Which pair of elements has the most similar properties?

• A. Li and B
• B. N and P
• C. K and He
• D. I and Ca

Answer: (B)

Elements that sit in the same column share similar properties because they have the same number of valence electrons and similar electron arrangements, which guide how they bond and react. Nitrogen and phosphorus both belong to the same group (the pnictogens) and each has five valence electrons. That common valence leads to similar bonding patterns—mostly covalent bonds with nonmetals and similar types of oxides and compounds—and comparable reactivity trends within that group. Because of this shared electronic structure, nitrogen and phosphorus behave more alike with other elements than do the other pairs, which mix metals with nonmetals or pair elements from different families. So nitrogen and phosphorus show the most similar properties.