An atom of element X has an electronic configuration of $1s^22S^22p^63s^23p^3.$ In which group of the periodic table is X?

Analyze the question: We're given the electronic configuration 1s²2s²2p⁶3s²3p³ and asked for the group of the periodic table.

Concept: For main-group (s- and p-block) elements, the group number is determined by the number of electrons in the outermost shell (valence electrons). Elements with 5 valence electrons are in Group 5 (Group 15 in the modern IUPAC numbering).

Solution:

1. Count the total electrons: 2 + 2 + 6 + 2 + 3 = 15, so the atomic number is 15. This is phosphorus (P).
2. The outermost shell is n = 3, containing 3s² 3p³.
3. Valence electrons = 2 + 3 = 5.
4. 5 valence electrons means Group 5 (Group VA).

Answer: E. 5

Tip: For s- and p-block elements, add up the electrons in the highest principal shell (s + p). That total is the group number in the older Roman-numeral system (I to VII, with 0 for noble gases). Nitrogen, phosphorus and arsenic all have ns²np³ and all sit in Group 5.