If a neutral atom has $2, 8,$ and $5$ electrons in $K, L,$ and $M$ shells respectively,what is its atomic number?

Give the definition and examples of isobars and isotopes.

Match the following atomic species with their correct classification:

$(i) \ { }_{26} Fe ^{54}, { }_{26} Fe ^{56}, { }_{26} Fe ^{57}, { }_{26} Fe ^{58}$	$(a)$ Isotopes
$(ii) \ { }_1 H ^3, { }_2 He ^3$	$(b)$ Isotones
$(iii) \ { }_{32} Ge ^{76}, { }_{33} As ^{77}$	$(c)$ Isodiaphers
$(iv) \ { }_{92} U ^{235}, { }_{90} Th ^{231}$	$(d)$ Isobars
$(v) \ { }_1 H ^1, { }_1 D ^2, { }_1 T^3$

$A$ sodium cation $(Na^{+})$ has a different number of electrons from:

$_{20}X^{40}$ and $_{21}X^{40}$ are

Give the relation between the following pairs of species:
$(i)$ $_{20}^{40}Ca$ and $_{19}^{40}K$
$(ii)$ $_{17}^{35}Cl$ and $_{17}^{37}Cl$
$(iii)$ $_{18}^{40}Ar$,$_{20}^{40}Ca$ and $_{19}^{40}K$
$(iv)$ $_{38}^{87}Sr$ and $_{38}^{90}Sr$