For an element with atomic number 35, which one of the following will be the correct number of electrons in its valence shell based on Bohr’s model of an atom?

1. Atomic Structure: The Fundamentals (basic)

To understand the Periodic Table, we must first look at the very building blocks of matter: the atom. An atom is the smallest particle of an element that still exhibits all the characteristics of that element Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100. Structurally, every atom consists of a tiny, dense, and positively charged center called the atomic nucleus, surrounded by a cloud of negatively charged particles. While the nucleus contains nearly all the atom's mass in the form of protons and neutrons, it occupies only a tiny fraction of the atom's total volume Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100.

Electrons do not just swarm randomly; they reside in specific energy levels or shells (often labeled K, L, M, and N). The arrangement of these electrons is what we call the electronic configuration. For instance, an atom like Sodium (Na) has its electrons distributed across three shells, but it is the valence shell—the outermost one—that determines how the atom behaves in the real world. If an atom has a full outer shell, it is stable; if not, it will react with other atoms to achieve a "stable octet" (eight electrons) Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46.

Understanding the number of electrons in this outermost shell is the "master key" to the Periodic Table. For example, Chlorine is known for having seven electrons in its outermost shell, which explains why it is so eager to react and gain one more electron to reach stability Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46. This fundamental structure is the reason why elements are grouped together—they often share the same number of valence electrons.

Particle	Location	Charge	Significance
Proton	Nucleus	Positive (+)	Determines the identity of the element.
Neutron	Nucleus	Neutral (0)	Adds mass and provides nuclear stability.
Electron	Shells	Negative (-)	Determines chemical reactivity and bonding.

Sources: Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46

2. Bohr's Model and Electron Shells (basic)

To understand the periodic table, we must first look inside the atom. Neils Bohr proposed a model where electrons do not just swarm randomly around the nucleus; instead, they revolve in discrete, fixed orbits called energy levels or shells. Think of these like the floors of a building, where electrons must occupy a specific floor rather than hovering in between.

These shells are named alphabetically starting from the one closest to the nucleus: K, L, M, and N (representing shell numbers n=1, 2, 3, and 4). Each shell has a maximum capacity for electrons defined by the formula 2n². For instance, the first shell (K) can hold 2(1)² = 2 electrons, while the second shell (L) can hold 2(2)² = 8. As we move to heavier elements, the distribution becomes more complex, but the goal remains the same: stability.

Shell Name	n Value	Maximum Capacity (2n²)
K	1	2 electrons
L	2	8 electrons
M	3	18 electrons
N	4	32 electrons

The most important part of this structure is the outermost shell, also known as the valence shell. The electrons living here are called valence electrons, and they are the "social butterflies" of the atom—they determine how the element reacts with others Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. Elements are most stable when their outermost shell is completely full, often referred to as a stable octet (having 8 electrons). For example, Sodium (Na) has 11 electrons distributed as 2, 8, 1. To reach stability, it prefers to lose that single electron from its M shell, leaving the L shell as its new, stable outermost layer 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; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46

3. The Modern Periodic Table Structure (intermediate)

The transition to the Modern Periodic Table marked a fundamental shift in how we understand the building blocks of the universe. While early scientists attempted to organize elements by their atomic masses, the modern table is organized by Atomic Number (Z)—the number of protons in the nucleus. This structural choice is brilliant because the atomic number determines the electronic configuration, which in turn dictates how an element behaves chemically. Elements are arranged in 18 vertical columns called Groups and 7 horizontal rows called Periods Science, Class X (NCERT 2025 ed.), Chapter 3, p. 47. To understand the structure, we look at how electrons occupy shells (K, L, M, N). The Period number tells us the total number of electron shells an atom possesses. For instance, any element in Period 4 has four occupied shells. On the other hand, the Group number (specifically for the main-group elements) relates to the number of valence electrons—those in the outermost shell. For example, Bromine (Atomic Number 35) has an electronic configuration of 2, 8, 18, 7. Because it has four shells, it sits in Period 4; because it has 7 valence electrons, it belongs to Group 17 (the Halogens) Science, Class X (NCERT 2025 ed.), Chapter 3, p. 47.

Feature	Periods (Horizontal Rows)	Groups (Vertical Columns)
Quantity	7 Periods	18 Groups
Significance	Indicates the number of occupied electron shells.	Indicates the number of valence electrons (for main groups).
Trend	Elements in a period have different properties but the same number of shells.	Elements in a group share similar chemical properties due to identical valence counts.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.47

4. Valence Electrons and Valency (intermediate)

To understand how elements react, we must look at their Valence Shell—the outermost shell of an atom. The electrons residing here are called Valence Electrons. These electrons are the "negotiators" in chemical reactions; they determine how an atom will bond with others. According to the Octet Rule, atoms are most stable when they have a completely filled outer shell, similar to the Noble Gases like Neon (2, 8) or Argon (2, 8, 8) Science, Metals and Non-metals, p.47.

While valence electrons represent the total count in the outer shell, Valency refers to the combining capacity of an atom. It is the number of electrons an atom needs to lose, gain, or share to achieve a stable octet. For example, Sodium (Na) has an electronic configuration of 2, 8, 1. It is easier for Sodium to lose that 1 electron than to gain 7. Thus, its valency is 1 Science, Metals and Non-metals, p.46. Conversely, Chlorine (Cl) has 7 valence electrons (2, 8, 7). It needs just 1 more to be stable, so its valency is also 1 Science, Carbon and its Compounds, p.60.

Element	Configuration	Valence Electrons	Valency
Magnesium (Mg)	2, 8, 2	2	2 (Loses 2)
Oxygen (O)	2, 6	6	2 (Gains 2)
Carbon (C)	2, 4	4	4 (Shares 4)

For larger atoms, we follow the same logic using the Bohr model and shell distribution (K, L, M, N...). Take an element like Bromine (Atomic Number 35). Its electrons are distributed as 2, 8, 18, 7. Even though it has 35 electrons in total, only the 7 electrons in its fourth (N) shell are valence electrons. Because it needs 1 electron to reach an octet, its valency is 1.

Sources: Science, Metals and Non-metals, p.46; Science, Metals and Non-metals, p.47; Science, Carbon and its Compounds, p.60

5. Electronic Configuration: Beyond the Basics (exam-level)

When we move beyond the first twenty elements of the periodic table, the simple Bohr-Bury rule of 2n² electrons per shell becomes a bit more nuanced. To master Electronic Configuration at an exam level, we must transition from thinking only about shells (K, L, M, N) to understanding subshells (s, p, d, f). Electrons don't just fill shells in a straight line; they follow the Aufbau Principle, which states that electrons occupy the lowest energy orbitals available first. This is why, surprisingly, the 4s subshell fills up before the 3d subshell begins, even though the 3d orbital is technically in the third shell.

Consider Bromine (Z=35). To find its configuration, we distribute 35 electrons according to increasing energy levels: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵. While the 3d subshell is filled after the 4s, it is physically "inside" the fourth shell. When we group these by their principal quantum number (the shell number), we get the distribution: 2, 8, 18, 7. Understanding this distribution is crucial for predicting chemical behavior, as seen in how atoms share electrons to reach a stable state Science, class X (NCERT 2025 ed.), Chapter 4, p.60.

Shell Number (n)	Subshells Included	Total Electrons for Bromine
1 (K)	1s²	2
2 (L)	2s², 2p⁶	8
3 (M)	3s², 3p⁶, 3d¹⁰	18
4 (N) - Valence	4s², 4p⁵	7

The Valence Shell is defined as the outermost shell of an atom (the shell with the highest value of 'n'). For elements in the p-block, like Bromine, the valence electrons are the sum of electrons in the outermost s and p subshells. In Bromine's case, the fourth shell (n=4) contains 2 electrons in 4s and 5 electrons in 4p, totaling 7 valence electrons. This explains why it behaves similarly to Fluorine and Chlorine, as they all seek one electron to complete their octet Science, class X (NCERT 2025 ed.), Chapter 3, p.47.

Sources: Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.60; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.47

6. The Halogen Family: Group 17 (exam-level)

Group 17 of the periodic table, popularly known as the Halogen Family, consists of five naturally occurring elements: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). The name "halogen" comes from the Greek words halo (salt) and gen (producer), referring to their ability to react with metals to produce salts, such as sodium chloride (NaCl). These elements are among the most reactive non-metals because they are just one electron away from achieving a stable, noble gas electronic configuration.

The defining characteristic of halogens is their electronic configuration. Every halogen has 7 electrons in its valence (outermost) shell ($ns^2 np^5$). For example, Chlorine (Atomic Number 17) has a configuration of 2, 8, 7, while Bromine (Atomic Number 35) follows a distribution of 2, 8, 18, 7 Science, class X (NCERT 2025 ed.), Metals and Non-metals, p. 47. This commonality in valence electrons explains why they share similar chemical properties. To achieve stability, they typically gain one electron to form a negative ion (anion) with a charge of -1, or they share a pair of electrons to form a single covalent bond. In organic chemistry, halogens often act as "heteroatoms," replacing hydrogen atoms in hydrocarbon chains to form functional groups that determine the compound's reactivity Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p. 66.

Halogens exhibit a unique trend in physical states at room temperature: Fluorine and Chlorine are gases, Bromine is one of the few elements that exist as a liquid at room temperature, and Iodine is a solid Science, class X (NCERT 2025 ed.), Metals and Non-metals, p. 39. They naturally exist as diatomic molecules (such as F₂, Cl₂, and Br₂) where two atoms are bonded together Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p. 60. Beyond basic chemistry, these elements have significant environmental impacts; for instance, Bromine and Chlorine compounds are potent ozone-depleting substances, with Bromine being significantly more destructive to ozone molecules than Chlorine Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p. 269.

Element	Atomic Number	Electronic Configuration (Shell-wise)	Valence Electrons
Fluorine (F)	9	2, 7	7
Chlorine (Cl)	17	2, 8, 7	7
Bromine (Br)	35	2, 8, 18, 7	7

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.39, 47; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60, 66; Environment, Shankar IAS Academy (ed 10th), Ozone Depletion, p.269

7. Solving the Original PYQ (exam-level)

Now that you have mastered the Bohr-Bury scheme and the 2n² rule, this question serves as a perfect application of those building blocks. To solve this, you must synthesize your knowledge of shell capacities and the definition of a valence shell. For an atom with atomic number 35, you are distributing 35 electrons across energy levels. Following the sequence described in Science, class X (NCERT 2025 ed.), the K shell (n=1) takes 2 electrons, the L shell (n=2) takes 8, and the M shell (n=3) takes 18. This accounts for 28 electrons, leaving exactly 7 electrons to occupy the N shell (n=4), which is the outermost or valence shell.

As your coach, I want you to focus on the layering process. The valence shell is defined by the highest principal quantum number (n=4 in this case). Even though we fill the 3d subshell after the 4s, the 4s and 4p electrons together constitute the fourth shell's total count. This gives Bromine its characteristic properties as a Halogen (Group 17), similar to Fluorine and Chlorine. Therefore, by calculating the shell-wise distribution as 2, 8, 18, 7, we logically arrive at (D) 7 as the correct number of valence electrons.

UPSC often uses options like (A) or (C) to catch students who make common calculation errors or subshell mistakes. Option (A) 1 might tempt someone confusing the element with an Alkali metal, while Option (C) 5 is a classic trap for those who only count the electrons in the 4p subshell and forget the 4s electrons. Always remember: the valence shell includes all electrons in the highest energy shell, not just the last subshell filled. By sticking to the Bohr model's shell distribution, you avoid these pitfalls and confirm the answer is 7.