The number of unpaired electrons in the valence shell of an atom depends on its electronic configuration.
Oxygen (O) has an atomic number of 8, which means it has 8 electrons. The electronic configuration of oxygen is 1s2 2s2 2p4. In the valence shell (the outermost shell), there are a total of 6 electrons. The 2p orbital, which can hold a maximum of 6 electrons, has four electrons in oxygen's case. Among these four electrons, two of them are paired (spin-paired) in one of the 2p orbitals, while the other two are unpaired in separate 2p orbitals. Therefore, oxygen has two unpaired electrons in its valence shell.
On the other hand, nitrogen (N) has an atomic number of 7, meaning it has 7 electrons. The electronic configuration of nitrogen is 1s2 2s2 2p3. In the valence shell, there are a total of 5 electrons. The 2p orbital has three electrons in nitrogen's case. Among these three electrons, two of them are paired (spin-paired) in one of the 2p orbitals, while the remaining electron is unpaired in a separate 2p orbital. Therefore, nitrogen has only one unpaired electron in its valence shell.
The difference in the number of unpaired electrons between oxygen and nitrogen arises due to the distribution of electrons in their respective atomic orbitals, governed by the Pauli exclusion principle and Hund's rule.