Question

Lithium(Li), sodium(Na), and potassium(K) were put in the same group on the basis of similar properties.

(a) What are the similarities in their properties.

(b) If the atomic weight of lithium and potassium are 7 and 39 respectively, what is the atomic weight of sodium?

Answer 1

(a) Similar properties of Lithium, sodium, and potassium:

1. They all are soft metals.
2. They all have one electron in their outer shell.
3. They all can form a uni-positive ion.
4. They all are reducing agents.
5. They all impart color to the flame.

(b) Atomic weight of sodium:

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)

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.