Periodic classification of Elements
Periodic classification of Elements
Father of periodic table – Mendeleev.
The arrangement of the known elements in certain groups in such a way so that the elements with similar properties are grouped together is known as classification of elements.
At present 118 elements are known to us. Out of these 118, only 98 are natural occurring elements
Genesis of periodic classification :
1. Lavoisier classified the elements into metals and nonmetals.
2. Dobereinier’s Triads: In 1829, Dobereinier, a German chemist arranged certain elements with similar properties in groups of three in such a way that the atomic mass of the middle element was nearly the same as the average atomic masses of the first and third elements.
But only few elements could be covered under triads.
3. Newland’s law of octaves: In 1866, John Newland, An English Chemist proposed the law of octaves by stating that, When elements are arranged in order to increasing atomic masses, every eighth element has properties similar to the first, just like musical notes.
He started with element having the lowest atomic mass (hydrogen) and ended at thorium which was the 56th element.
4. Lother’s-Mayer’s atomic volume curve: In 1869 Lother Mayer plotted a graph betweeen atomic volume of the elements and their atomic mass and he pointed that the elements with similar properties occupy similar position in the curve.
5. Mendeleev’s periodic law: The physical and chemical properties of the elements are the periodic function of their atomic masses.
Mendeleev arranged all the elements known at that time in increasing order of atomic mass and this arrangement become periodic table.
In a periodic table :
Horizontal line is called periods and Vertical line is called group.
In Mendeleev’s periodic table: Total number of periods are seven and total number of groups are nine i.e.,
Period – 7
Group – 9 (I, II, III, IV, V, VI, VII, VIII, Zero)
Defects in Mendeleev’s Periodic Table elements
1. Mendeleev’s arrangement of was based on increasing atomic masses hence some elements appeared to be out of place in periodic table.
For example:
Cobalt with atomic mass 58.9 proceeds nickel with atomic mass 58.6
2. According to Mendeleev’s periodic law, the isotopes must be given separate places in periodic table, since they have different atomic masses. But they were not assigned separate places.
3. Elements such as copper, and silver bear no resemblance with alkali metals (Li, Na, K, Rb, Cs) but they have been placed together.
4. Elements which are chemically similar such as gold and platinum have been placed in separate groups.
5. Hydrogen was not given a fixed position. It was considered in group IA as well as in group VII Abecause it form positive ion as in HCl and also negative ion as in NaH.
Mendeleev’s periodic table became immensely popular. So, he was regarded as “father of periodic table”. The element with atomic number 101 has been named as mendelevium in the honour of mendeleev.
6. Modern Periodic law: Modern periodic law was given by Moseley.
According to Moseley: “The physical and chemical properties of the elements are periodic function of their atomic numbers.”
In modern periodic table: There are seven periods and eighteen groups i.e.,
s-block elements are known as Alkali & Alkaline earth metals. They belong to group 1-2.
p-block elements are known as Chalcogen, Picogens, Halogens and inert gases. They belong to group 13-18.
d-block elements are known as Transition elements. They belong to group 3-12.
f-block elements are known as Inner transition elements.
⇒ These are also called rare earth metals. The first rare earth metal is Cerium (Ce).
The periodic table shows two horizontal rows each containing 14 elements at the bottom. The first row contains 14 elements from atomic number 58 to 71 and is called Lanthanides series. The second row also contains 14 elements from atomic number 90 to 103 and is called Actinide series.
Elements in a period have same number of shell (Orbit) but in a group number of shells (Orbits) changes.
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