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

According to the law of triads :

  1. the properties of the middle element were in between those of the other two members.

  2. three elements arranged according to increasing weights have similar properties.

  3. the elements can be grouped in the groups of six elements.

  4. every third element resembles the first element in periodic table.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

According to Dobereiner's law of triads, the properties of the middle element were in between those of the other two members. The elements were arranged in a group of three (called triads). The atomic weight of a middle element was almost the average of the other two. Three elements of the triad had similar chemical properties.

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

The law of triad is applicable to a group of :

  1. $Cl, Br, I$
  2. $C, N, O$
  3. $Na, K, Rb$
  4. $H, O, N$
Reveal answer Fill a bubble to check yourself
A,B Correct answer
Explanation

For Dobereiner's triads, the atomic weight of middle element is almost average of other two. Thus, the law of triad is applicable to (1) $Cl, Br, I$ and (2) $C, N, O$. However, it is not applicable to (3) $Na, K, Rb$ and (4) $H, O, N$. 


(1) For $Cl, Br$ and $I$, $\displaystyle \dfrac {35.5+127}{2} = 81.25 \simeq 80$ 


(2) For $C, N, O$ $\displaystyle \dfrac {12+16}{2}=14$ 

(3) For $Na, K, Rb$ $\displaystyle \dfrac {23+85.5}{2}=54.25 \neq 39$ 

(4) For $H,O,N$ $\displaystyle \dfrac {1+14}{2}=7.5 \neq 14$

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

Example of Dobereiner's triad is _________.

  1. $Li , Al , Ca$
  2. $Li , Na , K$
  3. $Li , K , Na$
  4. $K , Al , Ca$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$Li,Na,K$ is the example of Dobereineer's traid. Here, average of atomic weights of  'Li' and 'K' equals atomic weight of 'Na'.


$\dfrac{Weight\ of\ Li+ weight\ of\ K}{2}$ =$\dfrac{7+39}{2}$ =$\dfrac{46}{2}$ =23


=weight of $Na$.

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

The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. Such an arrangement of elements called as :

  1. Dobereiner triad.

  2. Mendeleev triad.

  3. Modern triad.

  4. none of the above.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Dobereiner observed in few atoms that mass of one atom is equal to the average of two atoms. He defined almost all properties basing on atomic weight.Later on it was proved wrong any-ways. Hence, this is called Dobereiner triad. Lithium has 7, sodium has 23 where as potassium has 39 as atomic masses.
$ 23 = (7+39)/2 = 23$

Here, we could observe that sodium atomic mass is average of potassium and lithium masses. 

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.