## Chemistry 10th Edition

From the given data we can write the electronic configuration as; 1s$^{2}$2s$^{2}$2p$^{6}$3s$^{2}$3p$^{6}$4s$^{2}$. Given that the atom is neutral, so number of electrons= atomic number=20. Highest principal quantum number is 4, hence the atom is in 4th period. Since the last electron enters in s orbital and it is completely filled the atom belongs to alkaline earth metals. Number of electrons in each sub-shell is clear from electronic configuration.