Question

Explain the following :
1)Law of triads
2)Periodic law
3)Law of Octave

Answer 1

Answer:

1) Law of triads

  Dobereiner''s Triad- Law of Triads

Johann Wolfgang Döbereiner (December 13, 1780 – March 24, 1849) was a German Chemist who is best known for work that foreshadowed the periodic law for the chemical elements.

In 1829, Dobereiner, a German scientist made  some groups of three elements each and called  them triads.  

He observed that the atomic mass of the middle element of a triad was nearly equal to the arithmetic mean of  atomic masses of other two elements. All three elements of a triad were similar in their properties.

Let us take the example of three elements lithium, sodium and potassium. They form a Dobereiner’s triad.

Elements  

Atomic mass

Lithium (Li)

7

Sodium (Na)

23

Potassium(K)

39

Mean of the atomic masses of the first (Li) and the third (K) elements: 7+39/ 2 = 23 u

The atomic mass of the middle element, sodium, Na =23 u

Other example..Triads.

Elements  

Atomic mass

calcium(Ca)

40

barium(Ba)

88

Strontium(K)

137

Elements  

Atomic mass

Chlorine

35.5

Bromine  

80

Iodine

127

Mean of the atomic masses of the first and third elements =40+137/2 = 88.5 u= 88u

Mean of the first atomic masses of the and third elements =   35.5 +127/2 = 81.5 u= 80 u

Demerits:

Dobereneir’s idea of classification of elements into triads did not receive wide acceptance as he could arrange only a few elements in this manner.

Extra Information:  

Döbereiner also is known for his discovery of furfural , for his work on the use of platinum as a catalyst, and for a lighter, known as Dobereiner's Lamp.

2) Periodic Law

 Periodic law, also known as Mendeleev's law, is the concept that the chemical and physical properties of elements are based on an element's atomic weight when arranged by increasing periodic atomic number. Periodic law was first developed in 1869 when Dimitri Mendeleev and Lothar Meyer each developed a periodic table on their own and noticed similarities in elements with comparable atomic masses. Both men organized the elements by their masses and noticed certain properties recur periodically.  

Mendeleev organized his periodic table by listing the elements with like characteristics into columns called groups. He left blank spaces for the elements that had yet to be discovered, such as scandium, gallium, technetium and germanium. Noble gases were also left off of Mendeleev’s table because they had yet to be discovered. Meyer also organized his chart according to atomic mass, but based his periodic law on the molar volume of the elements, which is the atomic mass divided by the density. Mendeleev’s table, however, is still considered noteworthy because of the accuracy of the elements’ atomic masses.

Mendeleev assumed that atomic weight was inaccurate, which was his main reason for not organizing his periodic table by increasing atomic weight. In 1913, Henry Moseley hypothesized that the properties of the elements were based on its nuclear charge, not its nuclear weight. He X-rayed the elements and used the following formula to confirm his suspicions: V = A(Z-b)2.

(V is equal to X-ray frequency. Z is the atomic number, and A and b are constants.) After his experiment, Moseley revised periodic law to state the following: "Similar properties recur periodically when elements are arranged according to increasing atomic number."

3)Law of Octave

The Law of Octaves is about the patterns of elements in the Periodic Table, stating that when elements are aligned according to their atomic weight, every eighth element shares similar properties. For example, hydrogen, fluorine, chlorine, bromine and iodine are all on the same interval.

Dictionary.com points out that in the Law of Octaves, an interval of seven elements separate two with similar properties. For instance, the first line of elements, hydrogen, fluorine, chlorine, bromine and iodine, are all halogens. Halogens cannot be found in nature with just a single atom. A hydrogen molecule always has two hydrogen atoms attached together. Also, fluorine, chlorine, bromine and iodine are all in the form of salt. They are either highly toxic or volatile. Another group of similar elements is the noble gases, which include helium, neon, argon, krypton, xenon and radon. They are gaseous in their natural state and have low boiling temperatures.

The Law of Octaves was first founded by John Newlands, a chemist, in 1865. Although Newlands was the first person to group elements according to patterns, another chemist improved on the fledgling Periodic Table by leaving room for undiscovered elements, ushering in the modern Periodic Table. Later, Newlands was credited with discovering the Periodic Law.

Explanation: