Atomic Physics

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.

The number of orbitals in any shell is given by , where is the principal quantum number of the shell. For the N shell, the principal quantum number . Therefore, the number of orbitals in the N shell is: Thus, the total number of orbitals in the N shell is 16.