09. Very Short Question Answer: Atomic Foundations of Matter

Q1: State the Law of Conservation of Mass.

Ans: The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. This law was proposed by Antoine Lavoisier in 1789.

Q2: State the Law of Constant Proportions.

Ans: The Law of Constant Proportions states that in any compound formed by two or more elements, the elements combine in a fixed ratio by mass, irrespective of the source of the compound. It is also called the Law of Definite Proportions or Proust’s Law.

Q3: Water always contains hydrogen and oxygen in a mass ratio of 1:8. If 9 g of water is decomposed, how much hydrogen and oxygen will be obtained?

Ans: From 9 g of water, $1\text{ g}$ of hydrogen and $8\text{ g}$ of oxygen will always be obtained, as the ratio of hydrogen to oxygen by mass is $1:8$.

Q4: Who proposed Dalton’s Atomic Theory and in which year?

Ans: John Dalton proposed his atomic theory in 1808. It was based on scientific experiments and became the starting point for the modern understanding of atomic structure and chemical reactions.

Q5: State any two postulates of Dalton’s Atomic Theory.

Ans: (i) All matter is made up of very tiny particles called atoms, which participate in chemical reactions. 
(ii) Atoms are indivisible particles which cannot be created or destroyed in a chemical reaction.

Q6: Define a molecule.

Ans: A molecule is defined as an electrically neutral entity consisting of more than one atom that is capable of independent existence and shows all the properties of that substance.

Q7: What is a chemical bond?

Ans: The force that holds atoms together when they combine to form a molecule is called a chemical bond. Atoms combine to become stable by achieving a complete valence shell through sharing or transfer of electrons.

Q8: What is a covalent bond? Give one example.

Ans: A covalent bond is formed by the sharing of a pair of electrons between two atoms. For example, two hydrogen atoms each share one electron to form a hydrogen molecule $({\text{H}}_2)$, joined by a single covalent bond represented as H-H.

Q9: What is the difference between a single bond and a double bond?

Ans: A single bond is formed when two atoms share one pair of electrons (e.g., ${\text{H}}_2$, represented as H-H). A double bond is formed when two atoms share two pairs of electrons (e.g., ${\text{O}}_2$, represented as O=O).

Q10: What is an ionic bond? Give one example.

Ans: An ionic bond is the electrostatic force of attraction between oppositely charged ions that holds them together. For example, in sodium chloride (NaCl), ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions are held together by an ionic bond.

Q11: What are cations and anions?

Ans: A cation is a positively charged ion formed when an atom loses one or more electrons (e.g., ${\text{Na}}^{+}$). An anion is a negatively charged ion formed when an atom gains one or more electrons (e.g., ${\text{Cl}}^{-}$). Together, cations and anions are called ions.

Q12: What are polyatomic ions? Give two examples.

Ans: Ions formed by the combination of atoms of two or more elements are called polyatomic ions. Examples are hydroxide $({\text{OH}}^{-})$ and sulfate $({\text{SO}}_4^{2-})$.

Q13: Write the chemical formula of calcium chloride and aluminium oxide.

Ans: The chemical formula of calcium chloride is ${\text{CaCl}}_2$ (since ${\text{Ca}}^{2+}$ and ${\text{Cl}}^{-}$ criss-cross to give subscripts 1 and 2). The chemical formula of aluminium oxide is ${\text{Al}}_2{\text{O}}_3$ (since ${\text{Al}}^{3+}$ and ${\text{O}}^{2-}$ criss-cross).

Q14: What is valence shell and what are valence electrons?

Ans: The outermost shell of an atom is known as the valence shell. The electrons present in the valence shell are called valence electrons. They determine the combining capacity (valency) of the atom.

Q15: What is an octet? Why do atoms try to achieve an octet?

Ans: When the outermost shell of an atom has 8 electrons, it is called an octet. Atoms with a complete octet are stable and largely unreactive. Atoms with incomplete valence shells lose, gain, or share electrons to achieve an octet and become stable.

Q16: Calculate the molecular mass of water (H2O).

Ans: Atomic mass: $\text{H}=1\text{ u},\text{ O}=16\text{ u}$.$$\text{Molecular mass of }{\text{H}}_2\text{O}=(1\times 2)+(16\times 1)=18\text{ u}$$

Q17: Calculate the molecular mass of carbon dioxide (CO2).

Ans: Atomic mass: $\text{C}=12\text{ u},\text{ O}=16\text{ u}$.$$\text{Molecular mass of }{\text{CO}}_2=(12\times 1)+(16\times 2)=44\text{ u}$$

Q18: What is formula unit mass? How is it different from molecular mass?

Ans: Formula unit mass is the sum of the atomic masses of all atoms present in a formula unit (simplest whole number ratio of ions) of an ionic compound. It differs from molecular mass because ionic compounds do not form molecules – their ions form 3-D crystal structures instead.

Q19: Give two differences in properties between ionic and covalent compounds.

Ans: (i) Ionic compounds (e.g., NaCl) are generally soluble in water but insoluble in kerosene and petrol, whereas most covalent compounds (e.g., camphor) are insoluble in water but dissolve in kerosene and petrol. 
(ii) Ionic compounds have high melting and boiling points due to strong inter-ionic forces, whereas covalent compounds usually have low melting and boiling points.

Q20: Sodium chloride (NaCl) contains sodium and chlorine in a mass ratio of 23:35.5. If 46 g of sodium reacts completely, how much chlorine is needed?

Ans: By the Law of Constant Proportions, the mass of chlorine required is:$$\text{Mass of Cl}=\frac{35.5}{23}\times 46=71\text{ g}$$Therefore, 71 g of chlorine is needed to react with 46 g of sodium to form NaCl.