Question

From the list of elements given below make three triads and name the family of each triad.

Cl, Li, Ca, Sr, I, K, Na, Br, and Ba

Answer 1

From the list of elements, these are the following three triads:

Dobereiner’s law of triads

• This triads were groups of elements with similar properties that were identified by the German chemist Johann Wolfgang Dobereiner.
• He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties.
• Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad.

1.Li(7), Na(23), k(39) - They are called as alkali metals.

atomic no. of Li: 7

atomic no. of K: 39

So, according to Dobereiner's law: $\frac{7+39}{2}$

= $\frac{46}{2}$

= $23$

So, 23 is the atomic mass of the sodium(Na)

2. Ca(20), Sr(87), Ba(137) - They are called alkaline earth metals.

atomic no. of Ca: 7

atomic no. of Ba: 39

So, according to Dobereiner's law: $\frac{20+137}{2}$

= $\frac{157}{2}$

= $78.5$

But, the atomic no. of Sr is 87 which means they are not a triad.

3. Cl(17), Br(79), I(127) - They are called halogens.

atomic no. of Ca: 7

atomic no. of Ba: 39

So, according to Dobereiner's law: $\frac{17+127}{2}$

= $\frac{144}{2}$

= $72$

But, atomic no. of Br is 79 which means they are not a triad.