Chemical Bonding Class 9 Question Answer Advanced Science Chapter 7
Chapter-wise Class 9 Advanced Science Solutions and NCERT Class 9 Advanced Science Chapter 7 Chemical Bonding Question Answer are useful for focused study. Chemical Bonding Class 9 Questions and Answers Chemical Bonding Question Answer Class 9 Quick Check Question 1. What is meant by the octet rule? Answer: The octet rule states that atoms tend […]
Chapter-wiseClass 9 Advanced Science Solutionsand NCERT Class 9 Advanced Science Chapter 7 Chemical Bonding Question Answer are useful for focused study. Chemical Bonding Class 9 Questions and AnswersChemical Bonding Question Answer Class 9Quick CheckQuestion 1. What is meant by the octet rule?Answer:The octet rule states that atoms tend to gain, lose or share electrons during bond formation until their valence shell contains eight electrons, achieving a stable configuration similar to noble gases. Question 2. Why does hydrogen not follow the octet rule?Answer:Hydrogen has only one valence shell (the first shell), which can hold a maximum of two electrons. It achieves stability by completing a duplet (2 electrons), matching the configuration of helium. Question 3. Give one example each of a molecule with(i) incomplete octet(ii) expanded octet(iii) an odd electronAnswer:(i) Incomplete octet BF3(boron has only 6 electrons around it). (ii) Expanded octet SF6(sulphur has 12 electrons around it). (iii) Odd-electron NO (11 total valence electrons – one is always unpaired). Question 4. Why can boron form compounds with only six electrons around it?Answer:Boron has only 3 valence electrons and can form a maximum of 3 bonds. After forming 3 bonds, it has only 6 electrons around it. It lacks lone pairs that could complete the octet, so it remains with an incomplete octet and yet the compound is stable. Question 5. What is meant by a duplet configuration?Answer:A duplet configuration means an atom has two electrons in its valence shell, the stable configuration of helium. Hydrogen achieves this during bonding instead of following the octet rule. Question 6. Why is NO considered an exception to the octet rule?Answer:NO has a total of 11 valence electrons (5 from N + 6 from O). Since 11 is an odd number, it is impossible for both atoms to have 8 electrons each. At least one atom will always have an incomplete octet and there will always be one unpaired electron. Question 7. Draw the Lewis dot structure of BF3and explain why boron does not complete its octet. Answer:In BF3, boron (3 valence electrons) forms bonds with three fluorine atoms (7 valence electrons each). Each bond uses one electron from boron, giving boron only 6 electrons total. Boron has no lone pairs to donate, so it cannot complete the octet — resulting in an incomplete octet around boron. Question 8. Assertion (A): SF6violates the octet rule. Reason (R): Sulphur can accommodate more than eight electrons. (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion. (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion. (C) Assertion is true, but Reason is false. (D) Assertion is false, but Reason is true. Answer:(A) Both Assertion and Reason are correct, and the Reason correctly explains the Assertion. SF6 is an example of an expanded octet as sulphur, being a third-period element, can use d-orbitals to accommodate 12 electrons (more than 8), which is why it forms 6 bonds with fluorine. Check Your UnderstandingQuestion 1. What is meant by the term ‘electron sea’ in metals?Answer:The ‘electron sea’ refers to the collection of delocalised, freely moving electrons in a metal. When metal atoms come together, their outer electrons become free and move randomly throughout the entire metallic structure, forming what is described as a sea of electrons. Question 2. What type of particles are in a fixed position in a metal according to the electron sea model?Answer:Positive metal ions (cations) are in fixed positions, arranged in a regular lattice-like pattern. The free (delocalised) electrons move around these stationary ions. Question 3. Define metallic bonding. Answer:Metallic bonding is the force of attraction between positive metal ions (fixed in the lattice) and the sea of delocalised, freely moving electrons surrounding them. This attraction holds the metal together as a stable structure. Question 4. Why are metallic bonds called non-directional?Answer:Metallic bonds are called non-directional because the free electrons are shared collectively by all metal atoms, not between any specific pair of atoms. The bond has no fixed direction and can adjust when the positions of metal ions shift. Question 5. Name two properties of metals explained by the electron sea model. Answer:Electrical conductivity: Free electrons carry electric current when a field is applied. Malleability: Layers of ions can slide without breaking the structure, as electrons hold them together throughout. Question 6. Explain how the electron sea model accounts for electrical conductivity in metals. Answer:Metals contain free, delocalised electrons. When an electric field is applied across a metal, these electrons which are not attached to any specific atom start moving in one direction. This directed movement of electrons constitutes electric current, making metals good conductors. Question 7. How does the electron sea model explain thermal conductivity in metals?Answer:When one region of a metal is heated, the free electrons in that region gain energy and move faster. They carry this energy to other parts of metal as they move. At the same time, the metal ions vibrate more rapidly and also assist in transferring heat. This efficient energy transfer makes metals good conductors of heat. Question 8. Why can metals be beaten into thin sheets? Explain using the electron sea model. Answer:Metals are malleable because their layers of positive ions can slide over each other when force is applied. The free electrons are non-directional and continue to hold the ions together even after they shift. Since the metallic bond adjusts to the new arrangement, the metal changed into thin sheets instead of breaking. Question 9. What is meant by ductility? How is it explained by the electron sea model?Answer:Ductility is the ability of a metal to be drawn into thin wires. According to the electron sea model, when a metal is stretched, its ions slide past one another. The non-directional metallic bonds do not break and the free electrons maintain the attractive force between ions even as the shape changes, allowing the metal to stretch into wires. Question 10. How is metallic bonding different from covalent bonding?Answer:In covalent bonding, electrons are shared between specific pairs of atoms and the bond is directional. In metallic bonding, electrons are delocalised and shared collectively by all atoms and the bond is non-localised and non-directional. Covalent bonds are fixed in direction, whereas metallic bonds adjust freely to ion movements. Question 11. Explain the structure of a metal according to the electron sea model. Answer:According to the electron sea model, a metal consists of positive metal ions arranged in a regular, fixed lattice. These ions are surrounded by a “sea” of delocalised electrons that are free to move throughout the structure. The electrostatic attraction between the positive ions and the negative electron sea holds the entire structure giving it stability and characteristic properties such as conductivity. Question 12. If electrons in a metal were not free to move, which property would be most affected? Explain. Answer:Electrical conductivity would be most directly affected. Electric current in metals is due to the movement of free electrons under an applied field. If electrons were fixed and could not move, there would be no current flow, and the metal would not conduct electricity. Question 13. Explain why metals do not break when hammered but instead change shape. Answer:When hammered, layers of positive metal ions slide over past each other. The free electrons, being non-directional and mobile, continue to maintain the attraction between the ions in their new positions. Since no bonds are broken, only rearranged thus the metal changes shape rather than breaking. Question 14. Copper is used for electrical wiring, while rubber is not. Explain using the electron sea model. Answer:Copper is a metal with delocalised free electrons that can move throughout its structure. When an electric field is applied, these electrons flow which constituting an electric current. Rubber, being an insulator (polymer), has no free electrons, all its electrons are fixed in bonds. Therefore, rubber cannot conduct electricity. Question 15. Why are metals generally good conductors of heat as compared to non-metals?Answer:Metals have free, mobile electrons throughout their structure. When heated, these electrons quickly gain energy and transfer it across the metal. Non-metals lack such free electrons and the heat transfer depends only on atomic vibrations, which is much slower. Hence, metals conduct heat much more efficiently. Question 16. Assertion (A): Metals are good conductors of electricity. Reason (R): Metals contain free electrons that can move under an electric field. (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion. (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion. (C) Assertion is true, but Reason is false. (D) Assertion is false, but Reason is true. Answer:(A) Both Assertion and Reason are correct and the Reason is the correct explanation of Assertion. Free (delocalised) electrons in metals move under an applied electric field, constituting electric current, this is why metals conduct electricity. Question 17. Assertion (A): Metallic bonds are non- directional. Reason (R): Electrons in metals are localised between two atoms. (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion. (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion. (C) Assertion is true, but Reason is false. (D) Assertion is false, but Reason is true. Answer:(C) The Assertion is correct as metallic bonds are indeed non-directional. However, the Reason is wrong. Electrons in metals are not localised, they are delocalised and move freely throughout the entire metal, not just between two atoms. Question 18. Assertion (A): Metals are malleable. Reason (R): Layers of metal ions can slide while electrons continue to hold them together. (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion. (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion. (C) Assertion is true, but Reason is false. (D) Assertion is false, but Reason is true. Answer:(A) Both Assertion and Reason are correct, and the Reason correctly explains the Assertion. The non-directional nature of metallic bonds allows layers of ions to slide over each other. The free electrons maintain the attraction in the new arrangement, so the metal changes shape instead of breaking. Chemical Bonding Class 9 MCQQuestion 1. Which of the following molecules has an expanded octet around the central atom?(A) BF3– Boron has 6 electrons(B) H2O – Oxygen has 8 electrons(C) SF6– Sulphur has 12 electrons(D) NO — Nitrogen has 7 electronsAnswer:(C) SF6– Sulphur has 12 electronsIn SF6, one sulphur atom forms 6 bonds with 6 fluorine atoms, resulting in 12 electrons around the central sulphur atom. This is more than 8 electron and thus an expanded octet. BF3is an molecule with incomplete octet (6 electrons around B), H2O has exactly 8 electrons around O (complete octet), and NO is an odd-electron molecule. Question 2. In the electron sea model of metals, which of the following correctly describes the arrangement?(A) Free electrons are fixed at specific positions, metal ions move randomly. (B) Both metal ions and electrons are free to move throughout the structure. (C) Positive metal ions are arranged in a fixed lattice, electrons move freely around them. (D) Metal atoms share electrons only with their two nearest neighbours. Answer:(C) Positive metal ions are arranged in a fixed lattice, electrons move freely around them. According to the electron sea model, positive metal ions occupy fixed positions in a regular lattice, while the valence electrons become delocalised and move freely throughout the entire metallic solid, forming the ‘sea of electrons’. Metal ions do not move randomly and only electrons are free. Assertion-Reason QuestionsDirections (Q. Nos. 1 and 2): Each question has an Assertion (A) and a Reason (R). Choose the correct option. (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion. (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion. (C) Assertion is true, but Reason is false. (D) Assertion is false, but Reason is true. Question 1. Assertion (A): Hydrogen follows the duplet rule instead of the octet rule during bond formation. Reason (R): The first shell of hydrogen can hold a maximum of two electrons. Answer:(A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion. Hydrogen has only one shell, the first shell which has a capacity of only 2 electrons. It therefore achieves stability by completing a duplet (2 electrons) rather than an octet (8 electrons). Question 2. Assertion (A): Metals can be drawn into wires without breaking. Reason (R): In metals, the bonds between specific pairs of atoms are strong and directional, preventing displacement of ions. Answer:(C) Assertion is true, but Reason is false. However, the Reason is false. Metallic bonds are non-directional and non-localised. It is precisely because the bonds are not fixed between specific atom pairs that layers of ions can slide past each other without breaking the structure, allowing the metal to be drawn into wires. Chemical Bonding Class 9 Extra Questions and AnswersCase/Source Based QuestionsDirections: Answer the questions on the basis of your understanding of the following passage and related studied concepts. Question 1. A scientist is studying two substances – substance X and substance Y. Substance X is a shiny solid that conducts electricity, can be hammered into sheets, and melts at a very high temperature. Substance Y is a gas at room temperature and does not conduct electricity. When the scientist examines the bonding in substance X, she finds that the atoms have released their outermost electrons, which now move freely throughout the solid. In substance Y, each atom shares exactly one electron with another atom of the same element, completing a stable electron arrangement with two electrons each. i. Identify the type of bonding present in substance X. Explain using the electron sea model. ii. Identify substance Y and draw its Lewis dot structure. iii. Why does substance X conduct electricity but substance Y does not?iv. Substance X can be beaten into thin sheets without breaking. Which property is this, and why is it possible according to the bonding model?Answer:i. Substance X has metallic bonding. Its atoms release their outermost electrons, which become delocalised and move freely throughout the solid, forming a ‘sea of electrons’. The remaining positive metal ions arrange in a fixed lattice. The attraction between these positive ions and the surrounding electron sea constitutes the metallic bond and this is the electron sea model. ii. Substance Y is hydrogen gas (H2). Each hydrogen atom has 1 valence electron and shares 1 electron with the other hydrogen atom, forming a covalent bond. Both atoms achieve a duplet (2 electrons). Lewis dot structure, H:H (or H—H with one bond line, with no lone pairs on either atom). iii. In substance X (metal), the electrons are free and delocalised and they can move in response to an electric field, constituting electric current. In substance Y (H2), all electrons are fixed in bonds between specific atoms and cannot move freely. Therefore, H2does not conduct electricity. iv. This property is malleability, the ability to be beaten into thin sheets. In the electron sea model, when a force is applied, layers of positive metal ions slide over each other. The free electrons are non-directional and continuously maintain the attraction between ions in their new positions. Since, no bonds are broken, only the metal is rearranged, and thus metal changes shape instead of cracking. Very Short Answer Type QuestionsQuestion 1. State the octet rule. Which element is a common exception to it, and what rule does it follow instead?Answer:The octet rule states that atoms tend to gain, lose, or share electrons during bond formation until their valence shell contains 8 electrons. Hydrogen is a common exception as it follows the duplet rule, achieving stability with only 2 electrons (the configuration of helium) since its first shell can hold a maximum of only 2 electrons. Question 2. What are non-bonding electrons? How are they different from bonding electrons?Answer:Non-bonding electrons (also called lone pairs) are valence electrons that do not participate in bond formation between atoms and they remain on their respective atoms. Bonding electrons are shared between two atoms and are shown as dots or a line between atomic symbols. Example in HF, the pair of dots between H and F are bonding electrons; the three pairs of dots on F are non-bonding (lone pairs). Question 3. Give one example of a molecule with an odd number of valence electrons. Why can it never satisfy the octet rule completely?Answer:Nitric oxide (NO) has 5 electrons (from N) + 6 electrons (from O) = 11 total valence electrons which is an odd number. Since 11 is odd, it is mathematically impossible to distribute electrons so that both N and O have exactly 8 electrons each (which would require an even total). Therefore, at least one atom will always have an incomplete octet, and there will always be one unpaired electron in the molecule. Question 4. State one way in which metallic bonding is similar to covalent bonding, and one way in which it is different. Answer:Similarity:In both metallic and covalent bonding, electrons are shared (not fully transferred). Difference:In covalent bonding, electrons are shared between a specific pair of atoms and the bond is directional. In metallic bonding, electrons are delocalised and shared collectively by all atoms and thus the bond is non-directional and non-localised. Short Answer Type QuestionsQuestion 1. BF3and NF3are both compounds of fluorine with a second-period element. BF3has an incomplete octet, but NF3has a complete octet around the central atom. Explain why this difference exists using the number of valence electrons of boron and nitrogen. Answer:Boron has 3 valence electrons, so it forms 3 bonds with 3 fluorine atoms. After bonding, boron has only 6 electrons around it which is an incomplete octet. It has no remaining valence electrons to form a fourth bond, and no lone pairs available to complete the octet. Nitrogen, on the other hand, has 5 valence electrons. It forms 3 bonds with 3 fluorine atoms (using 3 electrons) and retains 1 lone pair (2 electrons). Total electrons around N = 6 (from bonds) + 2 (lone pair) = 8 thus a complete octet. Question 2. A wire of copper stretches easily and conducts electricity, while a piece of sulphur (a non-metal solid) is brittle and does not conduct electricity. Using the concepts of metallic bonding and the nature of electrons, explain this difference in properties. Answer:Copper has metallic bonding and its valence electrons are delocalised and free to move throughout the structure. This explains both its ductility (ions slide over each other) and its electrical conductivity (free electrons carry current). Sulphur is a non-metal with covalent bonds between specific atoms and its electrons are localised in fixed bonds. When force is applied, these rigid bonds break, making sulphur brittle. Without free electrons, sulphur cannot conduct electricity. Question 3. Explain the terms(i) Lewis symbol,(ii) bonding pair of electrons and(iii) lone pair of electrons. Illustrate each with a suitable example. Answer:(i) Lewis symbol: A representation of an atom in which its valence electrons are shown as dots around the atomic symbol. Example:Carbon (4 valence electrons) is represented as with 4 dots around it. (ii) Bonding pair of electrons:A pair of electrons shared between two atoms during bond formation. Example:In H2the two electrons between the two H atoms (H:H) form one bonding pair. (iii) Lone pair (Non-bonding pair):A pair of valence electrons that belongs to one atom and does not take part in bonding. Example:In HF, fluorine has three lone pairs (six non-bonding electrons) in addition to the one bonding pair shared with hydrogen. Long Answer Type QuestionsQuestion 1. Explain the three exceptions to the octet rule with one example each. For each exception, state why the octet rule is not satisfied and describe the structure of the molecule. Support your answer with Lewis dot structures. Answer:Three exceptions to the octet Rule are as follows:Incomplete octet:Occurs in atoms with fewer than 4 valence electrons (Li, Be, B). They cannot form enough bonds to acquire 8 electrons. Example:In BF3, boron has 3 valence electrons and forms 3 bonds, giving it only 6 electrons. Lewis structure shows B at the centre bonded to 3 F atoms, with 3 lone pairs on each F but only 3 bond pairs (6 electrons) around B. Expanded octet:Occurs in third-period and heavier elements (S, P, Cl) that can use d-orbitals to accommodate more than 8 electrons. Example:In SF6, sulphur forms 6 bonds with 6 fluorine atoms, resulting in 12 electrons around the central S atom. Lewis structure shows S at the centre with 6 single bonds to 6 F atoms. Odd electron molecules:Some molecules have an total odd number of valence electrons. Since, it is impossible to pair all electrons in bonds and complete octets for all atoms with an odd total, at least one atom always has an incomplete octet and one electron remains unpaired. Example NO , 5 (N) + 6 (O) = 11 valence electrons. Lewis structure with one double bond and one unpaired electron on nitrogen. Question 2. Describe the electron sea model of metallic bonding in detail. Using this model, explain four physical properties of metals, electrical conductivity, thermal conductivity, malleability, and ductility. How is metallic bonding fundamentally different from both ionic and covalent bonding?Answer:Electron sea model and properties of metals are as followsElectron sea model:In a metal, the outermost electrons of each atom are loosely held and become delocalised when many metal atoms come together. Positive metal ions arranged in a regular lattice, while the released electrons move freely throughout the entire structure as a ‘sea of electrons’. The electrostatic attraction between these positive ions and the electron sea is called metallic bonding. Electrical conductivity:Free electrons move randomly under room temperature. When an electric field is applied, they drift in one direction, this directed movement constitutes electric current. Since the electrons are not attached to any specific atom, metals are excellent conductors. Thermal conductivity:When one region is heated, the free electrons in that region gain kinetic energy and move faster. They collide with each other and transfer energy to electrons in cooler regions. Simultaneously, lattice ions vibrate more and pass on energy. This combination makes metals good thermal conductors. Malleability:When a metal is hammered, layers of positive ions slide part over each other. The electron sea instantly adjusts to hold the ions in their new positions. Since the metallic bond is non-directional, no bonds are broken and the metal simply changes shape. Ductility:When a metal is stretched, ions slide past each other along rows. The delocalised electrons maintains cohesive forces (attractive forces that bind particles of substance together) during this movement, allowing the metal to elongate in a wire without breaking, as the bonding is non-directional. The bond does not snap because it is not fixed between specific atoms. Comparison with other bonds:Unlike ionic bonding (electrons completely transferred), metallic bonding involves collective sharing by all atoms. Unlike covalent bonding (electrons shared between specific atom pairs, directional), metallic bonds are non-localised, non-directional, and allow ions to shift position and explains why metals are malleable and ductile while ionic solids are brittle.