The number of valence electrons in the O2- ion is.

1. Atomic Structure and Subatomic Particles (basic)

Everything around us—from the air you breathe to the device you are holding—is composed of matter, and the fundamental building block of all matter is the atom. While early scientists thought atoms were indivisible, we now understand that they are made of three primary subatomic particles: protons, neutrons, and electrons Science, Class VIII, Particulate Nature of Matter, p.115. These particles were even formed in the very early stages of our universe, roughly 300,000 years after the Big Bang Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2.

The structure of an atom is similar to a tiny solar system. At the center lies the nucleus, which houses the positively charged protons and the neutral neutrons. Orbiting this nucleus at high speeds are the negatively charged electrons. In a neutral atom, the number of protons exactly equals the number of electrons, ensuring the total electrical charge is zero.

Particle	Charge	Location
Proton	Positive (+)	Inside the Nucleus
Neutron	Neutral (0)	Inside the Nucleus
Electron	Negative (-)	Orbits/Shells

The identity of an element is determined by its Atomic Number, which is the total count of protons in its nucleus. Electrons are arranged in layers called shells (K, L, M, etc.). Atoms are most stable when their outermost shell is full—usually containing eight electrons, a principle known as the Octet Rule. To achieve this stability, atoms often gain or lose electrons. When an atom loses an electron, it gains a net positive charge and becomes a cation (like Na⁺). Conversely, if it gains an electron, it becomes a negatively charged anion Science, class X, Metals and Non-metals, p.46.

Sources: Science, Class VIII (NCERT Revised ed 2025), Particulate Nature of Matter, p.115; Physical Geography by PMF IAS (1st ed.), The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

2. Electronic Configuration and Electron Shells (intermediate)

At the heart of chemistry lies the electronic configuration—the specific arrangement of electrons around an atom's nucleus. Electrons reside in distinct energy levels called shells, labeled K, L, M, and N. Each shell has a maximum capacity: the first (K) can hold 2 electrons, while subsequent shells like L and M can hold up to 8 and 18 respectively. However, for most common elements, the most important rule is the Octet Rule: atoms are most stable when their outermost shell, or valence shell, contains 8 electrons Science, Class X (NCERT 2025 ed.), Chapter 3, p.46.

Elements that naturally possess a completely filled valence shell are known as noble gases (like Helium, Neon, and Argon). Because they are already stable, they show very little chemical activity. Most other atoms, however, have incomplete outer shells and are reactive because they "want" to achieve that stable noble gas configuration Science, Class X (NCERT 2025 ed.), Chapter 3, p.47. They do this in three ways: losing electrons, gaining electrons, or sharing them. For instance, Metals like Sodium (Na) have only one electron in their M shell; by losing it, their L shell becomes the new, stable outermost shell, forming a positive cation (Na⁺) Science, Class X (NCERT 2025 ed.), Chapter 3, p.46.

Conversely, Non-metals usually have 5, 6, or 7 electrons in their valence shell. Instead of losing many electrons, they find it easier to gain electrons to complete their octet. For example, an Oxygen atom has 6 electrons in its L shell and needs 2 more to reach the stable number of 8 Science, Class X (NCERT 2025 ed.), Chapter 4, p.60. When an atom gains electrons, it becomes a negatively charged anion. Whether an atom forms a bond by sharing (covalent) or transferring (ionic) electrons, the ultimate goal is always the same: achieving a completely filled valence shell.

Element	Atomic Number	K Shell	L Shell	M Shell	Valence Electrons
Neon (Noble Gas)	10	2	8	-	8 (Stable)
Sodium (Metal)	11	2	8	1	1 (Reactive)
Chlorine (Non-metal)	17	2	8	7	7 (Reactive)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.47; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.60

3. The Octet Rule and Chemical Stability (intermediate)

In the world of chemistry, atoms are driven by a single fundamental goal: stability. Most atoms are naturally unstable and highly reactive because their outermost electron shell, known as the valence shell, is incomplete. To achieve a stable, low-energy state, atoms interact with one another to mimic the electronic configuration of Noble Gases (like Neon or Argon), which possess completely filled outer shells and rarely react with other elements Science, Class X (NCERT 2025 ed.), Chapter 3, p. 47.

The Octet Rule is the principle that atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons. This "magic number" eight provides a highly stable arrangement. For example, Sodium (Na) has 11 electrons (2, 8, 1). By losing its single outermost electron, it forms a Sodium cation (Na⁺). Its new outermost shell (the L shell) now contains 8 electrons, making it stable and isoelectronic (having the same electron count) with the noble gas Neon Science, Class X (NCERT 2025 ed.), Chapter 3, p. 46. Conversely, non-metals like Chlorine (Cl), which has 7 valence electrons, seek to gain one electron to complete their octet.

Elements achieve this stability through two primary methods: transferring or sharing electrons. While metals and non-metals often form ionic bonds through transfer, elements like Carbon or Nitrogen often share electrons to reach an octet. For instance, a Nitrogen atom has 5 valence electrons and needs 3 more; in an N₂ molecule, two nitrogen atoms share three pairs of electrons, forming a triple bond so that each atom can "claim" eight electrons in its valence shell Science, Class X (NCERT 2025 ed.), Chapter 4, p. 60.

Method	Mechanism	Common Elements
Electron Transfer	Atoms lose or gain electrons to form ions (Cations/Anions).	Metals (Na, Mg) and Non-metals (Cl, O).
Electron Sharing	Atoms share electron pairs to form covalent bonds.	Non-metals (C, N, H).

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.47; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.60

4. Periodic Table Trends and Valency (exam-level)

To understand how elements behave in the physical world, we must look at their Valence Electrons—the electrons present in the outermost shell of an atom. These electrons are the "negotiators" of the chemical world; they determine how an atom will react, bond, or remain stable. The fundamental drive for any atom is to achieve a completely filled valence shell, a state of maximum stability seen in noble gases like Helium or Neon. As noted in Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46, noble gases show little chemical activity because their outermost shells are already full.

It is crucial to distinguish between Valence Electrons and Valency. While valence electrons tell us how many electrons are currently in the outer shell, valency represents the "combining capacity" of the atom—essentially, how many electrons it needs to lose, gain, or share to reach that stable state. For instance, a neutral Oxygen atom (Atomic Number 8) has 6 valence electrons (configuration 2, 6). To achieve an octet (8 electrons), it is easier for Oxygen to gain 2 electrons than to lose 6. When it gains these 2 electrons, it becomes the Oxide ion (O²⁻). In this ionic state, its valence shell is now completely filled with 8 electrons, making it isoelectronic with the noble gas Neon (Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60).

In the Periodic Table, we observe distinct trends in these properties. Down a group, the number of valence electrons remains the same, which is why elements in the same group (like Lithium, Sodium, and Potassium) exhibit similar chemical properties. However, across a period (from left to right), the number of valence electrons increases from 1 to 8. Interestingly, the valency first increases from 1 to 4 and then decreases to zero as the shells get closer to being full. This reactivity also dictates the Activity Series, where metals are ranked by their tendency to lose electrons and react with other reagents (Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45).

Feature	Metals (e.g., Sodium)	Non-Metals (e.g., Chlorine)
Outer Shell Goal	Lose electrons to empty the outer shell	Gain electrons to fill the outer shell
Ion Formed	Positive Ion (Cation)	Negative Ion (Anion)
Valency Example	Na has 1 valence electron; Valency = 1	Cl has 7 valence electrons; Valency = 1

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45-46; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60

5. Chemical Bonding: Ionic vs Covalent (exam-level)

At the heart of chemistry is a simple quest for stability. Most atoms are naturally unstable because their outermost electron shells are incomplete. To find peace, they strive to attain a noble gas configuration—a state where their outermost shell is completely filled, usually with eight electrons (the octet rule). This journey toward stability happens through two primary paths: Ionic and Covalent bonding.

Ionic bonding occurs via the complete transfer of electrons from one atom to another, typically between a metal and a non-metal. Metals like Sodium (Na) or Magnesium (Mg) have a few extra electrons in their outer shell and find it easier to donate them. Conversely, non-metals like Oxygen (O) are just a few electrons short of a full shell. For instance, a neutral Oxygen atom has an atomic number of 8, meaning it has 6 valence electrons. To reach stability, it gains two electrons to form the oxide ion (O²⁻). In this ionic state, its outer shell is now perfectly filled with 8 electrons, making it isoelectronic with the noble gas Neon Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46. These oppositely charged ions then exert a strong electrostatic force on each other, leading to compounds with high melting points Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.49.

Covalent bonding, however, is about sharing rather than giving. This is the preferred path for elements like Carbon. Carbon has 4 valence electrons; gaining 4 more would be difficult because the nucleus (6 protons) would struggle to hold onto 10 total electrons, and losing 4 would require a massive amount of energy Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. Instead, Carbon shares its electrons with other atoms. Oxygen can also bond covalently; in an O₂ molecule, two oxygen atoms share two pairs of electrons to form a double bond, ensuring both atoms complete their octets through cooperation Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60.

Feature	Ionic Bond	Covalent Bond
Mechanism	Complete transfer of electrons.	Mutual sharing of electrons.
Typical Elements	Metals + Non-metals.	Non-metals + Non-metals.
Forces	Strong electrostatic attraction.	Shared electron pairs.

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46, 48-49; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59-60

6. Anions, Cations, and Ion Electronic Configuration (intermediate)

At the heart of chemical reactivity is the pursuit of stability. Atoms are most stable when they possess a completely filled outer shell, often referred to as a noble gas configuration. To achieve this "octe state," atoms engage in a give-and-take of electrons, transforming from neutral entities into charged particles known as ions Science , class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p. 59. When an atom loses or gains electrons, the delicate balance between the positive charge of the nucleus (protons) and the negative charge of the electron cloud is shifted, resulting in a net electrical charge.

Ions are categorized into two primary types based on their charge:

Feature	Cation	Anion
Charge	Positive (+)	Negative (–)
Formation	Loss of electrons	Gain of electrons
Proton/Electron Ratio	Protons > Electrons	Electrons > Protons
Example	Sodium (Na⁺)	Chloride (Cl⁻)

For instance, a neutral Sodium atom has 11 protons and 11 electrons. By losing one electron, it becomes a cation (Na⁺) with 11 protons but only 10 electrons, achieving the stable configuration of the noble gas Neon Science , class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p. 47. Conversely, atoms like Chlorine or Oxygen gain electrons to fill their valence shells. An anion like the oxide ion (O²⁻) forms when a neutral Oxygen atom (which has 8 protons and 8 electrons) gains two additional electrons. While the neutral atom has 6 valence electrons, the resulting O²⁻ ion has 8 valence electrons in its outermost shell, making its total electronic configuration 1s² 2s² 2p⁶ Science , class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p. 46.

Sources: Science , class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.59; Science , class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46-47; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Thunderstorm, p.348

7. Solving the Original PYQ (exam-level)

You’ve just mastered the fundamentals of atomic structure and electronic configuration; now, this question asks you to apply those building blocks to an ion. Recall that the atomic number of Oxygen is 8, which tells us a neutral atom has 8 electrons. As explained in Science, class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds, these electrons are distributed in shells, with 2 in the inner K-shell and 6 in the L-shell. Because the L-shell is the outermost shell, a neutral oxygen atom possesses 6 valence electrons.

To solve for the O^2- ion, you must apply the concept of chemical stability and ion formation. The "2-" superscript indicates that the oxygen atom has gained two additional negative charges (electrons) to achieve a stable noble gas configuration. By adding these 2 electrons to the original 6 valence electrons, the outermost shell becomes completely filled. This results in a stable count of 8 valence electrons, fulfilling the octet rule as described in Science, class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals. Therefore, the correct answer is (C) 8.

UPSC often uses distractors to test your attention to detail. Option (B) 6 is a classic trap for students who identify the valence electrons of a neutral oxygen atom but forget to account for the ionic charge. Option (D) 10 is another common pitfall; while the O^2- ion does have 10 total electrons (2 in the first shell + 8 in the second), the question specifically asks for valence electrons only. Always ensure you distinguish between the total electron population and the specific count in the outermost shell to avoid these common Prelims errors.