Answer: N (atomic number 7)
Explanation:
1) Due the electron distribution of each, taking into account Hund's rule to deal with pairing of electrons.
2) Hund's rule: each subshell orbital is occupied with single electrons of parallel spin before double occupation occurs.
That is, before any one orbital is doubly occupied, every orbital in a subshell is singly occupied with one electron, and all the electrons in singly occupied orbitals have the same spin, which means that they are unpaired.
3) Distributions:
a) C (atomic number 6) ⇒ 6 electrons:
⇒ 1s² 2s² 2p² ⇒ 1s² 2s² 2p ↑ ↑ : as per Hund's rule the two electrons on 2p are unpaired.
b) O (atomic number 8) ⇒ 8 electrons
⇒ 1s² 2s² 2p⁴ ⇒ 1s² 2s² 2p⇅ ↑ ↑: as per Hund's rule the two 2p electrons are unpaired
c) N (atomic number 7) ⇒ 7 electrons
1s² 2s² 2p³ ⇒ 1s² 2s² 2p↑ ↑ ↑: as per Hund's rule, the three 2p electrons are unpaired, so this is the answer to the question.
c) Be (atomic number 4) ⇒ 4 electrons
⇒ 1s² 2s² ⇒ none p electron.