The total number of sigma bonds is between (C2H6) is

1. The Versatile Nature of Carbon (basic)

Carbon is often called the 'building block of life' because of its extraordinary ability to form a vast array of compounds—from the simple carbon dioxide (CO₂) in the atmosphere to the complex DNA in our cells. This uniqueness doesn't happen by chance; it arises from two fundamental chemical properties: Tetravalency and Catenation. Unlike many other elements that are restricted in how they connect, carbon acts like a universal connector in the molecular world. Science, Chapter 4, p.77

The first pillar of carbon's versatility is Tetravalency. Carbon has four electrons in its outermost shell. To achieve stability (a full octet), it shares these four electrons with other atoms through covalent bonding. Because it needs four bonds to be satisfied, it can link up with four different monovalent atoms (like Hydrogen) or form multiple bonds (double or triple) with atoms like Oxygen or Nitrogen. This ability to 'reach out' in four directions allows for the creation of complex, three-dimensional structures. Science, Chapter 4, p.59

The second pillar is Catenation, which is carbon’s unique ability to form long, stable chains by bonding with other carbon atoms. While elements like Silicon can form chains with hydrogen, they are often unstable and highly reactive. In contrast, the Carbon-Carbon (C–C) bond is exceptionally strong and stable, allowing carbon to form straight chains, branched trees, or even closed rings. Science, Chapter 4, p.62

Feature	Tetravalency	Catenation
Definition	Having 4 valence electrons available for bonding.	Self-linking property to form long chains or rings.
Impact	Allows bonding with various elements (O, N, H, Cl).	Allows for immense structural variety and size.

Sources: Science (NCERT 2025 ed.), 4: Carbon and its Compounds, p.59, 62, 77

2. Classification of Hydrocarbons: Alkanes, Alkenes, and Alkynes (basic)

At the heart of organic chemistry are hydrocarbons—compounds composed entirely of carbon and hydrogen atoms Science, Class X (NCERT 2025 ed.), Chapter 4, p.65. Based on how these carbon atoms are linked together, we classify them into three primary families: Alkanes, Alkenes, and Alkynes. This classification is not just naming; it tells us about the "saturation" of the molecule. A saturated hydrocarbon is one where all carbon atoms are connected by single covalent bonds, meaning the carbon atoms are "saturated" with the maximum possible number of hydrogen atoms. These are known as Alkanes Science, Class X (NCERT 2025 ed.), Chapter 4, p.65.

When carbon atoms form double or triple bonds with each other, they are called unsaturated hydrocarbons because they contain fewer hydrogen atoms than an alkane with the same number of carbons. Alkenes contain at least one double bond (C=C), while Alkynes contain at least one triple bond (C≡C) Science, Class X (NCERT 2025 ed.), Chapter 4, p.65. These multiple bonds are chemically "reactive sites"; for instance, unsaturated fats found in vegetable oils can undergo hydrogenation in the presence of a catalyst like nickel to become saturated fats Science, Class X (NCERT 2025 ed.), Chapter 4, p.71.

Understanding the general formulas of these series helps us predict their molecular structure quickly:

Type	Bond Type	Saturation	General Formula	Example
Alkane	Single (C–C)	Saturated	CₙH₂ₙ₊₂	Ethane (C₂H₆)
Alkene	Double (C=C)	Unsaturated	CₙH₂ₙ	Ethene (C₂H₄)
Alkyne	Triple (C≡C)	Unsaturated	CₙH₂ₙ₋₂	Ethyne (C₂H₂)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.64-66, 71

3. The Concept of Chemical Bonding (intermediate)

At the heart of all chemistry is the drive for stability. Atoms are essentially seeking a state of balance, which they achieve by filling their outermost electron shells—a concept known as the Octet Rule. In the world of organic chemistry, this stability is reached through Chemical Bonding, primarily of two types: Ionic and Covalent.

Ionic bonds are formed when one atom completely transfers electrons to another, creating charged particles (ions) held together by intense electrostatic attraction. These compounds, like Sodium Chloride, are typically hard solids with high melting points because it takes massive energy to break those attractions Science, Metals and Non-metals, p.49. However, carbon is unique. With four electrons in its outer shell, carbon would require an immense amount of energy to either lose four electrons or gain four more. Instead, it chooses a middle path: Covalent Bonding. In this process, atoms share pairs of electrons so that both atoms can "claim" a full outer shell Science, Carbon and its Compounds, p.59.

Feature	Ionic Compounds	Covalent Compounds (Carbon)
Mechanism	Transfer of electrons	Sharing of electrons
Forces	Strong electrostatic attraction	Weak intermolecular forces
Conductivity	Good (in molten/solution state)	Poor (no ions formed)
Melting Point	High	Low

In organic molecules, these shared electrons form specific structures. When one pair of electrons is shared, we call it a single bond. When multiple pairs are shared, we see double or triple bonds (like the triple bond in N₂ where each nitrogen atom contributes three electrons Science, Carbon and its Compounds, p.60). In hydrocarbons like Ethane (C₂H₆), every single bond formed by sharing is known as a sigma (σ) bond. This includes the bond between the two carbon atoms and the bonds between carbon and hydrogen. Because covalent molecules are held together by internal sharing rather than external electrostatic grids, the forces between molecules are relatively weak, explaining why many carbon compounds are gases or liquids at room temperature Science, Carbon and its Compounds, p.59.

Sources: Science, Carbon and its Compounds, p.59; Science, Metals and Non-metals, p.49; Science, Carbon and its Compounds, p.60

4. Geometry and Hybridization in Carbon (intermediate)

To understand how carbon builds complex molecules, we must look at the 3D space its atoms occupy. Carbon has four valence electrons, allowing it to form four covalent bonds. In saturated hydrocarbons (alkanes), carbon undergoes sp³ hybridization. This means one 's' orbital and three 'p' orbitals mix to form four identical hybrid orbitals. To minimize electron repulsion, these orbitals point toward the corners of an imaginary pyramid, resulting in a tetrahedral geometry with bond angles of approximately 109.5°. This 3D structure is the reason why even simple molecules like methane (CH₄) are not flat. Science, Class X (NCERT 2025 ed.), Chapter 4, p. 62

When we move to Ethane (C₂H₆), the same principles apply. To construct the molecule, we first link the two carbon atoms with a single bond. Each carbon then uses its three remaining hybrid orbitals to bond with hydrogen atoms Science, Class X (NCERT 2025 ed.), Chapter 4, p. 63. These single covalent bonds, formed by the direct "head-on" overlap of orbitals, are known as sigma (σ) bonds. Because every bond in an alkane is a single bond, every single one of them is a sigma bond. In ethane, this results in one C–C sigma bond and six C–H sigma bonds, totaling seven.

Feature	Methane (CH₄)	Ethane (C₂H₆)
Hybridization	sp³	sp³
Geometry	Tetrahedral	Tetrahedral (at each C)
Total Sigma Bonds	4	7

As we increase the number of carbon atoms in a chain — such as in propane (C₃H₈) or butane (C₄H₁₀) — the tetrahedral framework continues, creating a "zig-zag" appearance in the carbon backbone rather than a straight line Science, Class X (NCERT 2025 ed.), Chapter 4, p. 64. This geometry is fundamental to the stability and chemical behavior of saturated hydrocarbons, which are generally unreactive under normal conditions but can undergo substitution reactions in sunlight Science, Class X (NCERT 2025 ed.), Chapter 4, p. 71.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.62-64, 71

5. Isomerism: Structural Diversity in Organic Chemistry (exam-level)

In the vast world of organic chemistry, carbon’s unique ability to link with itself (catenation) and form four bonds (tetravalency) leads to a fascinating phenomenon called Isomerism. At its heart, isomerism occurs when compounds share the exact same molecular formula (the same number and types of atoms) but possess different structural arrangements. These are known as structural isomers. Think of it like having the same set of LEGO bricks but building a tall tower with some and a wide bridge with others; the components are identical, but the final structures—and their properties—are different.

To understand this from first principles, let’s look at the alkane series. While Methane (CH₄), Ethane (C₂H₆), and Propane (C₃H₈) have only one possible way to arrange their atoms, larger molecules like Butane (C₄H₁₀) offer more variety. Butane can exist as a straight chain (n-butane) or as a branched structure (isobutane or 2-methylpropane). As the number of carbon atoms in the chain increases, the number of possible structural isomers grows exponentially Science, Carbon and its Compounds, p.64.

A classic exam-level example is Pentane (C₅H₁₂). You can arrange these five carbon atoms in three distinct ways: a straight line, a chain of four with one branch, or a cross-shaped structure with a central carbon bonded to four others Science, Carbon and its Compounds, p.68. This structural diversity is a primary reason why there are millions of carbon-based compounds in existence.

Feature	Molecular Formula	Structural Arrangement
Isomers	Identical (e.g., both are C₄H₁₀)	Different (e.g., straight vs. branched)
Homologous Series	Different (differ by –CH₂ units)	Similar pattern of bonding

Sources: Science, Carbon and its Compounds, p.64; Science, Carbon and its Compounds, p.68

6. Sigma (σ) and Pi (π) Bonds (exam-level)

To understand the architecture of organic molecules, we must look at how atoms actually "overlap" to share electrons. A covalent bond is formed when two atoms share an electron pair to reach a stable configuration Science, Chapter 4, p.60. However, not all overlaps are the same. In organic chemistry, we classify these overlaps into two primary types: Sigma (σ) and Pi (π) bonds.

A Sigma (σ) bond is the strongest type of covalent bond. It is formed by the "head-on" or axial overlap of atomic orbitals. Think of it as a firm handshake. In any molecule, the first bond formed between two atoms is always a sigma bond. Because of their direct overlap, sigma bonds allow for the free rotation of atoms around the bond axis. In saturated compounds—those containing only single bonds—every single bond you see is a sigma bond Science, Chapter 4, p.62.

A Pi (π) bond, on the other hand, occurs only when multiple bonds (double or triple) are present. These are formed by the "sideways" or lateral overlap of orbitals. If a sigma bond is a handshake, a pi bond is like two people standing side-by-side with their arms touching. These bonds are generally weaker than sigma bonds and prevent the atoms from rotating. Unsaturated compounds are defined by the presence of these double or triple bonds Science, Chapter 4, p.62.

Feature	Sigma (σ) Bond	Pi (π) Bond
Overlap Type	Head-on (Axial)	Sideways (Lateral)
Bond Order	First bond formed	Second or third bond
Rotation	Allows free rotation	Restricts rotation
Strength	Stronger	Weaker

Let’s apply this to Ethane (C₂H₆). Ethane is a saturated hydrocarbon where each carbon atom is linked to the other by a single bond, and to three hydrogen atoms by single bonds. Since all these are single covalent bonds, they are all sigma bonds. Counting them up: one C–C sigma bond plus six C–H sigma bonds gives us a total of 7 sigma bonds Science, Chapter 4, p.77.

Sources: Science, Carbon and its Compounds, p.60; Science, Carbon and its Compounds, p.62; Science, Carbon and its Compounds, p.77

7. Analyzing the Structure of Ethane (C₂H₆) (exam-level)

To understand the structure of ethane (C₂H₆), we must start with the fundamental properties of carbon: its tetravalency. Ethane is the second member of the alkane homologous series, following methane. Unlike methane, which has a single central carbon, ethane introduces a carbon-carbon backbone. To construct its structure, we first link the two carbon atoms with a single covalent bond. This single bond uses one valency from each carbon, leaving three valencies per carbon atom unsatisfied. To achieve stability, these remaining six valencies are filled by six hydrogen atoms, resulting in a saturated hydrocarbon where every atom is connected by single bonds Science, Carbon and its Compounds, p.63.

In terms of electronic arrangement, each carbon atom in ethane undergoes sp³ hybridization, meaning it organizes its orbitals to form four equivalent bonds directed toward the corners of a tetrahedron. This results in the formation of sigma (σ) bonds, which are the strongest type of covalent chemical bond. When we analyze the complete molecule, we find:

• One C–C bond: A single sigma bond connecting the two carbon atoms.
• Six C–H bonds: Three sigma bonds for each carbon atom connecting it to hydrogen.

Because every single bond in a saturated hydrocarbon is a covalent bond, the total count for ethane is exactly seven covalent bonds Science, Carbon and its Compounds, p.77. This structural integrity makes ethane relatively stable and less reactive compared to unsaturated compounds like ethene.

Understanding this 'chain-building' is crucial because it forms the template for all higher alkanes. By adding a –CH₂– unit to ethane, we arrive at propane (C₃H₈), and continuing this pattern creates the diverse range of carbon chains found in organic chemistry Science, Carbon and its Compounds, p.64, 66.

Sources: Science, Carbon and its Compounds, p.63; Science, Carbon and its Compounds, p.64; Science, Carbon and its Compounds, p.66; Science, Carbon and its Compounds, p.77

8. Solving the Original PYQ (exam-level)

Now that you have mastered the basics of sp3 hybridization and the structure of saturated hydrocarbons, this question brings those building blocks together. In an alkane like ethane (C2H6), every single covalent bond represents a sigma (σ) bond. To solve this, you must visualize the molecular geometry: two carbon atoms are linked by a single C–C sigma bond, and each carbon is further bonded to three hydrogen atoms to complete its octet. As noted in Science, Class X (NCERT), this arrangement ensures that the carbon atoms achieve a stable, saturated state where all available valencies are filled by single bonds.

To arrive at the correct answer, simply walk through the connectivity of the molecule. Each carbon atom forms four bonds: three with hydrogen atoms and one with the neighboring carbon atom. This results in six C–H bonds and one C–C bond. Adding these together (6 + 1) gives us a total of seven sigma bonds, which makes (C) Seven the correct choice. Thinking structurally rather than just memorizing formulas allows you to verify the count logically every time.

UPSC often includes distractor options to test your precision. Option (B) Six is the most common trap; students frequently focus only on the C–H bonds and neglect the central C–C bond that holds the molecule together. Options (A) Five and (D) Four are incorrect because they fail to satisfy the tetravalency of carbon in a saturated system. Always remember to account for every single line in the structural formula to avoid these simple counting errors.