Q.Electronic configuration of species M$^{2+}$ is 1s$^2$ 2s$^2$ 2p$^6$ 3s$^2$ 3p$^6$ 3d$^6$ and its atomic weight is 56. The number of neutrons in the nucleus of species M is

An atom's electronic configuration - the complete distribution of its electrons among orbitals - is written using the notation $nl^x$, where $n$ is the shell number, $l$ is written as the subshell's letter (s, p, d, f for $l=0,1,2,3$), and $x$ is the number of electrons in that subshell. Building up a configuration applies the Aufbau principle (fill orbitals in increasing-energy order), Pauli's exclusion principle (max. two electrons per orbital, opposite spins) and Hund's rule (one electron in each degenerate orbital before any pairing) together - for example carbon works out to $1s^2\,2s^2\,2p^2$ with the two 2p electrons unpaired in separate orbitals.

Two elements are the standard textbook exceptions to the configuration the Aufbau principle alone would predict: chromium ($Z=24$) is observed as $[\text{Ar}]3d^5\,4s^1$ rather than the "expected" $[\text{Ar}]3d^4\,4s^2$, and copper ($Z=29$) is observed as $[\text{Ar}]3d^{10}\,4s^1$ rather than $[\text{Ar}]3d^9\,4s^2$. In both cases a 4s electron shifts into 3d so the d-subshell becomes exactly half-filled ($d^5$) or exactly completely filled ($d^{10}$), because those particular configurations carry extra stability (from symmetry and exchange energy) that outweighs the energy cost of vacating the otherwise lower-energy 4s orbital.