ICSE – Grade 10 – Chemistry – Ch 02 – QA

1. What is a chemical bond? Explain its importance.
    Answer:
    A chemical bond is the force of attraction that holds atoms or ions together in a molecule or compound. It is formed due to the tendency of atoms to attain a stable electronic configuration, usually the octet configuration. Chemical bonds are essential because they result in the formation of compounds with entirely new properties and enable the existence of stable matter in nature.

2. Define an ionic bond. Explain with an example.
   Answer:
    An ionic bond is a chemical bond formed by the complete transfer of electrons from a metal atom to a non-metal atom, resulting in the formation of oppositely charged ions that are held together by strong electrostatic force of attraction.
    Example: In NaCl, sodium (Na) loses one electron to become Na⁺, and chlorine (Cl) gains that electron to become Cl⁻. These ions attract each other to form an ionic bond.

3. What is a covalent bond? Give an example.
   Answer:
    A covalent bond is formed when two non-metal atoms share one or more pairs of electrons to achieve a stable configuration. No ions are formed in covalent bonding.
    Example: In a water molecule (H₂O), the oxygen atom shares one electron each with two hydrogen atoms, forming two single covalent bonds.

4. Compare the properties of ionic and covalent compounds.
   Answer:
    Ionic Compounds:

• High melting and boiling points
• Solid and crystalline in structure
• Conduct electricity in molten or aqueous state
• Soluble in water
• Hard and brittle

Covalent Compounds:

• Low melting and boiling points
• Usually gases or liquids
• Do not conduct electricity
• Soluble in organic solvents
• Soft in solid form

5. Explain the formation of sodium chloride (NaCl) using electron configuration.
   Answer:
    Sodium (Na) has the electronic configuration 2,8,1 and chlorine (Cl) has 2,8,7. Sodium loses one electron to form a Na⁺ ion, achieving a stable octet (2,8). Chlorine gains this electron to form Cl⁻, also attaining an octet (2,8,8). The Na⁺ and Cl⁻ ions attract each other electrostatically, forming an ionic bond in NaCl.

6. How is a covalent bond formed in a hydrogen molecule (H₂)?
   Answer:
    Each hydrogen atom has one electron and requires one more to complete its duplet. Two hydrogen atoms share their single electrons, forming one shared pair. This mutual sharing results in a single covalent bond, making a stable H₂ molecule with both atoms having two electrons.

7. Describe the electron dot structure of a water molecule.
   Answer:
    In a water molecule (H₂O), the oxygen atom has 6 valence electrons and requires 2 more to complete its octet. Each hydrogen atom has 1 electron and needs one more. Oxygen shares one electron each with two hydrogen atoms. The electron dot structure shows two shared pairs (bond pairs) between O and H atoms, and two lone pairs on oxygen.

8. Write differences between cation and anion.
   Answer:
    Cation:

• Positively charged ion
• Formed by loss of electrons
• Generally formed by metals
• Example: Na⁺, Ca²⁺

Anion:

• Negatively charged ion
• Formed by gain of electrons
• Generally formed by non-metals
• Example: Cl⁻, O²⁻

9. Explain the octet rule with an example.
   Answer:
    The octet rule states that atoms tend to gain, lose, or share electrons to attain eight electrons in their outermost shell, similar to the configuration of noble gases.
    Example: In MgCl₂, magnesium loses 2 electrons to achieve octet, and each chlorine atom gains 1 electron, forming stable ions with 8 outer electrons.

10. What is electrovalency? How is it different from covalency?
    Answer:
     Electrovalency is the number of electrons lost or gained by an atom to form an ion in ionic bonding.
     Covalency is the number of electron pairs an atom shares in covalent bonding.
     Difference: Electrovalency involves ion formation by transfer of electrons, while covalency involves sharing of electrons without ion formation.

11. What are valence electrons? Why are they important in bonding?
    Answer:
     Valence electrons are the electrons present in the outermost shell of an atom. They are important because chemical bonding occurs due to the interaction of valence electrons, as atoms strive to attain a stable electronic configuration.

12. Explain the formation of MgCl₂ with electron configuration.
    Answer:
     Magnesium (2,8,2) loses two electrons to form Mg²⁺, achieving a configuration of 2,8. Each chlorine atom (2,8,7) gains one electron to form Cl⁻ (2,8,8). Two Cl⁻ ions are needed to balance the charge of one Mg²⁺ ion, forming MgCl₂ through ionic bonding.

13. Why do ionic compounds conduct electricity in molten state?
    Answer:
     In molten state, the ionic bonds break, and the ions become free to move. These free-moving charged particles carry current, hence ionic compounds conduct electricity in molten or aqueous solution.

14. Draw the Lewis structure of CH₄ and explain its bonding.
    Answer:
     Carbon has 4 valence electrons and hydrogen has 1. Carbon shares one electron with each of the 4 hydrogen atoms, forming 4 single covalent bonds. The Lewis structure shows C in the center with 4 shared electron pairs connecting to H atoms.

15. Explain the term lattice as used in ionic compounds.
    Answer:
     A lattice is a three-dimensional regular arrangement of oppositely charged ions in an ionic compound. This strong electrostatic arrangement gives ionic compounds their characteristic hardness, brittleness, and high melting points.

16. What are the physical properties of covalent compounds?
    Answer:
     Covalent compounds generally have low melting and boiling points, are poor conductors of electricity, are usually gases or liquids, and are soluble in organic solvents but insoluble in water.

17. Define polar covalent bond with an example.
    Answer:
     A polar covalent bond is formed when electrons are shared unequally between atoms due to difference in electronegativity.
     Example: In HCl, the shared electrons are closer to Cl, making H slightly positive and Cl slightly negative.

18. Define non-polar covalent bond with an example.
    Answer:
     A non-polar covalent bond is formed when electrons are shared equally between identical atoms.
     Example: In Cl₂, both atoms have equal electronegativity, so the bond is non-polar.

19. How is a triple covalent bond formed in nitrogen (N₂)?
    Answer:
     Each nitrogen atom has 5 valence electrons and needs 3 more to complete its octet. They share 3 pairs of electrons, forming a triple covalent bond, making both atoms stable with 8 outer electrons.

20. Explain the significance of the Lewis dot structure in bonding.
    Answer:
     Lewis dot structure visually represents valence electrons and bonding between atoms. It helps in understanding the number and type of bonds and lone pairs, which predict molecule shapes and reactivity.

21. What is meant by a molecule? Give two examples.
    Answer:
     A molecule is a group of two or more atoms chemically bonded together, representing the smallest unit of a compound with independent existence.
     Examples: H₂O (water), O₂ (oxygen).

22. Why do atoms form chemical bonds?
    Answer:
     Atoms form chemical bonds to attain a stable electronic configuration, usually an octet or duplet in the outermost shell, similar to noble gases. This stability is achieved by gaining, losing, or sharing electrons.

23. Differentiate between ionic and covalent bonding based on formation.
    Answer:
     Ionic Bonding: Formed by complete transfer of electrons from a metal to a non-metal.
     Covalent Bonding: Formed by mutual sharing of electrons between non-metal atoms.

24. Explain the formation of the water molecule using the electron dot method.
    Answer:
     Oxygen has 6 valence electrons and needs 2 more to complete its octet. Each hydrogen atom has 1 electron. Oxygen shares one electron each with two hydrogen atoms, forming two single covalent bonds. The dot diagram shows two bond pairs and two lone pairs on oxygen.

25. Why do ionic compounds have high melting and boiling points?
    Answer:
     Ionic compounds have strong electrostatic forces of attraction between oppositely charged ions in a crystal lattice. A large amount of energy is required to break these bonds, resulting in high melting and boiling points.

26. How is the Cl⁻ ion formed?
    Answer:
     A chlorine atom has 7 valence electrons (2,8,7). It gains one electron to complete its octet, becoming Cl⁻, a negatively charged ion with a stable configuration of 2,8,8.

27. Describe the properties of ionic compounds.
    Answer:
     Ionic compounds are hard, brittle, have high melting and boiling points, conduct electricity in molten or aqueous state, and are soluble in water but insoluble in organic solvents.

28. Describe the properties of covalent compounds.
    Answer:
     Covalent compounds have low melting and boiling points, are usually gases or liquids, are poor conductors of electricity, and are often soluble in organic solvents but insoluble in water.

29. What is a lone pair? How is it different from a bond pair?
    Answer:
     A lone pair is a pair of valence electrons not involved in bonding, while a bond pair is a shared pair of electrons between two atoms that forms a covalent bond.

30. How does a calcium atom form a Ca²⁺ ion?
    Answer:
     Calcium has an electronic configuration of 2,8,8,2. It loses two electrons to achieve a stable octet configuration of 2,8,8, becoming a Ca²⁺ ion with a +2 charge.

31. Write the differences between polar and non-polar covalent bonds.
    Answer:
     Polar Covalent Bond: Unequal sharing of electrons; bond has partial charges (e.g., HCl).
     Non-polar Covalent Bond: Equal sharing of electrons; bond has no charge separation (e.g., Cl₂).

32. What is meant by the term ‘crystal lattice’?
    Answer:
     A crystal lattice is a regular, repeating three-dimensional arrangement of ions in an ionic solid. It is responsible for the hardness and high melting points of ionic compounds.

33. Explain the bonding in HCl molecule.
    Answer:
     Hydrogen has 1 valence electron and chlorine has 7. They share one pair of electrons, forming a single covalent bond. Due to chlorine’s higher electronegativity, the bond is polar, with a partial negative charge on Cl and partial positive on H.

34. What are the conditions necessary for ionic bonding?
    Answer:
     Ionic bonding occurs when one atom has low ionization energy (metal) and another has high electron affinity (non-metal), allowing electrons to be transferred to form oppositely charged ions that attract each other.

35. Describe the structure of a nitrogen molecule using electron dot symbols.
    Answer:
     Each nitrogen atom has 5 valence electrons and needs 3 more. They share 3 pairs of electrons, forming a triple bond. The electron dot structure shows six shared electrons and one lone pair on each nitrogen.

36. Explain why covalent compounds are poor conductors.
    Answer:
     Covalent compounds do not form ions in solution, and since electrical conductivity requires free-moving charged particles, they are poor conductors of electricity in all physical states.

37. Why does chlorine form Cl⁻ and not Cl²⁻?
    Answer:
     Chlorine needs only one electron to complete its octet. Gaining a second electron would result in repulsion and instability. Thus, it forms Cl⁻ with a stable 2,8,8 configuration.

38. What is meant by shared pair and unshared pair of electrons?
    Answer:
     A shared pair (bond pair) of electrons is used in bonding between atoms. An unshared pair (lone pair) is not involved in bonding and remains on one atom.

39. Describe the shape of methane molecule and explain why.
    Answer:
     Methane (CH₄) has a tetrahedral shape because carbon forms four single covalent bonds with hydrogen. The repulsion between bonding electron pairs arranges them as far apart as possible in three dimensions.

40. Define and explain the term ‘valency’.
    Answer:
     Valency is the combining capacity of an atom, defined by the number of electrons it needs to gain, lose, or share to attain a stable configuration. For example, oxygen has a valency of 2.

41. How is a covalent bond different from an ionic bond in terms of energy?
    Answer:
     Covalent bonds involve lower bond energy as they result from shared electrons, while ionic bonds are stronger due to the electrostatic attraction between ions, requiring more energy to break.

42. Explain the role of electronegativity in polar covalent bonding.
    Answer:
     Electronegativity is the ability of an atom to attract shared electrons. In polar covalent bonding, the atom with higher electronegativity pulls the electron pair closer, creating a dipole with partial charges.

43. Why does ionic bonding not occur between two non-metals?
    Answer:
     Non-metals tend to gain electrons and have high electronegativity. Since neither can lose electrons easily, electron transfer does not occur, so ionic bonding is not possible.

44. Why do metals form cations?
    Answer:
     Metals have 1–3 valence electrons and low ionization energy. They lose electrons easily to form positively charged cations and attain a stable noble gas configuration.

45. Describe how carbon tetrachloride (CCl₄) is formed.
    Answer:
     Carbon has 4 valence electrons and each chlorine has 7. Carbon shares one electron with each of the four chlorine atoms, and each Cl shares one, forming 4 single covalent bonds in a tetrahedral arrangement.

46. What is the role of noble gas configuration in chemical bonding?
    Answer:
     Atoms bond to achieve the stable, low-energy configuration of noble gases (octet or duplet). This drives them to gain, lose, or share electrons, forming chemical bonds.

47. Explain the difference in bonding between O₂ and N₂ molecules.
    Answer:
     O₂ forms a double covalent bond by sharing 2 pairs of electrons, while N₂ forms a triple covalent bond by sharing 3 pairs. This makes N₂ more stable and less reactive than O₂.

48. Write a note on the directional nature of covalent bonds.
    Answer:
     Covalent bonds are directional because shared electrons lie between the bonding atoms, creating specific bond angles and shapes. This gives molecules like H₂O and CH₄ defined geometries.

49. Describe how ionic compounds are formed using Mg and O.
    Answer:
     Magnesium (2,8,2) loses two electrons to form Mg²⁺. Oxygen (2,6) gains two electrons to form O²⁻. These oppositely charged ions attract electrostatically, forming the ionic compound MgO.

50. Explain the term ‘bond pair’ and give an example.
    Answer:
     A bond pair is a pair of valence electrons shared between two atoms to form a covalent bond.
     Example: In H₂, the shared pair between two hydrogen atoms is a bond pair.