Periodic Classification of Elements
Chapter: Periodic Classification of Elements
We have already learnt that matter around us is present in the form of elements. Do u know the number of elements found till date? Till now, 118 elements are known to us. Out of these , only 98 elements are naturally occurring.
The earliest attempt to classify the elements as metals and non-metals. Later further classifications were tried out as our knowledge of elements and their properties increased.
1. Dobereiner’s Triads
Johann Wolfgang Dobereiner, a German scientist, tried to arrange the elements with similar properties into groups. He identified some groups having three elements each. Thus he named them as ‘triads’.
He showed that when the elements in triads are written in the increasing order, the atomic mass of the middle element is the average of the atomic masses of the other two elements.
2. Newlands’ Law of Octaves
Newland’s law of octaves is stimulated from the attempts of Dobereiner and he arranged elements in order of increasing atomic masses. He started with Hydrogen, the elements with the lowest atomic mass and ended at Thorium the 56th element. He found out that every eighth element had properties similar to that of the first. He compared this to the octaves found in music. Therefore, he called it the ‘Law of Octaves’. It is known as ‘Newlands’ Law of Octaves’.
- Law of octaves was applicable only up to calcium as after calcium, every eighth element did not have properties similar to the first one.
- It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future. But, later on, several new elements were discovered, whose properties did not fit into the Law of Octaves.
Mendeleev's Periodic Table
The major acknowledgment for classifying elements goes to Dmitri Ivanovich Mendeléev, a Russian scientist. He arranged the elements on the basis of their fundamental property which is the atomic mass and also on a similarity of chemical properties.
He investigated the relationship between the atomic masses of the elements and their physical as well as chemical properties. Mendeleev determined on the compounds formed by elements with oxygen and hydrogen among chemical properties. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. Only 63 elements were known when he started his work. Hence he made a table of all these 63 elements picking up such elements which have similar properties. This table is called Periodic Table.
The vertical columns in Mendeleev’s periodic table are called groups and the horizontal rows are called periods.
Achievements of Mendeleev’s Periodic Table
- Mendeleev faced some instances where he had to place an element with a slightly greater atomic mass before an element with a slightly lower atomic mass.
- He left some gaps in his table as he predicted the existence of some elements that had not been discovered by that time.
Limitations of Mendeleev’s Classification
- No fixed position can be given to hydrogen in the Periodic Table.
- Isotopes were discovered long after Mendeleev had proposed his periodic classification of elements and they posed a challenge to Mendeleev.
- Another problem was that the atomic masses do not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements especially when we consider the heavier elements.
The Modern Periodic Table
Mendeleev periodic table was modified by Henry Moseley, by showing that the atomic number of an element is the more important fundamental property than its mass.
The Modern Periodic Law can be stated as follows:
‘Properties of elements are a periodic function of their atomic number.’
The Modern Periodic Table takes care of three limitations of Mendeleev’s Periodic Table.
Position of Elements in the Modern Periodic Table
The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.
What decides placing of an element in a certain group and period?
Let us consider group 1A of the table and we will find that elements present in this group contain the same number of valence electrons.
We also find that some of these elements do not have the same number of valence electrons, but they contain the same number of shells. We also observe that the number of valence shell electrons increases by one unit, as the atomic number increases by one unit on moving from left to right in a period or we can say that atoms of different elements with the same number of occupied shells are placed in the same period.
Trends in the Modern Periodic Table
Elements belonging to the same group have the same number of valence electrons and thus same valency. Valency in a particular period from left to right increases as positive valency and then decreases as negative valency.
2. Atomic size:
Atomic size increases as we move down the group and it decreases as we move from left to right in the period. Atomic size increases down the group due to increase in the number of shells and decreases along the period due to the increase in the nuclear charge which tends to pull electron closer to the nucleus and reduce the size of the atom.
3. Metallic and Non-metallic Properties
In the modern periodic table, metals are on the left side and non-metals are on the right side of the table. A zigzag line of metalloids separates metals from non-metals.
Metallic characters decrease from left to right in a period and increase while going down in a group whereas it is vice versa in case of non-metallic characters.