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NCERT Class 9 Science Important Chapter 9 Atomic Foundations of Matter
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Atomic Foundations of Matter
Chapter – 9
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. What does the Law of Conservation of Mass state?
Ans: It states that matter can neither be created nor destroyed in a chemical reaction.
2. Who is known as the Father of Modern Chemistry and proposed the Law of Conservation of Mass?
Ans: Antoine Lavoisier is known as the Father of Modern Chemistry and proposed this law in 1789.
3. What does the Law of Constant Proportions state?
Ans: It states that in any compound formed by two or more elements, the elements always combine in a fixed ratio by mass, irrespective of the source.
4. Who proposed the Law of Constant Proportions?
Ans: Joseph Proust proposed the Law of Constant Proportions.
5. What is the fixed mass ratio of hydrogen to oxygen in purified water?
Ans: Purified water always contains hydrogen and oxygen in a fixed mass ratio of 1:8.
6. According to Dalton’s Atomic Theory, what happens to atoms during a chemical reaction?
Ans: Atoms are indivisible particles that cannot be created or destroyed; they merely rearrange themselves during a chemical reaction.
7. How is a molecule defined in science?
Ans: A molecule is an electrically neutral entity consisting of more than one atom that is capable of independent existence and shows all the properties of that substance.
8. How do atoms achieve a stable electronic configuration if they have less than eight electrons in their valence shell?
Ans: They may share, gain, or lose electrons to complete their valence shell and become stable.
9. What is a covalent bond?
Ans: A chemical bond formed by the sharing of a few or all of the valence electrons between two atoms is called a covalent bond.
10. How many electrons are shared in a single covalent bond, such as in a chlorine molecule?
Ans: Two atoms are joined by sharing one electron each (a single pair of electrons) to form a single covalent bond.
11. What type of bond holds the two oxygen atoms together in an $O_2$ molecule?
Ans: The two oxygen atoms are held together by a double covalent bond because they share two pairs of electrons.
12. What is an ionic bond?
Ans: It is the electrostatic force of attraction between oppositely charged ions that holds them together, formed by the transfer of electrons from one atom to another.
13. What happens to a sodium atom when it loses its valence electron?
Ans: It acquires a positive charge and becomes a sodium cation, represented as $Na^+$.
14. What are polyatomic ions?
Ans: Ions that are formed by the combination of atoms of two or more elements are called polyatomic ions.
15. Why do ionic compounds generally have high melting and boiling points?
Ans: They have high melting and boiling points due to the strong inter-ionic attractions (electrostatic forces) holding the ions together in a crystal lattice.
16. Why do solid ionic compounds like sodium chloride fail to conduct electricity?
Ans: In the solid state, their ions are held in fixed positions by strong forces and cannot move freely to conduct electricity.
17. Which type of compounds are generally soluble in organic solvents like kerosene and petrol?
Ans: Covalent compounds, such as camphor and naphthalene, are generally soluble in kerosene and petrol.
18. What is the correct chemical name for the covalent compound N2O5?
Ans: Using the prefix system, it is named dinitrogen pentoxide.
19. How do you calculate the molecular mass of a covalent compound?
Ans: The molecular mass is the total mass of a molecule, calculated by adding up the atomic masses of all the individual atoms present in it.
20. What is the formula unit mass of an ionic compound?
Ans: It is the sum of the atomic masses of all the atoms present in a formula unit, which is the simplest whole-number ratio of ions in an ionic compound.
Fill in the Blanks:
1. The Law of Conservation of Mass was proposed by __________.
Ans: Antoine Lavoisier.
2. A chemical bond formed by sharing electrons is called a __________ bond.
Ans: Covalent.
3. A sodium atom becomes a __________ ion after losing one electron.
Ans: Positive (Na⁺).
4. The sum of the atomic masses of all atoms in a molecule is called its __________ mass.
Ans: Molecular.
5. Ions made up of two or more atoms are called __________ ions.
Ans: Polyatomic.
True or False:
1. Matter can neither be created nor destroyed during a chemical reaction.
Ans: True.
2. Covalent compounds generally conduct electricity in the solid state.
Ans: False.
3. Ionic compounds usually have high melting and boiling points.
Ans: True.
4. The two oxygen atoms in an O₂ molecule are joined by a double covalent bond.
Ans: True.
5. The atomic mass of chlorine is a whole number because it has only one isotope.
Ans: False.
Long Questions & Answers:
1. Explain the Law of Conservation of Mass and the Law of Constant Proportions with suitable examples.
Ans: The Law of Conservation of Mass, proposed by Antoine Lavoisier, states that matter can neither be created nor destroyed during a chemical reaction, meaning the total mass of the reactants is always exactly equal to the total mass of the products formed. For example, if solutions of sodium sulfate and barium chloride are mixed in a closed flask, the total mass of the resulting white precipitate of barium sulfate and sodium chloride solution will be perfectly identical to the initial mass of the reactants before mixing. The Law of Constant Proportions, proposed by Joseph Proust, states that in any chemical compound, the constituent elements are always present in a fixed and definite ratio by mass, regardless of the compound’s source or method of preparation. For instance, purified water collected from a river, borewell, or ocean will always contain hydrogen and oxygen in a fixed mass ratio of 1:8, meaning that decomposing 9 grams of pure water will always yield exactly 1 gram of hydrogen and 8 grams of oxygen.
2. What is Dalton’s Atomic Theory? List its major postulates and explain how it accounts for the laws of chemical combination.
Ans: John Dalton proposed his atomic theory to logically explain the experimental laws of chemical combination, suggesting that atoms are the ultimate, indivisible building blocks of matter that merely rearrange during chemical reactions rather than being created or destroyed. His major postulates state that all matter is made up of tiny particles called atoms; atoms of a specific element are identical in mass and chemical properties, while atoms of different elements differ in these aspects; and atoms combine in simple whole-number ratios to form compounds. Furthermore, he postulated that the relative number and kinds of atoms remain perfectly constant in any given compound. By stating that atoms cannot be created or destroyed, Dalton successfully provided a theoretical mechanism for the Law of Conservation of Mass, and by stating that they combine in fixed whole-number ratios, he established a logical foundation for the Law of Constant Proportions.
3. Describe the formation of covalent bonds using the examples of hydrogen and oxygen molecules, and explain the basic rules for naming these compounds.
Ans: A covalent bond is a type of chemical bond formed when two atoms share a few or all of their valence electrons to achieve a highly stable electronic configuration. For example, a hydrogen atom has only one electron in its K-shell and needs one more to become stable, so two hydrogen atoms share one electron each to form a single covalent bond, creating a stable hydrogen molecule (H2 ). Similarly, an oxygen atom possesses six valence electrons and requires two more to complete its octet, which is achieved when two oxygen atoms share two electrons each, resulting in a double covalent bond that forms a stable oxygen molecule (O2 ). When naming such covalent compounds, a specific prefix system (such as mono-, di-, tri-, tetra-) is used to clearly indicate the number of atoms of each element present in the molecule, which is why a compound like CO2 is named carbon dioxide and N 2O5 is named dinitrogen pentoxide.
4. Explain how an ionic bond forms between atoms, detailing the formation of sodium chloride and describing its overall structural arrangement.
Ans: An ionic bond is formed through the complete transfer of one or more valence electrons from one atom to another, resulting in the creation of oppositely charged ions that are held tightly together by strong electrostatic forces of attraction. For example, a neutral sodium atom has only one valence electron, which it readily donates to achieve a stable configuration, thereby becoming a positively charged sodium cation (Na + ). Concurrently, a chlorine atom has seven valence electrons and desperately needs one more to complete its octet, so it eagerly accepts the electron lost by sodium to become a negatively charged chloride anion (Cl − ). These oppositely charged Na + and Cl−ions strongly attract each other to form the ionic compound sodium chloride, which does not exist as isolated single molecules but rather arranges itself into a repeating, three-dimensional crystal lattice structure where each sodium ion is surrounded by six chloride ions and vice versa.
5. Compare the physical properties of ionic and covalent compounds, specifically focusing on their solubility, electrical conductivity, and melting or boiling points.
Ans: Ionic and covalent compounds exhibit distinctly different physical properties primarily due to the specific nature of the bonds holding their constituent particles together. Ionic compounds, such as sodium chloride and copper sulfate, are generally highly soluble in water but remain completely insoluble in organic solvents like kerosene and petrol. In sharp contrast, most covalent compounds, such as camphor and naphthalene, are insoluble in water but dissolve very readily in kerosene and petrol. Regarding electrical conductivity, solid ionic compounds cannot conduct electricity because their ions are held rigidly in fixed lattice positions by strong forces; however, they become excellent conductors when dissolved in water or melted, as the ions are then completely free to move and carry a charge. Covalent compounds generally do not conduct electricity in any state because they are formed by sharing electrons and do not contain free ions. Finally, ionic compounds generally possess very high melting and boiling points due to the enormous amount of thermal energy required to break their strong inter-ionic attractions, whereas covalent compounds usually have comparatively much lower melting and boiling points.
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