Final answer:
The element with the electron configuration 3s² 3p⁴ is sulfur (S), which corresponds to the 16th element in the periodic table, indicating six valence electrons in the third shell.
Step-by-step explanation:
The element with the electron configuration of 3s23p4 in its outermost shell is sulfur (S). This configuration indicates that there are two electrons in the 3s subshell and four electrons in the 3p subshell, leading to six valence electrons. Sulfur's atomic number is 16, which matches this electron configuration in its ground state. Other elements mentioned do not match this configuration: Silicon (Si) has the electron configuration 3s23p2, Chlorine (Cl) has 3s23p5, and Aluminum (Al) has 3s23p1.