups’.
• It also consists of 7 horizontal rows termed ‘periods’.
• The elements present in any one group have the same number of valence electrons. For instance, elements like fluorine (F) and chlorine (Cl), belong to group 17.
• There is an irregularity with the position of hydrogen as it can be placed either in group 1 or group 17 in the first period.
• The maximum number of electrons that can be filled in a shell can be calculated by the formula 2n2 where ‘n’ is the number of the given shell from the nucleus.

K Shell – 2 *  (1)² = 2, 
So the first period has 2 elements. 
L Shell – 2 * (2)² = 8, 
So the second period has 8 elements. 
M Shell – 2 *  (3)² = 18, 
As the outermost shell can have only 8 electrons, so the third period also has only 8 elements. 
Metals like Na and Mg occupy the left-hand side whereas the non-metals like sulfur and chlorine occupy the right-hand side of the Periodic Table. Silicon or some other metals exhibiting the properties of both metals and non-metals termed semi-metal or metalloid are positioned in the middle of the periodic table. 

Trends in the modern periodic table

• The number of shells increases on going down the group.
• The number of valence shell electrons increases with the increase in atomic number on moving from left to right in a period with each period marking the filling of a new electronic shell.
• Atomic size decreases in moving from left to right along a period due to an increase in nuclear charge pulling the electrons closer to the nucleus.
• The addition of new shells down the group increases the distance between the outermost electrons and the nucleus thereby increasing the atomic size down the group.
• Across a period effective nuclear charge acting on the valence shell electrons increases which decrease the tendency to lose electrons. Hence metallic character decreases and non-metallic character increases across a period.
• Down the group, the effective nuclear charge decreases which increase the tendency to lose electrons. Hence metallic character increases and non-metallic character decreases down a group.