Write the name, symbol and electronic configuration of an element $\mathrm{X}$ whose atomic number is 11.
OR
Can the following groups of elements be classified as Dobereiner's triad:
(a) $\mathrm{Na},\mathrm{Si},\mathrm{Cl}$
(b) $\mathrm{Be},\mathrm{Mg}$, $\mathrm{Ca}$
Atomic mass of Be-9, Na-23, Mg-24, Si-28, Cl$35,\mathrm{Ca}-40$. Justify your answer in each case.