Q.An atom of an element contains 35 electrons and 45 neutrons. Deduce

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. …