The number of electron shells in an atom can be determined by using the periodic table and the atomic number of the element. Each electron shell is represented by a principal quantum number (n), and the value of n corresponds to the shell number. The formula for finding the number of shells is:
Number of Shells = Principal Quantum Number (n) = Atomic Number
In simpler terms, the number of shells in an atom is equal to its atomic number. For example, hydrogen (atomic number 1) has one electron shell, helium (atomic number 2) has two electron shells, lithium (atomic number 3) has three electron shells, and so on.