Periodic Table

Since ancient times, many elements were known to us. Elements like copper and iron were used since the early civilization. By 1800 around 30 elements were known to the chemists. Many elements were discovered later and now 114 elements are known to us. Chemists have tried to classify the elements based on their chemical and physical properties like metals and non-metals, atomic mass, atomic number.

Law of Triads

Dobereiner a German scientist classified tri-element on basis of same property like metal, non-metal or gas. Johann Wolfgang Döbereiner put forward his law of triads in 1817. Following elements were grouped on the basis of their atomic weights.

Element	Symbol	Atomic weight
Calcium	Ca	40.1
Strontium	Sr	87.6
Barium	Ba	137.3

Element	Symbol	Atomic weight
Suphur	S	32.1
Selenium	Se	79
Tellurium	Te	128

Newlands’ Law of Octaves

In 1864, John Newlands an English scientist put forward his law of octaves. He found that properties of elements repeat periodically. He arranged 56 elements (known at that time) starting from H to Th according to their atomic weight.

Mendeleev’s periodic table

In 1869, Dmitri Mendeleev a Russian chemist arranged elements (known at that time) according to atomic weight. Mendeleev’s periodic table played foundation for modern periodic table. He grouped elements with similar chemical and physical properties vertically and arranged them periodically in a table. He also predicted properties of missing (undiscovered) elements.

Modern Periodic Table

Henry Mosley proposed modern periodic table. In the Modern Periodic Table the elements have been arranged in a tabular format on the basis of their atomic number. The Modern Periodic Table has 18 vertical columns known as groups and 7 horizontal rows known as periods. The table has been arranged in blocks. The s (sharp) – block with 2 elements occupies the left portion, the d (diffused) – block with 10 elements occupies the middle portion and p (principal) – block with 6 elements occupy the right portion of the table. The f (fundamental) – block with 14 elements are the lanthanides and actinides.

Comparison of modern Periodic table and Mendeleev periodic table

1. Mendeleev’s periodic table has property of only 63 known elements where as modern table has known 114 elements
2. Mendeleev left some space for undiscovered elements but modern table has known elements all well-organized.
3. Mendeleev placed cobalt, nickel, tellurium and iodine in wrong position which was rectified.
4. Mendeleev made periodic table place according to atomic weight but modern table has elements according to atomic number.
5. Mendeleev’s periodic table has 9 vertical groups where as modern table has 18 vertical groups.

Defects of Mendeleev periodic table

1. Position of hydrogen is ambiguous.
2. Position of lanthanum not given.
3. Placement of rare earth ambiguous
4. Position of isotopes not given
5. Only 9 groups of elements.
6. Some similar in property placed in different group e.g. copper and mercury, silver and thallium, barium and lead.
7. Some dissimilar elements placed in same group e.g. coinage metal and basic metal manganese and halogen.
8. Placing  Ar (39.9) before K(39.1), Co(59.9) before Ni (58.7), Te(127.6) before Iodine(126.9), Th(232.1) before Pr(231).

Halogen

Chlorine (Cl), Bromine (Br), Fluorine (F), Iodine (I) are called halogen. They are univalent electron loving elements.

Noble gases

Helium, neon, argon, krypton, xenon and radon exist in free state in nature. They are called inert gas or noble gas. They are colorless, odorless and tasteless gas and their chemical reaction with other substances is extremely difficult.

Similarities between Hydrogen and Halogen

• Both can gain one electron in outward shell.
• Both bimolecular.
• Both non metallic bad conductor of electricity and heat.
• NaH and NaCl are both electronegative.
• Both collected in anode on electrolysis

Reducing Property

Reducing property increase from up to down and oxidation property decrease because electron donor capacity increased.