A monatomic species that has 18 electrons and a net charge of 2~ has

1. Structure of the Atom: Protons, Neutrons, and Electrons (basic)

At the most fundamental level, every substance around us is composed of atoms, which are the smallest particles of an element that retain its unique characteristics Environment and Ecology, Majid Hussain, Major Crops and Cropping Patterns in India, p.100. To understand how chemistry works, we must look inside the atom at its three primary subatomic particles: protons, neutrons, and electrons. These particles are arranged in a specific structure: a dense, positively charged atomic nucleus at the center, surrounded by a cloud of negatively charged electrons.

The nucleus contains the protons (which carry a positive charge) and neutrons (which carry no charge/neutral). Because protons and neutrons are much heavier than electrons, almost all of an atom's mass is concentrated in this tiny central core. The number of protons is the "ID card" of an element; it is known as the Atomic Number. For example, any atom with exactly 7 protons is nitrogen, regardless of how many neutrons or electrons it has Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60.

In a neutral atom, the number of protons and electrons is exactly equal, so the positive and negative charges cancel each other out. However, atoms often gain or lose electrons to achieve a stable state (often called an octet). If an atom loses an electron, it ends up with more protons than electrons, resulting in a net positive charge; this is called a cation. Conversely, if an atom gains electrons, it becomes a negatively charged anion Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46. Understanding this balance is the key to mastering chemical reactions.

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Major Crops and Cropping Patterns in India, p.100; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

2. Atomic Number (Z) and Mass Number (A) (basic)

To understand chemistry, we must first look at the "identity card" of an atom. Every atom is defined by two fundamental numbers: the Atomic Number (Z) and the Mass Number (A). Think of the Atomic Number as the unique fingerprint of an element; it tells us exactly which element we are dealing with.

Atomic Number (Z) is defined as the total number of protons present in the nucleus of an atom. For example, Hydrogen has an atomic number of 1 because it has one proton Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. Carbon always has 6 protons in its nucleus, giving it an atomic number of 6 Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59. In a neutral atom, the number of protons is exactly equal to the number of electrons, keeping the atom electrically balanced.

Mass Number (A), on the other hand, represents the total number of "nucleons"—the particles inside the nucleus. Since electrons have almost no mass, the weight of an atom comes from its protons and neutrons. Therefore, the Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n). For instance, if you look at a carbon atom with 6 protons and 6 neutrons, its mass number is 12 Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.66.

It is vital to remember that while atoms can gain or lose electrons to become ions (like the sodium cation Na⁺ which loses an electron but keeps its 11 protons), the Atomic Number (protons) never changes during ordinary chemical reactions Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46. If you change the number of protons, you have changed the element itself!

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.66; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.46

3. Electronic Configuration and the Octet Rule (intermediate)

To understand how matter behaves, we must look at the electronic configuration—the specific arrangement of electrons in layers or 'shells' around an atom's nucleus. These shells, labeled K, L, M, and N, have specific capacities. For instance, the K shell can hold 2 electrons, while the L and M shells typically accommodate up to 8 electrons in their stable states. As shown in Science, class X (NCERT 2025 ed.), Chapter 3, p.47, Noble gases like Helium, Neon, and Argon have completely filled outermost shells, making them chemically unreactive and highly stable.

Most other atoms are 'restless' because their outermost shells are incomplete. This leads us to the Octet Rule: the tendency of atoms to prefer having eight electrons in their valence (outermost) shell. To achieve this stable 'noble gas configuration,' atoms will either lose, gain, or share electrons. For example, a Sodium (Na) atom has 11 electrons arranged as (2, 8, 1). It is energetically easier for Sodium to lose its single outer electron than to gain seven more. Once it loses that electron, its new outermost shell (the L shell) has a stable octet, and the atom becomes a positively charged cation (Na⁺) Science, class X (NCERT 2025 ed.), Chapter 3, p.46.

Conversely, non-metals often gain electrons to complete their octet. Chlorine (Cl), with 17 electrons (2, 8, 7), requires just one more electron to reach the stable (2, 8, 8) configuration of Argon. By accepting an electron, it becomes a negatively charged anion (Cl⁻). In some cases, like with Carbon, atoms choose to share electrons with other atoms so that both can attain a stable configuration simultaneously Science, class X (NCERT 2025 ed.), Chapter 4, p.59. This 'giving and taking' of electrons is the fundamental reason behind chemical reactions and the formation of compounds.

Sources: Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46-47; Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.59

4. Classification: Metals vs. Non-metals in Reactivity (intermediate)

At the heart of chemical reactivity is a simple goal: stability. Atoms react with one another to achieve a stable electronic configuration, typically resembling the nearest noble gas (which has a full outer shell). The way metals and non-metals pursue this goal defines their unique chemical personalities. Metals are naturally generous; they have a few electrons in their outermost shell and find it easier to lose them to become positively charged ions (cations). In contrast, non-metals are "electron-hungry"; they possess more electrons in their outer shell and prefer to gain electrons to form negatively charged ions (anions) Science, Class X (NCERT 2025 ed.), Chapter 3, p.56.

To understand which metal is more "powerful" than another, we look at displacement reactions. If Metal A can kick Metal B out of its salt solution, Metal A is more reactive. For example, if iron is placed in a copper sulphate solution, it displaces the copper because iron is more reactive Science, Class X (NCERT 2025 ed.), Chapter 3, p.45. This allowed scientists to create a Reactivity Series, ranking elements from highly reactive (like Potassium) to very stable (like Gold) Science, Class X (NCERT 2025 ed.), Chapter 3, p.50.

Beyond just moving electrons, their reactions with oxygen reveal a fundamental divide in their nature. When these elements burn or oxidize, they create very different chemical environments, which is a classic diagnostic tool in the lab:

Science-Class VII, NCERT (Revised ed 2025), The World of Metals and Non-metals, p.54

Sources: Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.45; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.50; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.56; Science-Class VII, NCERT (Revised ed 2025), The World of Metals and Non-metals, p.54

5. The Noble Gases: Group 18 Elements (intermediate)

In the vast landscape of the periodic table, the Noble Gases (found in Group 18) occupy a position of unique stability. This group includes elements like Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). Often referred to as "inert gases," they are characterized by their remarkable lack of chemical reactivity under standard conditions. As we explore chemical principles, understanding these elements is crucial because they serve as the "gold standard" of stability that almost all other elements strive to achieve through chemical reactions Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p. 46.

The secret to their stability lies in their electronic configuration. Every element's reactivity is driven by the arrangement of electrons in its outermost shell, known as the valence shell. Noble gases possess a completely filled valence shell. While Helium is stable with just 2 electrons in its solitary K-shell (a duet), the others follow the Octet Rule, maintaining 8 electrons in their outermost shell Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p. 47. Because their shells are full, they have no urgent "need" to gain, lose, or share electrons with other atoms.

This state of "chemical contentment" is so desirable that other elements undergo chemical changes specifically to reach a similar configuration. For instance, a chlorine atom (with 17 electrons) will eagerly gain one electron to reach 18, making it isoelectronic (having the same electron count) with the noble gas Argon Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p. 60. This explains why metals and non-metals react: they are essentially trying to "mimic" the electronic architecture of the nearest noble gas to reach a lower energy, stable state.

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.60

6. Ions: Cations, Anions, and Net Charge (intermediate)

In the world of chemistry, electrical balance is the default state for a neutral atom, where the number of positive protons in the nucleus exactly matches the number of negative electrons orbiting it. However, atoms are rarely content in this neutral state; they often gain or lose electrons to achieve a stable electron configuration, similar to that of a noble gas. When this balance is disrupted, the atom becomes an ion—a particle with a net charge.

There are two primary types of ions you must distinguish for the UPSC exam:

• Cations: These are atoms or molecules that have lost one or more electrons. Because they now have more protons (positive) than electrons (negative), they carry a positive charge. Think of the "t" in Cation as a plus sign (+).
• Anions: These are atoms or molecules that have gained electrons. Since they possess a greater number of electrons than protons, they carry a negative charge Physical Geography by PMF IAS, Chapter 25, p. 348.

To calculate the net charge of an ion, you simply subtract the number of electrons from the number of protons (Net Charge = Protons - Electrons). For instance, a chlorine atom normally has 17 protons and 17 electrons. When it gains an electron to reach stability, it ends up with 18 electrons. With 17 protons and 18 electrons, the math (17 - 18) results in a net charge of 1-, forming the chloride anion (Cl⁻) Science, class X (NCERT 2025 ed.), Chapter 3, p. 47.

These oppositely charged ions don't just exist in isolation; they are strongly attracted to one another. This electrostatic force leads to the formation of ionic compounds, such as Sodium Chloride (NaCl), where the sodium cation and chloride anion aggregate together in a stable structure Science, class X (NCERT 2025 ed.), Chapter 3, p. 48.

Sources: Physical Geography by PMF IAS, Chapter 25: Thunderstorm, p.348; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.47; Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.48

7. Isoelectronic Species (exam-level)

In chemistry, the prefix 'iso-' means equal. Therefore, isoelectronic species are atoms, ions, or molecules that possess the same total number of electrons and the same electronic configuration, even though they belong to different elements. This phenomenon occurs because atoms often gain or lose electrons to achieve the highly stable 'octet' structure of the noble gases.

For example, a neutral atom of Argon (Ar) has 18 electrons. Other elements near Argon on the periodic table will strive to reach this same count to find stability. As detailed in Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.47, Sulfur (S) has an atomic number of 16, meaning it has 16 protons and 16 electrons. If sulfur gains two electrons, it becomes a sulfide ion (S²⁻) with 18 electrons—making it isoelectronic with Argon. Similarly, Chlorine (Cl) has 17 electrons; by gaining one, it becomes Cl⁻ (18 electrons). On the other side, Potassium (K) has 19 electrons; by losing one, it becomes K⁺ (18 electrons).

It is crucial to remember that while isoelectronic species share the same number of electrons, they have different numbers of protons in their nuclei. This difference in 'nuclear charge' is what causes them to have different physical sizes and chemical identities. A species with more protons will pull those 18 electrons closer, making the ion smaller than one with fewer protons.

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

8. Solving the Original PYQ (exam-level)

Now that you have mastered the building blocks of atomic structure and ionic charge, you can see how they converge in this question. The core concept here is the Charge-Electron-Proton Relationship: the net charge of an atom is determined by the balance between its positive protons and negative electrons. Since this species has 18 electrons and a net charge of 2-, we can deduce it must have 16 protons ($16 - 18 = -2$). As taught in Science, class X (NCERT 2025 ed.), this identifies the species as a Sulfide ion (S²⁻), which has gained two electrons to achieve the stable octet configuration of a noble gas.

To arrive at the correct answer, we compare our findings with the options. A neutral Argon atom has an atomic number of 18, meaning it naturally carries 18 electrons. Since our species also has 18 electrons, (A) is the correct answer because they are isoelectronic. This stability is a recurring theme in chemistry; because the electron shells are completely filled, there are zero unpaired electrons, which immediately invalidates option (C). Like the charging processes described in Physical Geography by PMF IAS, the movement of electrons is what creates these charged states.

UPSC frequently uses sign-reversal traps to test your precision. Option (B) is a classic distractor that describes a cation (more protons than electrons), whereas a negative charge always indicates an anion. Similarly, option (D) tempts students to think of Calcium (20 protons); however, a species with 20 protons and 18 electrons would result in a 2+ charge, not a 2- charge. By staying disciplined with your charge arithmetic, you can easily filter out these common misconceptions.