How does arsenic resemble nitrogen and phosphorus in its outer electron shell?

Arsenic resembles nitrogen and phosphorus because all three elements possess five electrons in their outermost shell. Specifically, this configuration consists of two electrons in an outer s orbital and one electron in each of the three outer p orbitals.12

This shared electronic structure has several chemical implications:

• Bonding: The three half-filled outer p orbitals allow each of these elements to form three covalent bonds with other atoms.2
• Lone Pairs: After forming these three bonds, each atom retains an unshared, or lone, pair of electrons. This lone pair can allow the atoms to act as electron pair donors in the formation of complexes, though this ability is generally less pronounced in arsenic than in nitrogen or phosphorus.12
• Oxidation States: Due to their valence electron configuration, these elements can exist in oxidation states of +3 or -3 in their compounds, depending on the relative electronegativity of the elements with which they are combined.12

While their outer shells are similar, arsenic differs from nitrogen and phosphorus by having 18 electrons in its penultimate shell, rather than two or eight.13