Which one of the following is the correct electronic configuration of chlorine?

1. Fundamentals of Atomic Structure (basic)

To master chemistry, we must first understand the atom: the smallest unit of an element that retains all its chemical properties Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100. While the universe began with simpler particles, it took approximately 300,000 years after the Big Bang for temperatures to cool enough for electrons to combine with protons and neutrons to form the very first atoms Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2.

Every atom consists of two main regions: the Atomic Nucleus and the electron shells. The nucleus is a tiny, dense, positively charged center containing protons (positive) and neutrons (neutral). The identity of an element is defined by its Atomic Number, which is simply the number of protons in its nucleus. Orbiting this nucleus are electrons (negative), which are arranged in specific energy levels called shells.

According to the Bohr model, these shells—labeled K, L, M, and N—have a maximum capacity for electrons. The K shell (the one closest to the nucleus) can hold only 2 electrons, while the L shell can hold 8. For an atom to be chemically stable, it generally seeks to have a full outermost shell, often referred to as a stable octet (8 electrons). This drive for stability explains why atoms react: for instance, Chlorine has 17 electrons arranged as 2, 8, 7. Because it has 7 electrons in its M shell, it is highly reactive as it seeks just one more electron to complete its octet Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46.

Sources: Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

2. Atomic Number and Mass Number (basic)

To understand chemistry, we must first look into the identity card of an atom. Every element is defined by two fundamental numbers: the Atomic Number and the Mass Number. Think of the Atomic Number as the unique fingerprint of an element—no two different elements can have the same atomic number.

The Atomic Number (Z) represents the number of protons present in the nucleus of an atom. In a neutral atom, this also tells us the number of electrons. For instance, Hydrogen has an atomic number of 1, meaning it has one proton and one electron Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. Similarly, Nitrogen has an atomic number of 7, which dictates its electronic configuration and how it forms bonds Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60.

While the atomic number defines the element, the Mass Number (A) tells us how "heavy" the atom is. Since electrons have negligible mass, the mass of an atom is concentrated in its nucleus, which contains protons and neutrons (collectively called nucleons). The Mass Number is simply the sum of these two. We can calculate the number of neutrons by subtracting the atomic number from the mass number (Neutrons = A - Z).

Understanding these numbers allows us to map out the Electronic Configuration of an atom. For example, Chlorine has an atomic number of 17. Following the Bohr model, its 17 electrons are distributed as 2 in the K shell, 8 in the L shell, and 7 in the outermost M shell Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46. This specific arrangement, derived directly from the atomic number, is what makes Chlorine a highly reactive non-metal.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46

3. Organization of the Periodic Table (intermediate)

To understand the Organization of the Periodic Table, we must first understand the concept of periodicity. Just as the phases of the moon are "natural periodic events" that repeat in a cycle Science Class VIII, Keeping Time with the Skies, p.178, the chemical properties of elements repeat at regular intervals when they are arranged by their atomic number. This arrangement is not random; it is dictated by the way electrons occupy space around the nucleus.

Electrons are organized into energy levels called shells (labeled K, L, M, and so on). Each shell has a specific capacity: the first shell (K) can hold 2 electrons, the second (L) can hold 8, and the third (M) typically seeks to reach 8 for stability. The chemical "personality" of an element is determined almost entirely by its valence electrons—the electrons in its outermost shell. Elements with the same number of valence electrons are placed in the same vertical Group because they react in similar ways.

Consider Chlorine (Cl), which has an atomic number of 17. In a neutral state, it has 17 electrons distributed as follows: 2 in the K shell, 8 in the L shell, and 7 in the M shell (2, 8, 7). Because it is just one electron short of a stable "octet" (8 electrons), it is highly reactive. While the Activity Series often ranks metals by how easily they lose electrons Science Class X, Metals and Non-metals, p.45, non-metals like Chlorine are characterized by their strong urge to gain electrons to achieve a stable configuration similar to a Noble Gas.

Sources: Science Class VIII, Keeping Time with the Skies, p.178; Science Class X, Metals and Non-metals, p.45

4. Valency and Chemical Reactivity (intermediate)

At the heart of chemistry is a simple quest for stability. We define chemical reactivity as the tendency of an atom to achieve a completely filled outermost shell, reaching a state of equilibrium similar to the noble gases like Helium, Neon, or Argon. These gases are chemically inert because their valence shells are already satisfied—Helium with a duplet (2 electrons) and others with an octet (8 electrons) Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46. For all other elements, reactivity is essentially the "work" they do to reach this stable configuration.

This brings us to the concept of Valency, which is the combining capacity of an atom. It is determined by the number of valence electrons—those residing in the outermost shell. For instance, consider Chlorine (Cl) with an atomic number of 17. Its electrons are distributed as 2 in the K shell, 8 in the L shell, and 7 in the M shell (2, 8, 7) Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.47. Since it only needs one more electron to complete its octet, its valency is 1. Rather than losing seven electrons (which would be energy-intensive), it prefers to gain or share one.

Elements follow different strategies to achieve this stability based on their electronic makeup:

In cases like Carbon, which has 4 electrons in its outermost shell, losing or gaining four electrons would require massive amounts of energy. Instead, carbon chooses to share its valence electrons with other atoms Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. This sharing creates covalent bonds, allowing both atoms to "count" the shared electrons toward their stable noble gas configuration.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.47; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60

5. Electron Distribution: Bohr-Bury Scheme (exam-level)

To understand how atoms interact, we must first understand how they organize their electrons. The Bohr-Bury Scheme provides the fundamental rules for this distribution. Electrons revolve around the nucleus in fixed circular paths called energy levels or shells, designated as K, L, M, N, and so on. The distribution follows a mathematical logic: the maximum number of electrons any shell can hold is given by the formula 2n², where 'n' is the orbit number.

According to this rule, the capacities are as follows:

However, capacity isn't the only rule. The Bohr-Bury Scheme adds two critical constraints: first, electrons are not accommodated in a given shell unless the inner shells are filled (a step-wise filling). Second, the outermost shell of any atom cannot accommodate more than 8 electrons, regardless of its total capacity. For example, Chlorine (Atomic Number 17) distributes its 17 electrons by filling the K shell with 2, the L shell with 8, and the remaining 7 electrons go into the M shell Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. This configuration is written as 2, 8, 7.

This distribution dictates an atom's chemical reactivity. Elements naturally strive to attain a "completely filled outer shell," known as a noble gas configuration, because this state is chemically stable Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. Atoms with nearly full or nearly empty outer shells, like Chlorine (7 valence electrons) or Sodium (1 valence electron), are highly reactive as they seek to gain or lose electrons to reach that stable octet Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59-60; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

6. Electronic Configuration of the First 20 Elements (exam-level)

To understand how elements behave, we must look at their electronic configuration—the specific arrangement of electrons around the nucleus. Think of an atom like a small stadium where electrons occupy specific rows or "shells." These shells are labeled K, L, M, and N, starting from the one closest to the nucleus. According to the Bohr model, each shell has a maximum capacity determined by the formula 2n² (where n is the shell number). Thus, the K-shell (n=1) can hold 2 electrons, while the L-shell (n=2) can hold 8 Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60.

The reactivity of an element is driven by its desire to achieve a noble gas configuration—a state where its outermost shell is completely filled, usually with 8 electrons (the octet rule). For example, Carbon has an atomic number of 6, meaning it has 6 electrons. These are distributed as 2 in the K-shell and 4 in the L-shell. Because it has 4 valence electrons (electrons in the outermost shell), it must gain, lose, or share electrons to reach stability Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. This "search for stability" explains why elements form bonds and participate in chemical reactions.

Let’s look at Chlorine (Atomic Number 17) as a key example. Following our rules, we fill the shells in order: the K-shell takes 2, the L-shell takes 8 (total 10), leaving 7 electrons for the M-shell. This configuration of 2, 8, 7 tells us Chlorine is just one electron short of a stable octet (2, 8, 8, like Argon). This makes Chlorine highly reactive; it will aggressively seek to pull an electron from another atom to complete its shell Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. In the upper atmosphere, this high reactivity allows even a single chlorine atom to break down thousands of ozone molecules Environment, Shankar IAS Academy (10th ed.), Ozone Depletion, p.268.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59-60; Environment, Shankar IAS Academy (10th ed.), Ozone Depletion, p.268

7. Solving the Original PYQ (exam-level)

Now that you have mastered the Bohr model and the rules for electron distribution, this question serves as a perfect application of those fundamental building blocks. To solve this, you must first recall that Chlorine (Cl) has an atomic number of 17, meaning a neutral atom possesses 17 electrons. Using the 2n² rule you recently studied, you know that electrons must fill shells in order of increasing energy: the first shell (K) accommodates a maximum of 2, and the second shell (L) accommodates 8. This accounts for 10 electrons, leaving 7 for the third shell (M), which leads us straight to the correct answer (B) 2, 8, 7.

UPSC often includes options that test your precision regarding chemical stability and shell capacity. For instance, option (C) 2, 8, 8 is a classic trap; while it represents a stable octet, this is the configuration of the Chloride ion (Cl⁻) or the noble gas Argon, not a neutral Chlorine atom. Option (A) 2, 7, 8 is incorrect because it suggests the outer shell is being filled before the second shell has reached its 8-electron capacity, violating the Aufbau principle. Understanding these nuances, as outlined in NCERT Class 9 Science, ensures you can distinguish between an element's ground state and its ionic form under exam pressure.