The number of electronic shells contained in an atom with electron configuration 1S$^2$2S$^2$2P$^6$3S$^2$3P$^6$4S$^2$ is

Analysis of the Question

The question asks for the number of electronic shells present in an atom based on its given electron configuration: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$.

Explanation of the Concept

In electron configuration, the principal quantum number ($n$) represents the main energy level or "shell" of an atom. These are written as the large numbers preceding the subshells ($s, p, d, f$).

• The configuration is written in order of increasing energy.
• The highest number appearing as a coefficient in the configuration indicates the outermost shell (valence shell) that contains electrons.
• The number of shells is simply the value of the highest principal quantum number found in the configuration.

Step-by-Step Solution

1. Examine the configuration: $1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^2$.
2. Identify the coefficients: The coefficients (the numbers before the letters) are 1, 2, 2, 3, 3, and 4.
3. Determine the highest coefficient: The highest number among these is 4.
4. Interpret the result: This means the electrons occupy energy levels 1, 2, 3, and 4. Therefore, there are 4 electronic shells in this atom.

Conclusion

Since the highest principal quantum number is 4, the atom has 4 electronic shells. This corresponds to option C.

Exam Tip: Always look at the largest number written before the $s, p, d,$ or $f$ orbitals. Don't be distracted by the exponents (the small numbers at the top), as those represent the number of electrons, not the shell number!