Electron configuration of Chlorine atom is

Analysis

To find the electron configuration of an atom, we must first determine its atomic number ($Z$). Chlorine ($Cl$) is a halogen located in Group 7 of the periodic table, and its atomic number is 17. This means a neutral chlorine atom has 17 electrons that must be arranged in orbitals according to the Aufbau principle.

Concept: The Aufbau Principle

The Aufbau principle states that electrons fill atomic orbitals in order of increasing energy levels. The sequence for filling orbitals is: $1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \dots$

Capacity of orbitals:

• s-orbital: holds a maximum of 2 electrons
• p-orbital: holds a maximum of 6 electrons

Step-by-Step Solution

We need to distribute 17 electrons into the orbitals following the sequence:

1. 1s orbital: Fill with 2 electrons ($1s^2$). Remaining: $17 - 2 = 15$.
2. 2s orbital: Fill with 2 electrons ($2s^2$). Remaining: $15 - 2 = 13$.
3. 2p orbital: Fill with 6 electrons ($2p^6$). Remaining: $13 - 6 = 7$.
4. 3s orbital: Fill with 2 electrons ($3s^2$). Remaining: $7 - 2 = 5$.
5. 3p orbital: Place the remaining 5 electrons here ($3p^5$).

Combining these, we get: $1s^2 2s^2 2p^6 3s^2 3p^5$

Verification

Adding the superscripts: $2 + 2 + 6 + 2 + 5 = 17$. This matches the atomic number of Chlorine.

Comparing this to the provided options:

• Option A: $1s^2 2s^2 2p^6 3s^2 3p^5$ (Matches our calculation)
• Option B: $1s^2 2s^2 2p^6 3s^2 3p^4$ (Total = 16 electrons)
• Option C: $1s^2 2s^2 2p^6 3s^2 3p^6$ (Total = 18 electrons, which is Argon)
• Option D: $1s^2 2s^2 2p^5 3s^2 3p^6$ (Total = 17 electrons, but violates the order of filling)

Exam Tip

Always remember the "2-8-7" shell structure for Chlorine. The $1s^2$ is the 1st shell (2 electrons), $2s^2 2p^6$ is the 2nd shell (8 electrons), and $3s^2 3p^5$ is the 3rd shell (7 electrons). If you ever forget the long-form configuration, just remember the shell distribution and fill the sub-shells accordingly!