Question

Given below are the atomic masses of the elements: 

Which of the following doesn't form triad? 
 

• A. Cl, Br, I
• B. Cl, K, Ca
• C. Li, Na, K
• D. Ba, Sr, Ca

Hint:

The triad rule is often used in understanding periodic trends. The middle element in a triad has an atomic mass that is nearly the average of the other two.

Answer 1

The Correct Option is B

In a triad, the atomic masses of the middle element is approximately the average of the other two.
- Cl, Br, I: The atomic masses of chlorine (Cl), bromine (Br), and iodine (I) are in a triad since the atomic mass of bromine is approximately the average of chlorine and iodine (35.5, 79.9, and 126.9, respectively).
- Cl, K, Ca: The atomic masses of chlorine (Cl), potassium (K), and calcium (Ca) do not form a triad as the middle element's mass (K = 39) is not the average of Cl (35.5) and Ca (40).
- Li, Na, K: The atomic masses of lithium (Li), sodium (Na), and potassium (K) form a triad as the atomic mass of sodium (Na = 23) is the average of Li (7) and K (39).
- Ba, Sr, Ca: The atomic masses of barium (Ba), strontium (Sr), and calcium (Ca) form a triad as the atomic mass of strontium (Sr = 88) is the average of Ba (137) and Ca (40).
Thus, the correct answer is Cl, K, Ca, which does not form a triad.