Which one of the following elements has valency three ?

1. Structure of the Atom: Subatomic Particles (basic)

Welcome to the beginning of our journey into the building blocks of the universe! To understand chemistry, we must first look inside the atom. While the word "atom" comes from the Greek word for "indivisible," we now know that atoms are actually composed of even smaller subatomic particles. These particles—protons, neutrons, and electrons—work together like a tiny solar system to create everything we see around us, from the iron in a bridge to the gold in a ring Science, Class VIII (NCERT), Particulate Nature of Matter, p.115.

At the center of every atom lies the nucleus, a dense core containing protons and neutrons. Protons carry a positive electrical charge (+), and the number of protons defines what element the atom is (for instance, every Carbon atom has exactly 6 protons). Neutrons are roughly the same size as protons but carry no charge—they act like "nuclear glue" to help keep the protons together. Interestingly, these particles formed just minutes after the Big Bang, eventually combining to form the first simple atoms like Hydrogen and Helium Physical Geography by PMF IAS, The Universe, p.2.

Whizzing around this nucleus at incredible speeds are the electrons. These are much smaller than protons and carry a negative electrical charge (-). In a stable, neutral atom, the number of negative electrons exactly matches the number of positive protons, creating an electrical balance. However, electrons are not fixed; they live in "shells" or energy levels. When an atom loses an electron, it loses a negative charge and becomes a positively charged cation Science, Class X (NCERT), Metals and Non-metals, p.46. Understanding how these electrons are arranged is the secret to understanding how chemicals react and bond.

Particle	Relative Mass	Electrical Charge	Location
Proton	1 unit	Positive (+)	Inside the Nucleus
Neutron	1 unit	Neutral (0)	Inside the Nucleus
Electron	Negligible (~1/2000th)	Negative (-)	Orbiting the Nucleus

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

2. Electronic Configuration and Shell Distribution (basic)

To understand how atoms interact, we must first look at how they organize their electrons. Electrons are not scattered randomly; they revolve around the nucleus in specific energy levels called shells (labeled K, L, M, N, and so on). The distribution of electrons across these shells is known as the electronic configuration. The innermost shell, the K shell, is the smallest and can hold only 2 electrons. Once it is full, electrons begin to fill the L shell (which holds up to 8), then the M shell, and so forth, following the 2n² rule where 'n' is the shell number.

The most important concept here is the valence shell, which is the outermost shell of an atom. The electrons residing there are called valence electrons. For example, Carbon has an atomic number of 6. Its electronic configuration is 2 in the K shell and 4 in the L shell. Since the L shell is its outermost shell, Carbon has 4 valence electrons Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p. 59. Similarly, Chlorine, with an atomic number of 17, distributes its electrons as 2, 8, 7 Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p. 60.

Why does this distribution matter? Because atoms are "lazy"—they want to reach a state of maximum stability. Stability is usually achieved when the outermost shell is completely full, often containing 8 electrons (known as an octet). This is called a noble gas configuration because noble gases like Neon or Argon naturally have full shells and rarely react with anything. Most other atoms have incomplete outer shells and will spend their "lives" trying to gain, lose, or share electrons with other atoms to reach that perfect, stable number Science, class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p. 60.

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

3. The Modern Periodic Table: Groups and Periods (intermediate)

In chemistry, periodicity refers to the recurring trends in the properties of elements. Just as we see periodic patterns in nature—like the changing phases of the Moon Science, Class VIII, Keeping Time with the Skies, p.178—the Modern Periodic Table organizes elements so that those with similar behaviors appear at regular intervals. The master key to this arrangement is the Atomic Number (the number of protons in an atom), which dictates how electrons are distributed around the nucleus.

The table is structured into a grid of Groups and Periods, each serving a distinct purpose in helping us understand an element's "personality":

• Groups (Vertical Columns): There are 18 groups. Elements in the same group have the same number of valence electrons (electrons in the outermost shell). Because chemical reactions involve these outer electrons, elements in a group share similar chemical properties. For example, Group 17 (Halogens) are all highly reactive non-metals.
• Periods (Horizontal Rows): There are 7 periods. The period number tells us the number of electron shells an atom possesses. For instance, any element in Period 3 has three shells of electrons. As you move from left to right across a period, the atomic number increases by one, and the valency changes systematically.

Feature	Groups (18)	Periods (7)
Direction	Vertical Columns	Horizontal Rows
Significance	Same valence electrons; similar chemistry	Same number of occupied electron shells
Example	Lithium and Sodium (Group 1)	Lithium and Carbon (Period 2)

Understanding an element's position allows us to predict its valency, or its combining capacity. For example, Carbon (atomic number 6) is located in Group 14 and has four valence electrons, making it tetravalent Science, Class X, Carbon and its Compounds, p.62. This specific placement is what makes the Modern Periodic Table the most powerful tool in chemistry—it isn't just a list; it is a map of reactivity.

Sources: Science, Class VIII, Keeping Time with the Skies, p.178; Science, Class X, Carbon and its Compounds, p.62

4. Chemical Bonding: Ionic and Covalent (intermediate)

In our journey through chemistry, we find that atoms are rarely happy being alone. Most atoms seek stability, which they achieve by mimicking the Noble Gases like Neon or Argon. These gases have a completely filled outermost shell, making them chemically inert. For most other elements, reactivity is simply a quest to attain this "perfect" electronic configuration, often referred to as the Octet Rule (having eight electrons in the valence shell) Science, class X (NCERT 2025 ed.), Chapter 3, p.46.

There are two primary ways atoms achieve this stability. The first is Ionic Bonding, which involves the complete transfer of electrons from one atom to another. Imagine Sodium (Na), which has one lonely electron in its outer shell (2, 8, 1). It finds it much easier to give that electron away to Chlorine (Cl), which has seven electrons (2, 8, 7) and is desperate for just one more Science, class X (NCERT 2025 ed.), Chapter 3, p.47. This transfer creates ions—charged particles—that stick together like magnets due to strong electrostatic forces. This is why ionic compounds typically have very high melting points and conduct electricity when dissolved in water Science, class X (NCERT 2025 ed.), Chapter 4, p.58.

The second method is Covalent Bonding, which is all about sharing. When atoms (usually non-metals) have similar needs, they contribute electrons to a shared pool so that both can count those electrons toward their octet. For instance, a Nitrogen atom (atomic number 7) has five valence electrons and needs three more. In a Nitrogen molecule (N₂), two nitrogen atoms share three pairs of electrons, creating a powerful triple bond Science, class X (NCERT 2025 ed.), Chapter 4, p.60. Because these molecules are neutral and the forces between them are relatively weak, covalent compounds like methane or water often have lower melting points and do not conduct electricity Science, class X (NCERT 2025 ed.), Chapter 4, p.59.

Feature	Ionic Bonding	Covalent Bonding
Mechanism	Transfer of electrons	Sharing of electrons
Constituents	Metals + Non-metals	Non-metals + Non-metals
Electrical Conductivity	High (in molten/solution state)	Poor (generally non-conductors)

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.58; 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

5. Isotopes and Isobars in Science & Tech (intermediate)

To understand isotopes and isobars, we must look at the identity card of an atom: its Atomic Number (Z), which is the number of protons, and its Mass Number (A), which is the sum of protons and neutrons. Isotopes are atoms of the same element that possess the same atomic number but different mass numbers. This means they have the same number of protons and electrons (leading to identical chemical properties) but a different number of neutrons. For example, Carbon exists as ¹²C and ¹⁴C. While ¹²C is stable, ¹⁴C is a radioactive isotope used in AMS (Accelerator Mass Spectrometry) dating to determine the age of ancient artifacts, such as those found at the Keeladi excavation site History, class XI (Tamilnadu state board 2024 ed.), Evolution of Society in South India, p.70.

On the other hand, Isobars are atoms of different chemical elements that have the same mass number but different atomic numbers. Because they are different elements, they have entirely different chemical properties. A classic example is Argon (atomic number 18) and Calcium (atomic number 20); both can have a mass number of 40. In the context of Physical Geography, you will also encounter the term 'isobars' in a different sense—referring to lines on a weather map connecting points of equal atmospheric pressure, where the closeness of these lines indicates the strength of the pressure gradient and wind speed Physical Geography by PMF IAS, Pressure Systems and Wind System, p.306.

Understanding these variations is crucial for Science & Tech because isotopes drive innovations in nuclear medicine (like Cobalt-60 for cancer treatment) and archaeology, while the concept of mass helps us understand the stability of matter.

Feature	Isotopes	Isobars
Atomic Number (Z)	Same (Same Element)	Different (Different Elements)
Mass Number (A)	Different	Same
Chemical Properties	Identical	Different
Neutron Count	Different	Different

Sources: History , class XI (Tamilnadu state board 2024 ed.), Evolution of Society in South India, p.70; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.306

6. Understanding Valency and Valence Electrons (exam-level)

To understand how atoms interact, we must look at their valence electrons—the electrons residing in the outermost shell of an atom. These electrons are the "active players" in chemical reactions because they determine how an atom will bond with others. While noble gases like Helium or Neon are chemically inert because their outermost shells are completely full, other elements react because they have a natural tendency to attain that same stable, completely filled valence shell Science, Class X, Chapter 3, p.46. This drive for stability is the foundation of all chemical bonding.

Valency is the "combining capacity" of an atom. It is defined by how many electrons an atom needs to lose, gain, or share to achieve a stable octet (eight electrons in the outermost shell, or two in the case of the K shell). For instance, if an atom has only one electron in its outer shell, like Lithium (Atomic Number 3), it is easier to lose that one electron than to gain seven; thus, its valency is 1. Conversely, if an atom like Oxygen (Atomic Number 8) has six valence electrons, it seeks to gain or share two more to reach eight, giving it a valency of 2 Science, Class X, Chapter 4, p.60.

It is crucial to distinguish between the number of valence electrons and the valency itself. For elements with 1 to 4 valence electrons, the valency is usually equal to the number of valence electrons. For elements with 5 to 7 valence electrons, the valency is calculated by subtracting that number from 8.

Element	Atomic Number	Electronic Configuration	Valence Electrons	Valency
Lithium	3	2, 1	1	1
Boron	5	2, 3	3	3
Carbon	6	2, 4	4	4 (Tetravalent)
Oxygen	8	2, 6	6	2

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

7. Solving the Original PYQ (exam-level)

To solve this question, you must synthesize the concepts of atomic number and electronic configuration that we recently covered. Valency is essentially the combining capacity of an atom, which is dictated by how many electrons it must lose, gain, or share to achieve a stable, full outer shell. As noted in Science, class X (NCERT 2025 ed.), this stability is the driving force behind chemical bonding. By applying the 2n² rule to distribute electrons, you can systematically determine the valency of any element on the periodic table.

Walking through the options, let’s look at Boron. With an atomic number of 5, its electrons are arranged as (2, 3). Because it has three electrons in its outermost shell, it typically shares these electrons to form bonds, giving it a valency of three. Therefore, (D) Boron is the correct choice. In your preparation, always associate Boron with this "trivalent" nature, just as you associate Carbon (atomic number 6) with being tetravalent, meaning it has a valency of four to complete its octet, a concept emphasized in Science, class X (NCERT 2025 ed.) > Chapter 4.

A common trap UPSC sets involves the distinction between valence electrons and valency. For example, Oxygen has an atomic number of 8 and a configuration of (2, 6). While it has six valence electrons, its valency is actually two because it needs to gain two electrons to reach a stable octet of eight. Similarly, Lithium (atomic number 3) has a configuration of (2, 1), resulting in a valency of one. Do not be misled by the number of electrons present; always calculate how many are needed or removed for stability.