Chemistry holds a very important position in NEET Exam, so make your strategy accordingly with reference to topics. Here is one of the important topic for chemistry which is Classification of Elements and Periodicity in Properties, in this article you will get the overview of this topic which will help you to make notes for NEET 2020 Exam.
Classification of Elements was essential since many elements had been being found within the 19th century and the research of those elements individually was proving tough.
There have been many attempts classifying elements including ‘Dobereiner’s Triads’ and ‘Newland’s Octaves’.
German chemist Johann Wolfgang Dobereiner tried to categorize elements with related properties into groups of three elements each. These groups had been known as ‘triads’.
Dobereiner advised that in these triads, the atomic mass of the element within the center can be roughly equal to the imply of the atomic masses of the opposite two elements within the triad.
An instance of such a triad can be one containing lithium, sodium, and potassium. The atomic mass of lithium 6.94 and that of potassium is 39.10. The element in the course of this triad, sodium, has an atomic mass of 22.99 which is kind of equal to the mean of the atomic masses of lithium and potassium (which is 23.02).
The Limitations of Dobereiner’s Triads are :
- All the elements recognized at the moment couldn’t be categorized into triads.
- Only 4 triads had been talked about – (Li,Na,K + Ca,Sr,Ba + Cl,Br,I + S,Se,Te).
English scientist John Newlands organized the 56 recognized elements in the rising order of atomic mass within the year 1866. He noticed a pattern whereby each eighth element exhibited properties much like the primary.
This similarity within the properties of each eighth element could be illustrated as figures in most of our textbooks.
Newland’s Law of Octaves states that when the elements are organized in the rising order of atomic mass, the periodicity in properties of two elements which have an interval of seven elements in between them can be related.
Limitations of Newland’s octaves are:
- It was only up until calcium that the classification of elements is completed through Newland’s Octaves.
- The invention of noble gases added to the constraints of this methodology since they couldn’t be included in this association without disturbing it fully.
Mendeleev’s Periodic Table
Russian chemist Dmitri Ivanovich Mendeleev put forth his periodic table in 1869. He noticed that the properties of elements, each physical and chemical, had been periodically associated with the atomic mass of the elements.
The Periodic Law (additionally known as Mendeleev’s Law), states that the chemical properties of elements are a periodic function of their atomic weights.
The advantages of Mendeleev’s Periodic table:
- The inclusion of those newly found elements didn't disturb the periodic table. Examples include germanium, gallium, and scandium.
- It was used to appropriate the mistaken atomic weights in use at the moment.
- A variance from the atomic weight order was offered by Mendeleev’s table.
The limitations of Mendeleev’s Periodic table are:
- Hydrogen’s place was within the group of alkali metals, however, hydrogen additionally exhibited halogen like qualities.
- Isotopes had been positioned in another way since this sort of classification of elements was finished by contemplating the atomic weight of the element. Subsequently – protium, deuterium, and tritium would occupy various positions in Mendeleev’s table.
- An anomalous positioning of some elements confirmed that the atomic masses didn't increase frequently from one element to the subsequent. An instance of this might be the location of the cobalt (atomic mass of 58.9) earlier than the nickel (atomic mass of 58.7).
All The Best To All The NEET 2020 Aspirants !