Question

in what period is selenium? in what group?
period =
group =

Explanation:

Step1: Recall selenium's atomic number and electron configuration. Selenium (Se) has an atomic number of 34. Its electron configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^4$.

Step2: Determine the period. The period number is equal to the highest principal quantum number (n) of the electron configuration. Here, the highest n is 4, so the period is 4.

Step3: Determine the group. For main - group elements, the group number can be determined by the number of valence electrons. Selenium is a main - group element, and the number of valence electrons in its outermost shell (n = 4) is $2 + 4=6$ (from $4s^24p^4$). For non - transition elements, the group number for elements in the p - block is equal to 10+ the number of valence electrons in the p - subshell plus the electrons in the s - subshell of the outermost shell. So, group number is 16 (or 6A in the old numbering system).

Answer:

Period = 4
Group = 16