Which one among the following is correct regarding 20Ne, 23Na+, 19F- and 24Mg2+?

1. Fundamental Structure of an Atom (basic)

Welcome to your first step in mastering chemistry! To understand how materials behave—from the oxygen we breathe to the silicon in our phones—we must look at the atom, the fundamental building block of matter. Every atom consists of a tiny, dense core called the nucleus, surrounded by a cloud of electrons. While the universe began with simple particles, it took nearly 300,000 years after the Big Bang for electrons to combine with protons and neutrons to form the first stable atoms like Hydrogen and Helium Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2.

The internal structure of an atom is defined by three subatomic particles:

The identity of an element is fixed by its Atomic Number (Z), which is simply the number of protons in its nucleus. For example, any atom with 7 protons is Nitrogen Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60. In a neutral atom, the number of negatively charged electrons exactly equals the number of positively charged protons, meaning the net charge is zero. However, atoms often strive for stability by achieving a full outer shell (an octet). To do this, they may lose or gain electrons, becoming ions. If a neutral sodium atom (11 protons) loses one electron, it becomes a sodium cation (Na⁺) with 11 protons but only 10 electrons, resulting in a net positive charge Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.46.

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.2; 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) vs. Mass Number (A) (basic)

In our journey to understand the building blocks of matter, we must distinguish between an element's identity and its weight. Think of the Atomic Number (Z) as an atom's unique ID card. It represents the number of protons present in the nucleus. This number is fundamental because it defines the element itself; for example, any atom with exactly 1 proton is Hydrogen Science, Class X (NCERT 2025 ed.), Chapter 4, p.59, and any atom with 7 protons is Nitrogen Science, Class X (NCERT 2025 ed.), Chapter 4, p.60.

While the Atomic Number tells us who the atom is, the Mass Number (A) tells us how heavy it is. Since electrons have negligible mass, the mass of an atom is concentrated in its nucleus, which contains both protons and neutrons (collectively called nucleons). Therefore, the Mass Number is the sum of protons and neutrons. For instance, a typical Carbon atom has 6 protons and 6 neutrons, giving it a mass number of 12 Science, Class X (NCERT 2025 ed.), Chapter 4, p.66.

It is important to remember that in a neutral atom, the number of protons equals the number of electrons. However, during chemical reactions, atoms can gain or lose electrons to form ions. Crucially, the Atomic Number remains unchanged during these processes. For example, a Sodium (Na) atom has 11 protons. Even when it loses an electron to become a sodium cation (Na⁺), the nucleus still contains 11 protons Science, Class X (NCERT 2025 ed.), Chapter 3, p.46. This is why it remains Sodium, just in a charged state.

To keep these straight, you can look at this comparison:

Sources: Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.59; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.60; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.66; Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46

3. Formation of Ions: Cations and Anions (intermediate)

In the world of chemistry, atoms are like people—they generally seek stability. Most atoms are naturally neutral because they have an equal number of protons (positive) and electrons (negative). however, they are often "unstable" because their outermost electron shell isn't full. To achieve a stable noble gas configuration, atoms participate in a give-and-take of electrons Science, class X (NCERT 2025 ed.), Chapter 4, p.59. This process transforms neutral atoms into charged particles called Ions.

When an atom loses electrons, it becomes a Cation. Since electrons carry a negative charge, losing them leaves the atom with more protons than electrons, resulting in a net positive charge. Metals, like Sodium (Na) or Magnesium (Mg), typically form cations. For instance, a neutral Magnesium atom has 12 protons and 12 electrons. To become stable, it sheds 2 electrons, becoming a Mg²⁺ cation with 12 protons but only 10 electrons Science, class X (NCERT 2025 ed.), Chapter 3, p.47. Conversely, when an atom gains electrons, it becomes an Anion. Non-metals, such as Chlorine (Cl) or Fluorine (F), tend to pull electrons toward themselves to complete their "octet." A Chlorine atom gains one electron to become a Cl⁻ anion, now possessing 18 electrons against its 17 protons Science, class X (NCERT 2025 ed.), Chapter 3, p.47.

Understanding these charges is crucial because oppositely charged ions attract each other through electrostatic forces, forming ionic compounds like common table salt (NaCl) Science, class X (NCERT 2025 ed.), Chapter 3, p.48. Interestingly, this principle of charge separation isn't just limited to test tubes; it even occurs in nature during thunderstorms, where the movement of particles causes the top of clouds to become positively charged (cations) and the bottom to become negatively charged (anions) Physical Geography by PMF IAS, Chapter 15, p.348.

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

4. Understanding Isotopes, Isobars, and Isotones (intermediate)

In our journey through chemistry, we often find that atoms of different elements or even the same element share surprising similarities. To classify these relationships, we look at three fundamental subatomic properties: the Atomic Number (Z), which is the number of protons; the Mass Number (A), which is the sum of protons and neutrons; and the number of neutrons (N) itself. Understanding how these vary or stay constant helps us predict the stability and reactivity of matter.

Isotopes are atoms of the same element (same atomic number) that have different mass numbers because they possess different numbers of neutrons. For instance, Carbon-12 and Carbon-14 are isotopes; both are Carbon because they have 6 protons, but Carbon-14 is heavier and radioactive. While carbon is often studied for its unique ability to form long chains or rings like cyclohexane (C₆H₁₂), its isotopic nature is what allows scientists to perform carbon dating Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.65. Conversely, Isobars are atoms of different elements that happen to have the same mass number (e.g., Argon-40 and Calcium-40). Finally, Isotones are species that have the same number of neutrons (calculated as A - Z), such as ¹⁴C and ¹⁶O, both of which contain 8 neutrons.

An advanced but high-yield concept for competitive exams is the isoelectronic species. These are atoms or ions that possess the same number of electrons, regardless of their atomic number. For example, Neon (Ne), which has 10 electrons, is isoelectronic with the Sodium ion (Na⁺). While Sodium naturally has 11 electrons, losing one electron to achieve a stable configuration leaves it with 10 Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.47. Similarly, Fluorine (9 electrons) gains one to become F⁻, also reaching 10 electrons Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46. This shared electronic configuration often leads to similar chemical stability, even though the species are physically different elements.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.65; 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

5. The Octet Rule and Noble Gas Configuration (intermediate)

In the world of chemistry, stability is the ultimate goal. Why do some elements, like Gold or Helium, remain remarkably indifferent to their surroundings while others, like Sodium or Fluorine, react almost explosively? The answer lies in the Octet Rule. This principle states that atoms are at their most stable state when their outermost (valence) shell is completely filled, typically with eight electrons. As noted in Science, Class X (NCERT 2025 ed.), Chapter 3, p.46, noble gases like Neon (Ne) and Argon (Ar) possess this completely filled valence shell, which explains their very low chemical activity.

Elements that lack this perfect arrangement are chemically "restless." They strive to attain the noble gas configuration through chemical reactions. This is achieved in three primary ways: losing electrons, gaining electrons, or sharing them. For instance, a Sodium (Na) atom has 11 electrons (2, 8, 1). By losing the single electron in its outermost M shell, its L shell—which already has 8 electrons—becomes the new outermost shell, creating a stable Sodium cation (Na⁺) Science, Class X (NCERT 2025 ed.), Chapter 3, p.46. Conversely, a Chlorine atom (2, 8, 7) needs just one more electron to complete its octet, which it gains to become a Chloride anion (Cl⁻).

When different atoms or ions end up with the exact same number and arrangement of electrons, we call them isoelectronic species. Even though their nuclei contain different numbers of protons (which defines their identity as different elements), their electron clouds look identical. For example, Neon (atomic number 10), Na⁺ (11 - 1 = 10), and Mg²⁺ (12 - 2 = 10) are all isoelectronic because they all share the stable electronic configuration of 2, 8 Science, Class X (NCERT 2025 ed.), Chapter 3, p.47.

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

6. Isoelectronic Species Explained (exam-level)

In chemistry, the prefix iso- means "equal" or "same." Therefore, isoelectronic species are atoms, ions, or molecules that possess the exact same number of electrons and, consequently, the same electronic configuration. While these species have the same number of electrons, they are usually different elements with different atomic numbers (number of protons) and different chemical properties.

To understand how different elements achieve the same electron count, we must look at how ions are formed. As we've learned, a cation is a positively charged ion formed when an atom loses electrons, while an anion is a negatively charged ion formed when an atom gains electrons Physical Geography by PMF IAS, Thunderstorm, p.348. Atoms often gain or lose these electrons to reach a stable state, typically mimicking the electron count of the nearest noble gas—this is known as the octet rule Science, class X (NCERT 2025 ed.), Chapter 3, p.47.

Consider the following group of species, all of which are isoelectronic because they share a total of 10 electrons:

It is crucial to distinguish these from isotopes. Isotopes share the same atomic number but have different mass numbers (neutrons). Isoelectronic species are the opposite: they have different atomic numbers but the same electron count Science, class X (NCERT 2025 ed.), Chapter 4, p.59. Even though they have the same number of electrons, their physical sizes (radii) differ because the varying number of protons in the nucleus exerts a different amount of pull on those 10 electrons.

Sources: Physical Geography by PMF IAS, Thunderstorm, p.348; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.47; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.59

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of atomic structure and ion formation, this question demonstrates how those building blocks come together. To solve this, you must apply your knowledge of atomic numbers (Z) and the behavior of electrons during the formation of cations and anions. The core task is to look past the different chemical symbols and mass numbers to calculate the total number of electrons for each species. As taught in Science, class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, atoms gain or lose electrons to achieve a stable noble gas configuration, which is exactly what we see happening with these ions.

Let’s walk through the logic: Neon (Ne) has an atomic number of 10, meaning it naturally possesses 10 electrons. Sodium (Na) has 11 electrons, but the Na+ cation indicates a loss of one electron (11 - 1 = 10). Fluorine (F) has 9 electrons, but the F- anion indicates the gain of one electron (9 + 1 = 10). Finally, Magnesium (Mg) has 12 electrons, and the Mg2+ cation indicates a loss of two electrons (12 - 2 = 10). Because all four species possess exactly 10 electrons, they are defined as isoelectronic. Therefore, the correct answer is (C) They are isoelectronic with each other.

UPSC often uses the "iso-" prefix to set traps for students who rely on surface-level memory. Isotopes (Option B) must be the same element (same atomic number) with different mass numbers; since these are four different elements, this is a clear distractor. Isomers (Option A) are molecules with the same formula but different structures, a concept typically reserved for organic compounds as discussed in Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds. By calculating the electron count systematically, you can avoid these common terminology traps and arrive at the correct conclusion.