Answer
Solutions for problem 1.1 Three rules must follow to write the ground state electronic configuration of an atom/ion
1) Aufbau principle which states that lowest energy orbital must be filled first than higher energy orbital
2) Hund's rule which states that if two or more orbitals of same energy are available then these must fill singly first before pairing in them.
3) Pauli's exclusion principle which states that no more than two electrons can be filled in an orbital and these two electrons must have opposite spins
So following these three principles (1) oxygen having 8 electrons will show ground state electronic configuration 1s² 2s² 2p⁴
(2) Nitrogen having atomic number 7 means having 7 electrons gives ground state electronic configuration 1s² 2s²2p³
(3) Sulfur with atomic number 16 shows electronic configuration 1s² 2s² 2p6 3s²3p⁴
Work Step by Step
$a)$ Oxygen has an atomic number of 8, and has a total of 8 electrons. So when filling in the proper order the ground state would be $1s^{2}2s^{2}2p^{4}$
$b)$ Nitrogen has an atomic number of 7, and has a total of 7 electrons. So when filling in the proper order the ground state would be $1s^{2}2s^{2}2p^{3}$
$c)$ Sulfur has an atomic number of 16, and has a total of 16 electrons. So when filling in
the proper order the ground state would be $1s^{2}2s^{2}2p^{6}3s^{2}3p^{4}$
