Periodic Table of Elements
More than 115 elements have been discovered which need to be arranged systematically, first elements were classified as metals and non metals.
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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 )
Prout’s Hypothesis
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).
Dobereiner’s Triads
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 3rdelement. These groups of three elements are known as Dobereiner’s triads.
Example-
Element
Li Na K
Li Na K
Atomic Mass
7 23 39
7 23 39
Mean of Atomic Masses of 1stand 3rd elements is (7+39)/2 = 46/2 = 23
Drawback of Dobereiner’s triads : All elements are not arranged in triads.
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
Drawback of Newland’s Law of Octaves :
Heavy elements do not follow this law, inert gases disturb the order of octave.
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”.
Periodic Table
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.
Groups of Mendeleev’s Periodic Table
1. In original Mendeleev’s Periodic Table only 8 groups present because Zero group contains noble gases is added later after discovery of noble gases.
2. So, total vertical column is 9 including group 1 to 8 and Zero group.
3. Group 1 to 7 are divided into two subgroups (A and B) each.
4. In eighth group, three elements found together in each period (from fourth period onward) known as transition triplet.
Periods of Mendeleev’s Periodic Table
1. Mendeleev’s Periodic Table has seven periods.
2. First period is known as very short period because contain only two elements.
3. Second and Third period is known as short period because contain only 8 elements each.
4. Fourth and Fifth period is known as long period because contain 18 elements each.
5. Sixth period is known as very long period because contain 32 elements each, 18 elements in table and remaining 14 elements called Lanthanides (from atomic no. 58 to 71)
6. Seventh period is known as incomplete period because it contains only 24 elements and some places are left vacant for elements as they are not known yet. 10 elements in the table and remaining 14 elements called Actinides (from atomic no. 90 to 103) placed outside at bottom of periodic table.
Demerits of Mendeleev’s Periodic Table
These below points are some Demerits of Mendeleev’s Periodic Table
1. Position of Hydrogen: Hydrogen is placed in 1st as well as in 7thgroup because its properties are similar to both groups, which is not explained well.
2. Elements which have similar properties are placed apart in different groups.
Example: Copper and Mercury with similar properties placed in group first and second.
3. Elements with different properties are placed in one group.
Example: Coin metals like Li, Na, K, Rb, Cs.
4. Atomic weights are not kept in serial order.
Example: Argon with higher atomic weight 39.948 is placed before potassium, which have lower atomic weight of 39.102
5. Position of isotope is not proper because all isotopes of elements are placed in one group though the atomic weights of isotopes are different.
6. Eighth group elements position is not appropriate because elements put in triplet form.
7. Rare earth elements (Lanthanides and Actinides) position is not proper so they placed outside at bottom of periodic table.
Modern Periodic Law and 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
1. 18 vertical columns known as groups.
2. Horizontal rows known as periods.
3. Light metals – These are elements of periodic table of group 1 and 2.
4. Heavy metals or Transition metals - These are elements of periodic table of group 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12.
5. Non-Metals – These are elements of periodic table of group 13, 14, 15, 16 and 17.
6. Zero group – These are elements of periodic table of group 18.
Properties of periods
1. All periods start with alkali metal and end with noble gases.
2. If we see table, it is very clear that from 1st to 7thperiod there is an addition of one shell in the each period like n=1,2,3,4,5………………
Properties of groups
1. In any group, outermost shell electron are known as valance electrons and these electrons are same so main properties of elements of group is similar.
2. Elements are divided into four blocks, which is s, p, d, f according to valance electrons.
3. s-block elements – elements of 1 and 2 group.
4. p-block elements – elements of 13 to 18 group.
5. d-block elements – elements of 3 to 12 group.
6. f-block elements – elements of the Lanthanide and Actinide series.
7. Representative Elements – elements of s-block and p-block collectively called as Representative elements also known as Normal elements or Typical elements.
8. Transition Elements – elements of d-block.
9. Inner Transition Elements – elements of f-block, also known as Rare Earth Elements.
10. Alkali Metals – elements of 1stgroup.
11. Alkaline Earth Metals – elements of 2ndgroup.
Solution for the demerits of Mendeleev’s Periodic Table in the Long Form of Periodic Table (or Modern Periodic Table)
1. Mendeleev’s Periodic Table contains some heavy elements placed before lighter elements, this problem is solved automatically in Modern Periodic Table
2. All isotopes of an element have same atomic number so no need to place them separate.
3. All inert gases (or noble gases) got their appropriate place in zero group (or 18thgroup).
4. Mendeleev’s Periodic Table have problem with 8th group but it is resolved in Modern Periodic Table as it is divided into three groups.
