Periodic classification of elements is the method by which elements are classified based on their properties and characteristics.
On the periodic classification of elements, earlier attempts were done.
- Dobereiner’s Triads
- Newland law of octaves
- Mandleev periodic table
According to Dobereiner, all elements occurred in groups of three when arranged in increasing order of atomic mass.
- Three elements having similar chemical properties called Triades.
Dobereiner’s law of triads states that the atomic mass of the central element of a triad is the arithmetic average of the atomic masses of the two remaining elements.
3 elements >grouping>properties
|Li, Na, K||7, 23,39||(7+39)/2=23|
|Cl ,Br ,I||35.5 ,80 ,127||(35.5+127)/2=81.25|
|Ca,Sr, Ba||40 ,88 ,137||(40+137)/2=88.5|
Dobereiner could only identify three triads based on known elements at that time, therefore this classification system was not found to be useful for classifying many other elements that were not able to form a triad like all three previous triads.
Newlands law of octaves
In this law, Newland arrange the elements in the increasing order of their atomic mass and observe that the properties of every 8th element were similar to the 1st one, like in the case of musical vowels notation.
- At that time inert gas were not known.
- It was found that octave law applied only up to atomic number 20 (calcium).
- It was assumed by Newlands that only 56 elements excited in nature and no more element would be discovered ,but later several element were discovered whose properties did not fit into the law of octaves.
- In order to fit elements into table, Newlands adjusted 2 elements in the same slot but also put some unlike element under the same note.
e.g; Co and Ni are placed in H, Fl, Cl table.
Mendeleev’s periodic law
The physical and chemical properties of the elements are the periodic function of their atomic mass.
- It was based on atomic weight.
- 63 elements were known at that time.
- Noble Gas were not discovered .
- He was the first scientist to systematically classify the elements in horizontal rows and vertical columns.
- Horizontal rows are called periods and there were 7 periods in table.
- Verticals column are called group and there were 9 groups in table .
- Each group unto VII in divided into A and B subgroups elements are called transition element.
- The VII groups contain 9 elements in three rows (transitional metal group).
- The element within the same groups has similar characteristics.
Achievements of Mendeleeve’s periodic table:
Mendeleev kept some empty spaces in the periodic table for the element which have not yet been discovered.
|Predicted element||Actual element|
-He also predicted the properties of some elements even before their discovery.
|Formula of O2||E2O3||Ga2O3|
|Formula of Cl2||ECl3||GaCl3|
-Mendeleev’s periodic table could accommodate noble gas when they were discovered.
- Element with large differences in properties were included in the same group.
- No proper position could be provided for the hydrogen of the element.
- The increasing order of atomic mass was violated in case of cobalt and nickel etc.
- As isotopes are atoms of the same elements having different atomic mass,they should be given different position, but this was not done.
Modern law of periodic classification of elements:
“The physical and chemical properties of elements are the periodic function of their atomic number instead of their atomic mass.”
- This law was given by a scientist named Henery Moseley.
- Using modern periodic table, Henery mosely along with Werner, Bohr and Bury made modern periodic table.
The periodic table is divided into four blocks (s,p,d and f) depending upon the subshell to which the valence electron enter into.
- The elements from groups 1 and 2 constitute the block s.
- Elements of group 3,4,5,6,7,8,9,10 ,11,12 constitute P-block.
- Elements of group group 13,14,15,16,17,18 constitute P-block.
- The F block constitutes two horizontal rows placed at the bottom of the periodic table to avoid its unnecessary expansion.
Element of S and P block are called elements, while d-block are called transition elements and F block elements are called inner transition elements.
The 18 vertical columns are called group., the elements belonging to a particular group is known as a family and is usually named after the first member.
Apart from this some of the groups are given typical names. eg;
- Elements of group 1 are called Alkali metals.
- Elements of group 2 are called Alkaline metals.
- Elements of group 15 are called Alkaline earth metal.
- Elements of group 16 are called chalcogen.
- Elements of group 17 are called Halogens.
- Elements of group 18 are called noble gas or inert gas.
- The horizontal rows are called period.
- There are seven periods within the long shape of the periodic table.
The reoccurrence of properties of elements after certain regular intervals, when they are arranged in the increasing order of their atomic numbers, is called Periodicity.
Cause of Periodicity:
- Properties of elements depend upon the number of valence electr.
- When the elements are arranged in the increasing order of their number then the elements having the same number of valence electrons are repeated at regular intervals of 2,8,18,32 ……
The properties which are directly or indirectly related to their electronic configuration and show gradual change when we move or from left to right in a period or from top to bottom in a group are called periodic properties.
- Atomic Size
- Ionisation energy
- Electron gain enthalpy
- Electron negativity
- Metallic and non-metallic character
- Nature of oxides
Anamalous behaviour of first element of a group:
- The first element of a group exhibits difference in its properties in certion respects from the rest of the elements of its group.
- This is due to its small size ,high electronegativity and non-availability of d elements.
- This anomalous behaviour is shown by the elements of the second row i.e Li to F.
Some important general periodic trends:
I hope this article on periodic classification of elements class 10 notes will help in your exams!
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