18 electrons
Shells of an element are denoted using letters K, L, M N, O, P... The expression for calculating the number of electrons in the shell of an element is expressed as:
[tex]e=2n^2[/tex]where:
n is the number of shells
Given the following parameters:
n = 3
Substitute the given value of "n" into the formula to have:
[tex]\begin{gathered} e=2(3)^2 \\ e=2(9) \\ e=18 \end{gathered}[/tex]Hence the maximum number of electrons that may be placed in the third shell is 18 electrons