Question

in the modern periodic table, which of the following represents the last pair of elements for which newlands law of octaves would hold true?
a. al and ga
b. al and si
c. ne and ar
d. mg and ca

Explanation:

Newlands' Law of Octaves states that when elements are arranged in order of increasing atomic mass, every eighth element has properties similar to the first. This law was valid only up to calcium (\(Ca\)). Magnesium (\(Mg\)) (atomic mass - 24.3) and calcium (\(Ca\)) (atomic mass - 40.1) follow the law as \(Ca\) is the eighth element after \(Mg\) in the order of increasing atomic mass (when considering elements known at that time).

• Option A: Gallium (\(Ga\)) was discovered later and Newlands' law was not applicable for it with Aluminium (\(Al\)) as it went beyond the scope (up to \(Ca\)) of the law.
• Option B: Aluminium (\(Al\)) and Silicon (\(Si\)) are adjacent elements and do not follow the octave rule.
• Option C: Neon (\(Ne\)) and Argon (\(Ar\)) are noble gases. Newlands' law was mainly focused on the elements known before noble gases' discovery in a significant way and also beyond the \(Ca\) limit for the law's validity.

Answer:

D. Mg and Ca