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Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

The law of triads was given by :

  1. Dobereiner

  2. Lewis

  3. Mendeleev

  4. Dalton
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In the year 1829, Johann Wolfgang Dobereiner, a German scientist, was the first to classify elements into groups based on John Dalton's assertions. He grouped the elements with similar chemical properties into clusters of three called 'Triads'. The distinctive feature of a triad was the atomic mass of the middle element. When elements were arranged in order of their increasing atomic mass, the atomic mass of the middle element was approximately the arithmetic mean of the other two elements of the triad.

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

If the two members of a Dobereiner's triad are chlorine and iodine, then the third member of this triad is :

  1. fluorine

  2. bromine

  3. sodium

  4. calcium
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

According to Dobereiner, the atomic mass of the middle element is nearly equal to the arithmetic mean of the other two. Therefore, the atomic mass of the third element is $=$ $\dfrac{{35.5}+{127}}{2}$ $=$ 81.25, which is the atomic mass of bromine.

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

If the two members of a Dobereiner triad are phosphorus and antimony, the third member of this triad is :

  1. arsenic

  2. sulphur

  3. iodine

  4. calcium
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

According to Dobereiner, the atomic mass of the middle element was nearly equal to the arithmetic mean of the other two. Therefore, the atomic mass of the third element is $=$ $\frac{{31}+{121.75}}{2}$ $=$ 76.37 which is nearly equal to the atomic mass of arsenic.

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

Which of the following sets of atomic masses do not correspond to Dobereiner's triad?

  1. 7, 23, 39

  2. 20, 38, 56

  3. 47, 91, 178

  4. 54, 95, 183
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

The atomic mass of the middle element should be the average of the atomic masses of the first and third elements. In case of C and D options, the atomic masses do not correspond to that rule.

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

Which of the following elements follow Dobereiner's law of Triads?

  1. Na = $23$, Li = $7$ and K = $39$
  2. Cl = $35$, Br = $80$ and F = $19$
  3. Ca = $40$, Mg = $24$ and Sr = $88$
  4. Na = $23$, K = $39$ and Rb = $85$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

According to Dobereiner's, the atomic weight of the middle element is nearly the same as the average of the atomic weights of the other two elements.


$Li(7)+K(39)\longrightarrow \cfrac{7+39}{2}=Na(23)$

So, $Na=23, Li=7$ and $K=39$ 

The correct option is ( A ) 

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

Following triads have approximately equal size

  1. ${ Na }^{ + },\quad { Mg }^{ 2+ },\quad { Al }^{ 3+ }$
  2. ${ F }^{ - },\quad Ne,\quad { O }^{ 2- }$
  3. $Fe,\quad Co,\quad Ni$
  4. ${ Mn }^{ + },\quad { Fe }^{ 2+ },\quad Cr$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Fe, Co, and Ni are transition metals located in the same period (Period 4) and have very similar atomic radii, making them a triad of approximately equal size.

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

The law of  traids is not applicable on

  1. $Cl, Br,I$
  2. $Na, K, Rb$
  3. $S, Se, Te$
  4. $Ca, Sr, Ba$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Dobereiner's Law of Triads states that the atomic weight of the middle element is approximately the average of the other two. Na, K, and Rb do not fit this pattern because Rb is significantly heavier than the average of Na and K.