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Q1. Molecular formula of chlorine gas is:
Chlorine is a non-metal that exists as a diatomic molecule in its elemental form. This means two chlorine atoms share one pair of electrons (a single covalent bond) to achieve a stable octet configuration. Therefore, its molecular formula is Cl₂, not monatomic Cl.
Q2. Which compound has the highest melting point among those listed?
Acetic acid has the highest melting point (16.6°C) because it forms strong hydrogen bonds between its molecules and also exists as dimers (two molecules linked together) in solid state. Ethanol also has hydrogen bonds but weaker. Methane and chloroform have only weak van der Waals forces.
Q3. All living structures are primarily:
Carbon is the backbone of all known life because it can form four stable covalent bonds, allowing it to create long chains, branched structures, and rings. These structures form carbohydrates, proteins, lipids, and nucleic acids—the building blocks of living organisms.
Q4. Oxygen completes its octet by sharing:
Oxygen has 6 valence electrons and needs 2 more to complete its octet (8 electrons). In O₂, each oxygen atom shares 2 electrons (forming a double bond). More simply, each atom contributes 2 electrons for sharing, so each oxygen effectively “gains” access to 2 shared electrons.
Q5. Atomic number of chlorine is:
Atomic number equals the number of protons in an atom’s nucleus. Chlorine has 17 protons. This gives its electron configuration as 2,8,7—which explains why it needs 1 electron to complete its octet and forms Cl⁻ ion or a Cl-Cl single bond.
Q6. Which test can confirm the presence of carbon dioxide?
When CO₂ is passed through limewater (calcium hydroxide solution), it turns milky due to formation of insoluble white calcium carbonate. If CO₂ is passed for a long time, the milkiness disappears forming soluble calcium bicarbonate. This is a specific confirmatory test.
Q7. Atmospheric carbon is present mainly as:
In Earth’s atmosphere, carbon is overwhelmingly found as carbon dioxide (CO₂) gas. Although methane (CH₄) is also a greenhouse gas, its concentration is far lower. Carbonates (like limestone) store carbon in Earth’s crust, not in the atmosphere.
Q8. Percentage of carbon dioxide in the atmosphere is about:
CO₂ makes up approximately 0.03% to 0.04% (about 420 parts per million) of the atmosphere by volume. While this seems tiny, it is crucial for photosynthesis and maintaining Earth’s temperature through the greenhouse effect.
Q9. Bond formed by sharing of electrons is called:
A covalent bond is defined as a chemical bond formed by mutual sharing of electron pairs between two atoms. Ionic bonds involve transfer of electrons, metallic bonds involve a “sea” of delocalized electrons, and hydrogen bonds are weaker intermolecular attractions.
Q10. Carbon compounds are poor conductors of electricity because they:
For a substance to conduct electricity, it needs mobile charged particles (ions or electrons). Most carbon compounds (e.g., glucose, methane) are covalent, meaning they do not dissociate into ions in solution or in molten state. They have no free electrons or ions to carry current.
Q11. A single covalent bond consists of:
A single covalent bond involves exactly one shared pair of electrons (two electrons, one from each atom). For example, in H₂, each hydrogen contributes its single electron to form one shared pair. Two shared pairs make a double bond, and three make a triple bond.
Q12. Atomic number of hydrogen is:
Hydrogen has 1 proton in its nucleus, so its atomic number is 1. It has one electron in its K-shell (1s¹). This makes it unique—it can lose one electron to become H⁺, gain one to become H⁻, or share one to form a single covalent bond.
Q13. Atomic number of carbon is:
Carbon has 6 protons. Its electronic configuration is 2,4 (K-shell = 2 electrons, L-shell = 4 valence electrons). This is why carbon can form four covalent bonds and is tetravalent.
Q14. Percentage of carbon present in the Earth’s crust is approximately:
Carbon makes up only about 0.02% by mass of the Earth’s crust. Most of this carbon is locked in carbonate rocks like limestone (CaCO₃), fossil fuels, and organic matter. Despite being low in abundance, carbon is essential for life.
Q15. Carbon needs to gain or lose how many electrons to attain noble gas configuration?
Carbon has 4 valence electrons (2,4). To achieve stable octet like neon, it could gain 4 electrons (to become C⁴⁻) or lose 4 electrons (to become C⁴⁺). Both require huge energy. So instead, carbon shares 4 electrons with other atoms.
Q16. Hydrogen atoms share electrons to attain configuration of:
Hydrogen has 1 electron and needs 1 more to achieve the duplet (2 electrons) of helium, which is a stable noble gas configuration. It does this by sharing one electron with another hydrogen atom to form H₂.
Q17. Most carbon compounds have relatively:
Most covalent carbon compounds (like methane, ethanol, sugar) are molecular substances. They have weak intermolecular forces (van der Waals or hydrogen bonds). Little energy is needed to overcome these forces, so they have low melting and boiling points compared to ionic compounds.
Q18. Which of the following allotropes of carbon is a good conductor of electricity?
In graphite, each carbon is bonded to three others (forming hexagonal layers), leaving one free delocalized electron per carbon. These electrons can move along the layers, making graphite a good conductor. Diamond has no free electrons; all four valence electrons are in covalent bonds.
Q19. Atomic number of oxygen is:
Oxygen has 8 protons. Its electronic configuration is 2,6 (K=2, L=6 valence electrons). It needs 2 more electrons to complete its octet, which is why it forms O²⁻ ion or double covalent bonds (e.g., O=O in oxygen gas).
Q20. Carbon compounds do not conduct electricity mainly because they:
Most carbon compounds are covalent and do not dissociate into ions in water or when melted. Without mobile ions (or free electrons in most cases), electricity cannot flow. This is why sugar solution or liquid ethanol does not conduct electricity.
Q21. Covalent compounds generally have:
While covalent bonds within molecules are strong, the forces between separate molecules (intermolecular forces) are weak. This explains why covalent compounds are often gases, liquids, or low-melting solids at room temperature.
Q22. Main reason for diversity of carbon compounds is:
Carbon’s ability to form four strong covalent bonds, combined with catenation (self-linking to form long chains, branches, and rings), leads to millions of organic compounds. No other element shows such diversity, which is why organic chemistry is a separate field.
Q23. Formation of C⁴⁻ ion is difficult because:
Carbon has only 6 protons. Adding 4 extra electrons to form C⁴⁻ would create a huge electron-electron repulsion. The small nucleus cannot efficiently attract and hold these 10 electrons (total), making this ion extremely unstable.
Q24. Which compound has the lowest boiling point among those listed?
Methane (CH₄) is a non-polar molecule with only weak London dispersion forces. Its boiling point is -161.5°C. Ethanol and acetic acid have hydrogen bonding; chloroform has dipole-dipole interactions. All these are stronger than dispersion forces, so they have higher boiling points.
Q25. Despite its small availability, carbon is important due to its:
Carbon’s unique bonding properties—catenation, tetravalency, and ability to form multiple bonds—allow it to create millions of complex molecules essential for life (DNA, proteins) and industry (plastics, fuels). Its abundance in the crust is low, but its bonding versatility makes it irreplaceable.
Q26. Water molecule contains bonds that are:
In H₂O, each hydrogen forms a single covalent bond with the oxygen atom (H–O–H). Oxygen shares one electron pair with each hydrogen, so both bonds are single bonds. There are no double or triple bonds in water.
Q27. Single bond is represented by:
In Lewis structures and structural formulas, a single covalent bond is represented by a single straight line between two atoms. For example, H–H for hydrogen molecule. Each line represents one shared pair of electrons (two electrons).
Q28. Which element is described as being of immense significance in both elemental and combined forms?
Carbon in elemental form exists as diamond (hardest natural substance), graphite (conductor, lubricant), and fullerenes. In combined form, it is found in all living matter, fossil fuels, carbonates, plastics, medicines, and thousands of industrial chemicals. No other element has such versatility.
Q29. Carbon overcomes bonding difficulty by:
Carbon cannot easily lose 4 electrons (too much energy required) nor gain 4 electrons (nucleus too weak to hold extra electrons). So it overcomes this by sharing its 4 valence electrons with other atoms, forming stable covalent bonds. This is the basis of organic chemistry.
Q30. Simplest molecule formed by sharing electrons is:
The hydrogen molecule (H₂) is the simplest covalent molecule. Each hydrogen atom has one electron; they share one pair of electrons to form a single bond. It has only two atoms and one bond, making it simpler than O₂ (double bond), N₂ (triple bond), or CO₂ (more atoms).
Q31. Carbon shares electrons to achieve:
By sharing four electrons with other atoms (or other carbon atoms), carbon achieves the stable electronic configuration of neon (2,8). This octet configuration gives the molecule lower energy and greater stability compared to individual atoms.
Q32. Nitrogen molecule contains:
Each nitrogen atom has 5 valence electrons. To achieve octet, each needs 3 more electrons. They share three pairs of electrons, forming a very strong triple bond (N≡N). This triple bond makes nitrogen gas chemically inert under normal conditions.
Q33. Number of electrons in K shell of helium is:
Helium has atomic number 2, meaning it has 2 electrons. Both occupy the first shell (K shell), which can hold a maximum of 2 electrons. This complete duplet makes helium stable and unreactive (a noble gas).
Q34. Bonding in carbon compounds is mainly:
The vast majority of carbon compounds (organic compounds) form covalent bonds. Carbon shares electrons with other non-metals like hydrogen, oxygen, nitrogen, and halogens. Ionic carbon compounds are rare (e.g., carbides like CaC₂).
Q35. Noble gas configuration means:
Noble gases (He, Ne, Ar, etc.) have their outermost electron shell completely filled—2 electrons for He (duplet) and 8 for others (octet). This is the most stable electronic arrangement. Other atoms react to achieve this configuration.
Q36. Most items used in daily life like food, clothes, and medicines are based on which element?
Carbohydrates, proteins, fats (food), cotton, wool, polyester (clothes), and most pharmaceutical drugs are organic compounds containing carbon. Even plastics, soaps, detergents, and fuels are carbon-based. Life itself is carbon-based.
Q37. A double bond consists of:
A double bond involves sharing of two pairs of electrons (4 electrons total). For example, in O₂: O=O. Each oxygen contributes two electrons. In structural formulas, a double bond is shown by two parallel lines (=).
Q38. Number of electrons shared by each oxygen atom in O₂ is:
In the O₂ molecule, oxygen atoms form a double bond. Each oxygen atom contributes 2 electrons to the bond. So each oxygen shares 2 of its electrons (total 4 shared electrons in the bond, but each atom “owns” 2 of them in the shared pair sense).
Q39. Bond between two oxygen atoms is:
The bond between two oxygen atoms in an O₂ molecule is a double covalent bond (O=O). Each oxygen has 6 valence electrons; they need 2 more each. Sharing two pairs of electrons satisfies both atoms’ octets.
Q40. Shared electrons in covalent bonding belong to:
In a covalent bond, the shared pair of electrons is attracted to the nuclei of both bonded atoms simultaneously. They spend time in the region between the atoms, effectively belonging to both. This mutual attraction holds the atoms together.
Q41. Example of a covalent bond is found in:
H₂ has a covalent bond (shared electron pair). The other options are ionic compounds: NaCl (Na⁺ Cl⁻), CaO (Ca²⁺ O²⁻), MgCl₂ (Mg²⁺ 2Cl⁻). Covalent bonds occur between non-metals; ionic bonds between metals and non-metals.
Q42. Formation of C⁴⁺ ion is difficult because:
Carbon has 4 valence electrons. Removing the first electron requires energy, but removing 4 electrons (especially the last two which are closer to the nucleus) needs an enormous amount of ionization energy. This makes C⁴⁺ highly unstable and unrealistic.
Q43. Methane has the molecular formula:
Methane is the simplest hydrocarbon. One carbon atom shares four single covalent bonds with four hydrogen atoms. Each hydrogen contributes one electron, and carbon contributes four electrons. The formula CH₄ represents this tetrahedral structure.
Q44. Chlorine forms a molecule that is:
Chlorine gas exists as Cl₂ molecules (two atoms bonded together). It is a diatomic molecule. Other halogens (F₂, Br₂, I₂) are also diatomic. Polyatomic means more than two atoms (e.g., O₃), monoatomic means single atom (e.g., noble gases).
Q45. Which compound has the lowest melting point among the given?
Methane has the lowest melting point (-182.5°C). It is a non-polar molecule with only weak London forces. Acetic acid (16.6°C), ethanol (-114°C), and chloroform (-63.5°C) have higher melting points due to hydrogen bonding or dipole interactions.
Q46. Hydrogen molecule is represented as:
Elemental hydrogen exists as diatomic molecules (H₂). Two hydrogen atoms share a pair of electrons to form a single covalent bond. H atom alone (monatomic hydrogen) is highly reactive and does not exist under normal conditions.
Q47. Oxygen has how many electrons in its outer shell?
Oxygen has atomic number 8: electronic configuration = 2,6. The outer shell (L shell) contains 6 valence electrons. It needs 2 more to complete the octet (8 electrons in outer shell). It can gain 2 (forming O²⁻) or share 2 (forming two single bonds or one double bond).
Q48. What is formed when a compound containing carbon is burnt?
Complete combustion of any carbon-containing compound (organic matter, fuels, wood, plastic) in sufficient oxygen produces carbon dioxide (CO₂) and water (H₂O). For example: CH₄ + 2O₂ → CO₂ + 2H₂O. Incomplete combustion produces carbon monoxide (CO).
Q49. Number of valence electrons in carbon is:
Carbon (atomic number 6) has electronic configuration 2,4. The four electrons in the outermost shell (L shell) are its valence electrons. These are responsible for bond formation. This is why carbon is tetravalent and forms four covalent bonds.
Q50. Carbon in the Earth’s crust mainly occurs in the form of:
Most carbon in Earth’s crust is found in carbonate minerals like limestone (calcium carbonate, CaCO₃), dolomite (CaMg(CO₃)₂), and marble. Fossil fuels (coal, petroleum, natural gas) contain carbon but are much less abundant than carbonate rocks.