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Q.
The correct arrangement of decreasing order of atomic radius among $Na, K, Mg$ and $Rb$ is
Classification of Elements and Periodicity in Properties
Solution:
$Na, K$ and $Rb$ are of group $1$ elements and size increase down the group $1$ elements $(v)$.
$Na$ and $Mg$ are of group $1$ and $2$ elements respectively but both belong to $3$ rd period. Along the period size decreases $(\to),$ i.e. size of $Na > Mg$.
Hence the order of sizes is
