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Q1. Molecular formula of chlorine gas is:
Chlorine is a non-metal from group 17 (Halogens). A single chlorine atom has 7 valence electrons and is highly unstable. To become stable like Argon (8 electrons), two chlorine atoms share one pair of electrons, forming a single covalent bond. This creates a diatomic molecule (two atoms), so the correct formula is Cl₂. Remember the “HOFBrINCl” rule — these elements (Hydrogen, Oxygen, Fluorine, Bromine, Iodine, Nitrogen, Chlorine) always come in pairs when found alone in nature.
Q2. Which compound has the highest melting point among those listed?
Melting point depends on intermolecular forces. Methane (CH₄) is a gas with very weak London forces. Chloroform (CHCl₃) is a liquid with dipole-dipole forces. Ethanol (C₂H₅OH) has hydrogen bonding but is a small molecule. Acetic acid (CH₃COOH) does something special: it forms dimers (two molecules joining together like a chain) through strong hydrogen bonds. This extra bonding requires more heat energy to break, giving it the highest melting point (16.6°C) compared to ethanol (-114°C), chloroform (-63°C), and methane (-182°C).
Q3. All living structures are primarily:
Life on Earth is called “carbon-based life” because carbon atoms can form four covalent bonds with other atoms (hydrogen, oxygen, nitrogen, etc.). This property, called tetravalency, allows carbon to build long chains, rings, and complex molecules like carbohydrates, proteins, fats, and DNA. No other element can form such a wide variety of stable, large molecules. While hydrogen and oxygen are also abundant in our bodies, carbon is the backbone of all organic chemistry.
Q4. Oxygen completes its octet by sharing:
An oxygen atom has 6 valence electrons (Group 16). It needs 2 more electrons to reach 8 (stable like Neon). Instead of stealing electrons (which requires too much energy), oxygen shares its electrons. In a water molecule (H₂O), oxygen shares one electron with each hydrogen atom (total 2 electrons shared). In an oxygen molecule (O₂), each oxygen shares two electrons (forming a double bond). So in all cases, oxygen completes its octet by sharing a total of two electrons from its own side.
Q5. Atomic number of chlorine is:
The atomic number (Z) is the number of protons in the nucleus of an atom. Chlorine (symbol Cl) has 17 protons. This number is fixed and never changes for an element. You can find it on the periodic table: Chlorine is in period 3, group 17. Atomic number 16 is sulfur, 8 is oxygen, and 18 is argon. Remember: Atomic number = number of protons = number of electrons (in a neutral atom).
Q6. Which test can confirm the presence of carbon dioxide?
Limewater is a solution of calcium hydroxide [Ca(OH)₂]. When carbon dioxide (CO₂) is passed through it, a chemical reaction occurs: CO₂ + Ca(OH)₂ → CaCO₃ (white precipitate) + H₂O. The clear limewater turns milky white due to the formation of insoluble calcium carbonate. This is the standard confirmatory test for CO₂. Burning test confirms combustible gases, Benedict’s test tests for sugars, and litmus test checks for acids or bases.
Q7. Atmospheric carbon is present mainly as:
In Earth’s atmosphere, carbon is found mostly in the form of carbon dioxide (CO₂) gas. While methane (CH₄) is also a greenhouse gas, its concentration is much lower (about 1.8 parts per million). Carbon monoxide (CO) is a pollutant from incomplete burning and is toxic but present in very tiny amounts. Carbonates (like limestone) contain carbon but are found in the Earth’s crust, not the atmosphere.
Q8. Percentage of carbon dioxide in the atmosphere is about:
The concentration of carbon dioxide in Earth’s atmosphere is approximately 0.03% to 0.04% (about 400-420 parts per million). While this seems very small, it is enough to trap heat and support photosynthesis. For comparison, nitrogen is about 78%, oxygen is about 21%, and argon is about 0.9%. Even this small percentage of CO₂ is critical for life and climate regulation.
Q9. Bond formed by sharing of electrons is called:
A covalent bond is formed when two atoms share one or more pairs of electrons. This usually happens between non-metal atoms. For example, in a hydrogen molecule (H₂), both atoms share their single electrons. In an ionic bond, electrons are completely transferred from one atom to another (e.g., NaCl). Metallic bonds involve a “sea of delocalized electrons” in metals. Hydrogen bonds are weak attractions between a hydrogen atom and an electronegative atom (like O or N) in different molecules.
Q10. Carbon compounds are poor conductors of electricity because they:
Electricity is conducted by the movement of charged particles (ions or electrons). Most carbon compounds are covalent compounds — they do not dissociate into ions when dissolved in water or melted. Without free-moving ions or free electrons, there is no charge flow. For example, sugar solution does not conduct electricity, but salt solution (which forms Na⁺ and Cl⁻ ions) does. The volatility, solid state, or acidity of carbon compounds does not directly explain poor conductivity.
Q11. A single covalent bond consists of:
A single covalent bond is formed when two atoms contribute one electron each to form a shared pair. This pair of electrons is counted as one bond. For example, in a hydrogen molecule (H₂), the bond is H:H (one shared pair). “Two shared pairs” means a double bond (e.g., O₂). “Four shared electrons” means two pairs (again a double bond). “One shared electron” is impossible because electrons always come in pairs in covalent bonds.
Q12. Atomic number of hydrogen is:
Hydrogen is the lightest and simplest element. Its atomic number is 1, meaning it has 1 proton in its nucleus. In a neutral hydrogen atom, it also has 1 electron. Atomic number 2 is helium, 3 is lithium, and 0 is not possible (atomic numbers start from 1). Hydrogen is unique because it can lose its one electron to become H⁺ (a proton) or gain one to become H⁻ (hydride ion).
Q13. Atomic number of carbon is:
Carbon has an atomic number of 6, meaning it has 6 protons and 6 electrons. Its electronic configuration is 2,4 (K shell = 2 electrons, L shell = 4 valence electrons). This arrangement allows carbon to form four covalent bonds, making it the basis of all organic chemistry. Atomic number 4 is beryllium, 8 is oxygen, and 12 is magnesium (mass number of carbon is also 12, but that’s different from atomic number).
Q14. Percentage of carbon present in the Earth’s crust is approximately:
Carbon is not very abundant in the Earth’s crust. It makes up only about 0.02% by weight. Most of this carbon is locked in carbonate rocks like limestone (CaCO₃), dolomite, and marble, as well as in fossil fuels (coal, oil, natural gas) and organic matter in soil. For comparison, oxygen is about 46%, silicon 28%, aluminum 8%, and iron 5%. Despite its low percentage, carbon is extremely important due to its unique bonding properties.
Q15. Carbon needs to gain or lose how many electrons to attain noble gas configuration?
Carbon has 4 valence electrons (electronic configuration 2,4). To achieve the stable noble gas configuration of Neon (2,8), it can either: Lose 4 electrons (to become C⁴⁺) — but this requires too much energy and is very difficult. Gain 4 electrons (to become C⁴⁻) — but the nucleus cannot hold 4 extra electrons due to repulsion. So carbon shares its 4 electrons with other atoms instead of completely gaining or losing them. This is why carbon forms covalent bonds.
Q16. Hydrogen atoms share electrons to attain configuration of:
Hydrogen has 1 electron in its K shell. The K shell can hold a maximum of 2 electrons (duplet rule, not octet rule for hydrogen). By sharing one electron with another hydrogen atom, each hydrogen gets 2 electrons in its outer shell, which is exactly the stable configuration of Helium (He). Helium is a noble gas with a completely filled K shell. Hydrogen does not aim for Argon, Krypton, or Neon because those have more than 2 electrons and require filling higher shells.
Q17. Most carbon compounds have relatively:
Most carbon compounds are covalent compounds. Covalent bonds are strong within a molecule, but the forces between molecules (intermolecular forces) are weak (London forces, dipole-dipole forces, or hydrogen bonds). These weak forces require little heat energy to overcome. Therefore, most carbon compounds (like wax, sugar, kerosene, methane) have low melting and boiling points compared to ionic compounds (like NaCl, which has a high melting point of 801°C). There are exceptions like diamond and graphite, but they are allotropes, not typical molecular compounds.
Q18. Which of the following allotropes of carbon is a good conductor of electricity?
Graphite is a good conductor of electricity because of its unique structure. In graphite, each carbon atom is bonded to three other carbon atoms, leaving one free electron that is delocalized (able to move freely) between the layers. These free electrons can carry an electric current. In diamond, all four valence electrons are used in strong covalent bonds, leaving no free electrons. Fullerenes (like C₆₀) are poor conductors. Amorphous carbon (like charcoal) conducts very poorly.
Q19. Atomic number of oxygen is:
Oxygen has an atomic number of 8, meaning it has 8 protons and 8 electrons. Its electronic configuration is 2,6 (K shell = 2, L shell = 6 valence electrons). It needs 2 more electrons to complete its octet, which is why it forms two bonds (like in H₂O) or a double bond (like in O₂). Atomic number 6 is carbon, 7 is nitrogen, and 16 is sulfur.
Q20. Carbon compounds do not conduct electricity mainly because they:
This is similar to
Q21. Covalent compounds generally have:
Within a covalent molecule, atoms are held together by strong covalent bonds. But between different molecules, the forces of attraction (intermolecular forces) are weak — they are either London dispersion forces, dipole-dipole interactions, or hydrogen bonds. Because these forces are weak, covalent compounds have low melting and boiling points. Ionic and metallic compounds have strong intermolecular (or interionic) forces, which is why they are solids at room temperature with high melting points.
Q22. Main reason for diversity of carbon compounds is:
Carbon’s ability to form covalent bonds is the main reason for the millions of carbon compounds. Specifically, two properties make carbon unique: Tetravalency — Carbon can form 4 bonds. Catenation — Carbon atoms can bond with other carbon atoms to form long chains, branched chains, and rings. High atomic mass (12) is not special; metallic nature and radioactivity are not properties of carbon. Covalent bonding allows carbon to combine with H, O, N, S, Cl, and many other elements in countless ways.
Q23. Formation of C⁴⁻ ion is difficult because:
Carbon has only 6 protons in its nucleus. If it tries to gain 4 extra electrons to become C⁴⁻, the electrostatic repulsion between the negatively charged electrons becomes very strong. The small nucleus with 6 positive charges is not strong enough to hold 10 electrons (6 original + 4 extra) tightly. This makes the C⁴⁻ ion highly unstable. For the same reason, losing 4 electrons to become C⁴⁺ also requires too much energy. That’s why carbon prefers sharing.
Q24. Which compound has the lowest boiling point among those listed?
Methane (CH₄) is a non-polar molecule with only weak London dispersion forces between its molecules. It is a gas at room temperature and boils at -161.5°C. Ethanol has hydrogen bonding (boiling point 78°C). Acetic acid has hydrogen bonding plus dimer formation (boiling point 118°C). Chloroform has dipole-dipole forces (boiling point 61°C). Methane has the weakest intermolecular forces, so it has the lowest boiling point.
Q25. Despite its small availability, carbon is important due to its:
Carbon is only 0.02% of the Earth’s crust and 0.03% of the atmosphere, yet it is the basis of all known life. This is because of its unique bonding properties — catenation (ability to form long chains) and tetravalency (ability to form four strong covalent bonds). These properties allow carbon to form millions of organic compounds, from DNA and proteins to plastics and medicines. Reactivity, high density, and metallic nature are not special features of carbon.
Q26. Water molecule contains bonds that are:
A water molecule (H₂O) has two single covalent bonds. Each hydrogen atom shares one pair of electrons with the oxygen atom. The bonds are represented as O–H (single bonds). There are no double or triple bonds in water. The bonds are covalent, not ionic. Oxygen does form a double bond in O₂, but not in H₂O.
Q27. Single bond is represented by:
In chemical structures, a single covalent bond is represented by a single straight line between two atoms. For example, H–H represents a hydrogen molecule. A double bond is represented by two lines (H₂C=CH₂), and a triple bond by three lines (N≡N). Arrows (→) are used for coordinate bonds or reaction directions. Dots (·) represent lone pair electrons. Circles are not used for bonds.
Q28. Which element is described as being of immense significance in both elemental and combined forms?
Carbon in its elemental form includes diamond (hardest natural substance), graphite (used in pencils and lubricants), fullerenes, and charcoal. In its combined form, carbon is found in millions of organic compounds (food, fuels, medicines, plastics, DNA, proteins). No other element matches this versatility. Oxygen and nitrogen are important but mainly in combined forms in living systems. Hydrogen is important but not as diverse in elemental forms.
Q29. Carbon overcomes bonding difficulty by:
Carbon has 4 valence electrons. It cannot easily lose 4 electrons (requires too much energy) nor gain 4 electrons (nucleus cannot hold them). So carbon overcomes this difficulty by sharing electrons with other atoms (including other carbon atoms) to form covalent bonds. This sharing gives each carbon atom a share in 8 electrons (octet) without fully gaining or losing any. This is the foundation of organic chemistry.
Q30. Simplest molecule formed by sharing electrons is:
The simplest covalent molecule is H₂ (hydrogen gas). Each hydrogen atom has 1 electron. They share a single pair of electrons to form a stable molecule. The molecule has only two atoms and one bond. Oxygen (O₂) is diatomic but has a double bond. Nitrogen (N₂) has a triple bond. Carbon dioxide (CO₂) has three atoms and double bonds. So H₂ is the smallest and simplest.
Q31. Carbon shares electrons to achieve:
Carbon atoms share electrons with other atoms to achieve the stable electronic configuration of the nearest noble gas (Neon, 2,8). By sharing four pairs of electrons, carbon gets a share in 8 electrons in its outer shell. This stability lowers the energy of the system and makes the molecule stable. Conductivity, magnetism, and radioactivity are unrelated to why carbon shares electrons.
Q32. Nitrogen molecule contains:
A nitrogen molecule (N₂) has a triple covalent bond. Each nitrogen atom has 5 valence electrons. To achieve an octet, they share three pairs of electrons (represented as N≡N). This triple bond is very strong (bond energy ~941 kJ/mol), which is why nitrogen gas is very unreactive. For comparison: O₂ has a double bond, H₂ and Cl₂ have single bonds.
Q33. Number of electrons in K shell of helium is:
The K shell is the first shell (n=1). It can hold a maximum of 2 electrons (using the formula 2n², where n=1 gives 2). Helium (atomic number 2) has exactly 2 electrons, and both are in the K shell. This is a completely filled K shell, making helium a noble gas (stable). Hydrogen has 1 electron in its K shell.
Q34. Bonding in carbon compounds is mainly:
Carbon is a non-metal with 4 valence electrons. It rarely forms ionic bonds because it would have to gain or lose 4 electrons, which is energetically very difficult. Instead, carbon almost always forms covalent bonds by sharing electrons. This includes simple molecules like CH₄, CO₂, and C₂H₅OH, as well as giant covalent structures like diamond and graphite. Electrovalent/ionic bonds are found in salts like NaCl, not typical carbon compounds.
Q35. Noble gas configuration means:
Noble gases (Helium, Neon, Argon, etc.) are stable because their outermost electron shell is completely filled. For Helium, the K shell has 2 electrons (duplet). For others (Neon, Argon), the outermost shell has 8 electrons (octet). A completely filled shell gives the atom low energy and high stability. Half-filled shells (like in nitrogen) are relatively stable but not as stable as a completely filled shell. Unstable or empty shells are found in reactive elements.
Q36. Most items used in daily life like food, clothes, and medicines are based on which element?
Food (carbohydrates, proteins, fats), clothes (cotton, wool, polyester — all carbon-based polymers), and medicines (most drugs are organic compounds) are all primarily made of carbon compounds. Even soaps, plastics, paper, wood, and fuels are carbon-based. Silicon is used in electronics and glass, aluminium in utensils and foils, and iron in construction and tools — but for the items listed (food, clothes, medicines), carbon is the key element.
Q37. A double bond consists of:
A double covalent bond is formed when two atoms share two pairs of electrons (total 4 electrons). For example, in oxygen gas (O₂), the bond is O=O (one sigma bond and one pi bond). A single bond = one shared pair. A triple bond = three shared pairs. Four shared pairs would be a quadruple bond, which is extremely rare.
Q38. Number of electrons shared by each oxygen atom in O₂ is:
In an oxygen molecule (O₂), each oxygen atom has 6 valence electrons. To achieve an octet, they share two electrons from each atom (forming a double bond). So each oxygen atom contributes 2 electrons to the sharing. The shared pair is counted as belonging to both atoms. This gives each oxygen atom a share in 8 electrons. The key point: each atom puts in 2 electrons.
Q39. Bond between two oxygen atoms is:
In an oxygen molecule (O₂), the two oxygen atoms are joined by a double covalent bond (O=O). Each oxygen needs 2 more electrons to complete its octet. By sharing two pairs of electrons, both achieve stability. A single bond would give each oxygen only 7 electrons (not enough). A triple bond would give 10 electrons (too many). So double bond is perfect.
Q40. Shared electrons in covalent bonding belong to:
In a covalent bond, the shared pair of electrons is in a region of space called the molecular orbital, which is associated with both atoms. This means the electrons “belong” to both atoms simultaneously. They spend part of their time near one nucleus and part near the other. This mutual sharing is what holds the atoms together. Electrons do not belong to the nucleus or only one atom in a covalent bond.
Q41. Example of a covalent bond is found in:
The hydrogen molecule (H₂) is the classic example of a covalent bond — two hydrogen atoms sharing one pair of electrons. Sodium chloride (NaCl) has an ionic bond (Na⁺ Cl⁻). Calcium oxide (CaO) has an ionic bond (Ca²⁺ O²⁻). Magnesium chloride (MgCl₂) has ionic bonds (Mg²⁺ and two Cl⁻ ions). So only the hydrogen molecule is purely covalent.
Q42. Formation of C⁴⁺ ion is difficult because:
To form C⁴⁺, carbon would have to remove all 4 valence electrons. The first ionization energy (removing one electron) is high, the second is even higher, and the third and fourth are extremely high because removing an electron from a positively charged ion becomes harder and harder. The total energy required is enormous, making C⁴⁺ impossible under normal conditions. This is why carbon shares electrons instead.
Q43. Methane has the molecular formula:
Methane is the simplest alkane (hydrocarbon). One carbon atom forms four single covalent bonds with four hydrogen atoms. The molecular formula is CH₄. C₂H₆ is ethane, CH₂ is methylene (unstable), and CH₃ is methyl group (not a stable molecule alone). Methane is the main component of natural gas and is a greenhouse gas.
Q44. Chlorine forms a molecule that is:
Chlorine gas exists as Cl₂, which is a diatomic molecule (two atoms). Like other halogens (F₂, Br₂, I₂), chlorine is never found as a single atom (monoatomic) in nature because the atom is highly reactive. It is also not polyatomic (more than 2 atoms) or triatomic (3 atoms). Diatomic means two atoms chemically bonded.
Q45. Which compound has the lowest melting point among the given?
This is similar to
Q46. Hydrogen molecule is represented as:
A hydrogen molecule consists of two hydrogen atoms covalently bonded together. Its correct representation is H₂. H represents a single hydrogen atom (not stable alone). H⁺ is a hydrogen ion (proton). H₃ is an unstable ion (H₃⁺ exists in space but not as a normal molecule). So H₂ is the correct formula for hydrogen gas.
Q47. Oxygen has how many electrons in its outer shell?
Oxygen has atomic number 8, so its electronic configuration is 2,6. The first shell (K shell) has 2 electrons. The second shell (L shell) is the outer shell and has 6 electrons (valence electrons). It needs 2 more electrons to complete the octet (8 electrons). So oxygen has 6 electrons in its outer shell, not 8 (that would be a complete octet like Neon).
Q48. What is formed when a compound containing carbon is burnt?
When a carbon compound (like wood, paper, petrol, methane) undergoes complete combustion in sufficient oxygen, the carbon atoms react with oxygen to form carbon dioxide (CO₂) and hydrogen forms water (H₂O). For example: CH₄ + 2O₂ → CO₂ + 2H₂O. If oxygen is insufficient, carbon monoxide (CO) or soot (carbon) may also form. But in general, complete burning of a carbon compound produces carbon dioxide.
Q49. Number of valence electrons in carbon is:
Carbon has atomic number 6, so its electronic configuration is 2,4. The electrons in the outermost shell (L shell) are called valence electrons. Carbon has 4 valence electrons. This is why carbon can form 4 covalent bonds. Valence electrons determine the combining capacity (valency) of an element. For carbon, valency is 4.
Q50. Carbon in the Earth’s crust mainly occurs in the form of:
Most of the carbon in the Earth’s crust is locked up in carbonate minerals such as limestone (calcium carbonate, CaCO₃), dolomite (CaMg(CO₃)₂), and marble. These are called minerals. While carbon also occurs as oxides (CO₂ in the atmosphere) and in fossil fuels (coal, petroleum), the largest reservoir in the crust is as carbonate minerals. Carbon does not occur as metals or nitrates in significant amounts in the c