What is Periodic Table ?
Periodic table is arrangement of chemical elements in the form of table. According to order of their atomic number, electronic configuration or chemical properties.
Why we need Periodic Table ?
Their was more than 115 elements have been discovered, which need to be arranged systematically.
How scientists arranged elements in Periodic Table ?
Scientists work hard to arrange elements in periodic table, today's periodic table is the result of different historical arrangements...
History of classification of elements
1. Prout’s Hypothesis – 1815
2. Dobereiner’s Triads – 1829
3. Newland’s Law of Octaves – 1863
4. Lother Meyer Curves – 1869
5. Mendeleev’s Periodic Table – 1869
6. Modern Periodic Laws of Moseley -1913
7. Modern Periodic Table ( of 18 columns )
According to this hypothesis atomic mass of elements is multiple of mass of hydrogen atom. So elements are related to each other on the basis of their atomic masses. It fails due to the reason that some elements do not have atomic mass in whole no. (Like chlorine have atomic mass of 35.5).
According to Dobereiner’s triads elements arranged in the group of three elements so that atomic mass of central element is appropriate mean of 1st and 3rd element. These groups of three elements are known as Dobereiner’s triads.
Li Na K
Li Na K
7 23 39
7 23 39
Mean of Atomic Masses of 1st and 3rd elements is (7+39)/2 = 46/2 = 23
Newland’s Law of Octaves
According to Newland’s Law of Octaves elements arranged in the group of 8 elements and every 8th element is similar to the 1st element.
Li Be B C N O F
Na Mg Al Si P S Cl
Lother Meyer Curves
In 1869, Lother Meyer derived following conclusion by obtaining information from graph between the atomic weight at x-axis and atomic volume at y-axis.
1. Elements which have similar properties found on the same position on the graph curves.
2. All alkali metals like Li, Na, K, Rb, Cs, and Fr found at highest peak of curve.
3. Halogens like F, Cl, Br and I are found on ascending portion of curve.
4. Alkaline earth metals like Mg, Ca, Sr, Ba and Ra are found on descending portion of curve.
5. Elements which do not melt easily like Be, B, C, Al, Si, Cu etc. are found at lowest point of curve.
On the basis of above conclusions Lother Meyer give periodic Law:
According to Lother Meyer periodic law – “Atomic volume of elements are periodic function of their atomic weights”.
To classify elements a chart is prepared in which elements are arranged in rows and columns. These elements are arranged in table in such a way that the elements with similar properties are repeat after some intervals; also elements with similar properties are put in same column below one another. In periodic table vertical columns are called group while horizontal rows are called periods. In periodic table this repetition of properties in regular interval is known as periodicity. Read below chemistry notes of periodic table like Mendeleev periodic table, Mosley periodic table, Modern periodic table etc.
Mendeleev’s Periodic Table
In Mendeleev’s Periodic Table, Mendeleev classify elements according to their atomic masses and arranged these elements in table according to their increasing order of atomic masses.
Mendeleev’s Periodic Law
According to Mendeleev’s Periodic Law – “Physical and chemical properties of elements are periodic function of their atomic masses”.
Mendeleev’s Periodic Table contains seven horizontal rows known as periods and nine vertical columns known as groups.
Modern Periodic Table
Moseley in 1913, after doing many experiments comes to a conclusion that the elements should be arranged as per their atomic no. and not according to atomic masses. As atomic no. is fundamental property of all elements of periodic table. So Mosley gives a new law which is called is Modern Periodic Law.
Modern Periodic Law
According to Modern Periodic Law- “Physical and Chemical properties of elements are periodic function of their atomic numbers”.
Long Form of Periodic Table
Bohr Bury invented new periodic table, which is called as Long Form of Periodic Table. This Long Form of Periodic Table is also known as Modern Periodic Table.
Special features of Modern or Long Form of Periodic Table
These below are Special features of Modern or Long Form of Periodic Table
I. 18 vertical columns known as groups.
II. Horizontal rows known as periods.
III. Light metals – These are elements of periodic table of group 1 and 2.
IV. Heavy metals or Transition metals - These are elements of periodic table of group 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12.
V. Non-Metals – These are elements of periodic table of group 13, 14, 15, 16 and 17.
VI. Zero group – These are elements of periodic table of group 18.