09. Short and Long Answer Questions: Atomic Foundations of Matter

Short Answer Type Questions

Ques 1: State the Law of Conservation of Mass. Who proposed it and when? Does this law hold true for physical changes as well?

Ans: The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. In other words, the total mass of the reactants before a chemical reaction is always equal to the total mass of the products after the reaction.

This law was proposed by Antoine Lavoisier in 1789. He is known as the Father of Modern Chemistry. He stated that in every operation, an equal quantity of matter exists both before and after the operation.

Yes, this law also holds true for physical changes. For example, when salt is dissolved in water, the mass of the resulting solution equals the sum of the masses of water and salt taken. There is practically no change in mass during the formation of a solution.

Ques 2: In a chemical reaction, 4.0 g of calcium carbonate reacts with 2.92 g of hydrochloric acid to produce 1.76 g of carbon dioxide, 0.72 g of water, and 4.44 g of calcium chloride. Verify whether the Law of Conservation of Mass is obeyed.

Ans: According to the Law of Conservation of Mass, the total mass of reactants must equal the total mass of products.

Mass of reactants:$$\text{Mass of calcium carbonate}+\text{Mass of hydrochloric acid}=4.0\text{ g}+2.92\text{ g}=6.92\text{ g}$$

Mass of products:$$1.76\text{ g}+0.72\text{ g}+4.44\text{ g}=6.92\text{ g}$$

Since the total mass of reactants $(6.92\text{ g})$ equals the total mass of products $(6.92\text{ g})$, the Law of Conservation of Mass is obeyed.

Ques3: State the Law of Constant Proportions. By what other names is this law known? Illustrate it with the example of water.

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 or how it was prepared.

This law is also known as the Law of Definite Proportions or Proust’s Law, after the French chemist Joseph Louis Proust who proposed it.

Illustration with water: Water collected from various sources – rivers, borewells, or the ocean – is always purified and analysed to contain hydrogen and oxygen in a mass ratio of 1 : 8. This means that if 9 g of purified water from any source is decomposed, it always yields 1 g of hydrogen and 8 g of oxygen, regardless of where the water came from.

Ques 4: Sodium chloride (NaCl) contains sodium and chlorine in the mass ratio of 23 : 35.5. If 46 g of sodium reacts completely, how much chlorine is needed to form NaCl? Also find the total mass of NaCl formed.

Ans: Given: mass ratio of sodium to chlorine in NaCl = $23:35.5$

Mass of chlorine required:$$\text{Mass of chlorine}=\frac{35.5}{23}\times 46=35.5\times 2=71\text{ g}$$

Total mass of NaCl formed (by the Law of Conservation of Mass):$$\text{Mass of NaCl}=46\text{ g (Na)}+71\text{ g (Cl)}=117\text{ g}$$

Therefore, 71 g of chlorine is needed and 117 g of NaCl is formed.

Ques 5: State the postulates of Dalton’s Atomic Theory. How did this theory explain the Law of Conservation of Mass?

Ans: John Dalton proposed his atomic theory in 1808. Its main postulates are:

• All matter is made up of very tiny particles called atoms, which participate in chemical reactions.
• Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
• Atoms of a given element are identical in mass and chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in the ratio of simple whole numbers to form compounds.
• The relative number and kinds of atoms are constant in a given compound.

Explanation of Law of Conservation of Mass: Since atoms are indivisible and merely rearrange during a chemical reaction without being created or destroyed, the total number of atoms – and hence the total mass – remains the same before and after the reaction. This directly explains the Law of Conservation of Mass.

Ques 6: What is a molecule? Define a covalent bond. How is a covalent bond formed in a hydrogen molecule (H2)?

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. Some elements, such as helium, exist only as atoms because their atoms are already stable.

A covalent bond is the type of chemical bond formed by the sharing of a pair of electrons between two atoms. The shared pair of electrons attracts both nuclei and holds the atoms together as a molecule.

Formation of H2 molecule: Hydrogen has atomic number 1 and has only one electron in its K-shell. The K-shell can hold a maximum of two electrons, so a hydrogen atom needs one more electron to become stable. Therefore, two hydrogen atoms each share their single electron with the other, forming a shared pair of electrons. This shared pair holds the two atoms together and forms a hydrogen molecule (${\text{H}}_2$). Since one pair of electrons is shared, this is a single covalent bond, represented as H-H.

Ques7: Distinguish between a single bond and a double bond with one example of each. How is the double bond depicted?

Ans: The difference between a single bond and a double bond is as follows:

Feature	Single Bond	Double Bond
Definition	Formed when two atoms share one electron each (one pair of electrons shared)	Formed when two atoms share two electrons each (two pairs of electrons shared)
Representation	A single line between the atoms	Two lines between the atoms
Example	Hydrogen molecule $({\text{H}}_2)$ – depicted as H-H	Oxygen molecule $({\text{O}}_2)$ – depicted as O=O

Formation of double bond in O2: Oxygen has atomic number 8 and an electronic configuration of 2, 6. Its valence shell has 6 electrons and requires 2 more to complete its octet. So, two oxygen atoms each share two electrons with each other, forming two shared pairs – a double bond, represented as O=O.

Ques 8: What are cations and anions? How is a sodium cation (Na+) and a chloride anion (Cl−) formed?

Ans: When atoms lose or gain electrons to attain a stable electronic configuration, they become electrically charged species called ions. Positively charged ions are called cations and negatively charged ions are called anions.

Formation of Sodium cation (Na+): Sodium has atomic number 11 and electronic configuration 2, 8, 1. Its valence shell has only one electron. Since it has fewer than four valence electrons, the sodium atom loses this one electron to achieve the stable electronic configuration of neon (2, 8). After losing one electron, sodium has 11 protons but only 10 electrons, so it carries a net positive charge of $+1$ and is represented as ${\text{Na}}^{+}$.

Formation of Chloride anion (Cl−): Chlorine has atomic number 17 and electronic configuration 2, 8, 7. Its valence shell has 7 electrons and needs one more to complete its octet. So, a chlorine atom gains one electron from another atom. After gaining one electron, it has 17 protons but 18 electrons, giving it a net negative charge of $-1$. It is represented as ${\text{Cl}}^{-}$.

Ques9: What is an ionic bond? How are cations and anions held together in an ionic compound? Give one example.

Ans: An ionic bond is the electrostatic force of attraction between oppositely charged ions (a cation and an anion) that holds them together to form an ionic compound. Ionic bonds are formed by the transfer of electrons from one atom to another.

When a metal atom (which tends to lose electrons) transfers one or more valence electrons to a non-metal atom (which tends to gain electrons), a positively charged cation and a negatively charged anion are formed. The strong electrostatic attraction between these oppositely charged ions constitutes the ionic bond.

Example – Sodium Chloride (NaCl):

• Sodium ($\text{Na}$) loses one electron to form ${\text{Na}}^{+}$ (cation).
• Chlorine ($\text{Cl}$) gains that electron to form ${\text{Cl}}^{-}$ (anion).
• The electrostatic attraction between ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ forms the ionic bond in sodium chloride (NaCl).

Ques10: Explain the rules for naming covalent compounds. Give three examples to illustrate these rules.

Ans: Covalent compounds are named by indicating the number of atoms of each element in the compound using a prefix system. The rules are:

• The first element retains its regular name, while the second element’s name ends in -ide.
• Prefixes – mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), etc. – are used before each element’s name to indicate the number of atoms. However, mono- is generally omitted for the first element.
• If a prefix ends with ‘o’ or ‘a’ and the element name starts with a vowel, the last vowel of the prefix is dropped. For example: monoxide (not monooxide), pentoxide (not pentaoxide).
• If a prefix ends with ‘i’, it is kept for pronunciation. For example: dioxide, trioxide.
• When hydrogen is the first element, no prefix is added before it, irrespective of how many hydrogen atoms are present.

Three examples:

• ${\text{CO}}_2$ is named carbon dioxide (not monocarbon dioxide).
• ${\text{SF}}_6$ is named sulfur hexafluoride, showing six fluorine atoms.
• ${\text{N}}_2{\text{O}}_4$ is named dinitrogen tetroxide (not tetraoxide).

Ques 11: What are polyatomic ions? Give the formulae and valencies of any four common polyatomic ions .

Ans: Some ions are formed by the combination of atoms of two or more elements. These are called polyatomic ions. Unlike simple (monoatomic) ions, the names of polyatomic ions generally do not end with -ide.

Four common polyatomic ions with their formulae and valencies are:

Name of Ion	Formula	Valency
Hydroxide	${\text{OH}}^{-}$	1
Nitrate	${\text{NO}}_3^{-}$	1
Carbonate	${\text{CO}}_3^{2-}$	2
Sulfate	${\text{SO}}_4^{2-}$	2

Ionic compounds are named by writing the name of the cation first, followed by the name of the anion. When two or more polyatomic ions of the same type are present in a formula, brackets are used around the polyatomic ion – for example, ${\text{Mg(OH)}}_2$.

Ques 12: State two differences each between ionic and covalent compounds based on (a) solubility and (b) electrical conductivity.

Ans: (a) Solubility:

Ionic Compounds	Covalent Compounds
Generally soluble in water (e.g., sodium chloride, copper sulfate)	Most covalent compounds are insoluble in water (e.g., camphor, naphthalene)
Generally insoluble in organic solvents like kerosene and petrol	Generally soluble in organic solvents like kerosene and petrol

(b) Electrical conductivity:

Ionic Compounds	Covalent Compounds
Do not conduct electricity in the solid state because their ions are held in fixed positions by strong forces	Do not conduct electricity in solid or dissolved state because they do not form ions in solution
Conduct electricity when dissolved in water because ions become free to move in solution	Some covalent compounds like sugar are soluble in water but still do not conduct electricity as they do not provide ions

Ionic compounds also generally have high melting and boiling points due to strong inter-ionic attractions, whereas covalent compounds usually have low melting and boiling points.

Long Answer Type Questions

Ques1: Compare the Law of Conservation of Mass and the Law of Constant Proportions. Who proposed each law? Illustrate the Law of Constant Proportions with a numerical example using carbon and oxygen.

Ans: The two fundamental laws of chemical combination are compared below:

Feature	Law of Conservation of Mass	Law of Constant Proportions
Statement	Mass can neither be created nor destroyed in a chemical reaction	In any compound, elements combine in a fixed ratio by mass, irrespective of source or preparation
Proposed by	Antoine Lavoisier (1789)	Joseph Louis Proust (also called Proust’s Law or Law of Definite Proportions)
Focus	Total mass before and after reaction	Ratio of masses of elements in a compound
Applicability	Every chemical and physical change	Compounds only (not mixtures)
Example	In NaCl formation: 23 g Na + 35.5 g Cl gives 58.5 g NaCl; no mass is gained or lost	Water always contains H and O in ratio 1:8 by mass, regardless of source

Numerical example using carbon and oxygen (Law of Constant Proportions):

From the textbook: 12 g of carbon reacts with 32 g of oxygen to form 44 g of carbon dioxide.$$\text{Carbon}+\text{Oxygen}\to \text{Carbon dioxide}$$

If 2.4 g of carbon reacts completely with oxygen, the mass of carbon dioxide produced is calculated as follows:$$1\text{ g of carbon gives}=\frac{44}{12}\text{ g of }{\text{CO}}_2$$$$2.4\text{ g of carbon gives}=\frac{44}{12}\times 2.4=8.8\text{ g of }{\text{CO}}_2$$

The ratio of carbon to oxygen in carbon dioxide is always $12:32=3:8$ by mass, irrespective of how much carbon dioxide is made or from where. This confirms the Law of Constant Proportions.

Ques 2: Explain all the postulates of Dalton’s Atomic Theory. How does this theory explain both the Law of Conservation of Mass and the Law of Constant Proportions?

Ans: John Dalton presented his atomic theory in 1808 based on combined earlier experimental observations. A postulate is a fundamental assumption accepted as truth without formal proof, from which further ideas are developed. Dalton’s postulates are:

• All matter is made up of very tiny particles called atoms, which participate in chemical reactions.
• Atoms are indivisible – they cannot be created or destroyed in a chemical reaction.
• Atoms of a given element are identical in mass and chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in the ratio of simple whole numbers to form compounds.
• The relative number and kinds of atoms are constant in a given compound.

Explanation of the Law of Conservation of Mass:

Since atoms cannot be created or destroyed in a chemical reaction (postulate 2), and they merely rearrange to form new substances, the total number of atoms – and therefore the total mass – remains the same before and after the reaction. For example, when hydrogen and oxygen combine to form water, the hydrogen and oxygen atoms are not destroyed; they only rearrange. Hence mass is conserved.

Explanation of the Law of Constant Proportions:

Because atoms of a given element are all identical in mass (postulate 3) and atoms combine in fixed whole number ratios (postulate 5), the mass ratio of elements in a compound is always constant. For example, in water (${\text{H}}_2\text{O}$), two hydrogen atoms always combine with one oxygen atom. Since atoms have fixed masses, the mass ratio of hydrogen to oxygen in water is always $1:8$, regardless of the source or method of preparation.

Ques 3: Explain step by step the formation of a water molecule (H2O) through covalent bonding. What type of bond is present? Also explain how atoms combine to form compounds through the two general ways described in the textbook.

Ans: Atoms combine to form compounds to achieve a stable electronic configuration (complete octet or duplet). This takes place in two general ways:

• Sharing of electrons: One or more valence electrons are shared between atoms, forming a covalent bond.
• Transfer of electrons: One or more valence electrons are transferred from one atom to another, forming an ionic bond (cations and anions are created).

When atoms combine, the total energy of the system becomes lower, making the arrangement more stable. The force holding atoms together is called a chemical bond.

Step-by-step formation of water (H2O) by covalent bonding:

• Step 1 – Electronic configuration of hydrogen: Atomic number of hydrogen is 1. It has 1 electron in its K-shell. The K-shell can hold 2 electrons, so hydrogen needs 1 more electron to become stable (complete its duplet).
• Step 2 – Electronic configuration of oxygen: Atomic number of oxygen is 8. Its electronic configuration is 2, 6. The outermost (L) shell has 6 electrons. Oxygen needs 2 more electrons to complete its octet.
• Step 3 – Sharing of electrons: One oxygen atom cannot share its two electrons with just one hydrogen atom (which needs only one electron). Therefore, two hydrogen atoms each share one electron with the oxygen atom. Each hydrogen atom contributes one electron and the oxygen atom contributes two electrons – one to each hydrogen atom.
• Step 4 – Result: Two shared pairs of electrons are formed – one between oxygen and each hydrogen atom. Each pair forms a single covalent bond. The water molecule is thus represented as ${\text{H}}_2\text{O}$, with two O-H single bonds.

Since hydrogen and oxygen share electrons to form water, water is a covalent compound. The type of bond between each hydrogen and oxygen atom is a single covalent bond.

Ques4: Describe the formation of sodium chloride (NaCl) through ionic bonding. What is the crystal structure of NaCl? Write the chemical formulae of calcium chloride (CaCl2) and aluminium oxide (Al2O3) using the criss-cross method.

Ans: Formation of NaCl by ionic bonding:

• The atomic number of sodium is 11 and its electronic configuration is 2, 8, 1. Its valence shell has only one electron. Since it has fewer than four valence electrons, sodium tends to lose this one electron.
• When sodium loses one electron, it becomes a sodium cation (Na+) with 11 protons and 10 electrons, carrying a charge of $+1$.
• Chlorine has atomic number 17 and electronic configuration 2, 8, 7. Its valence shell has 7 electrons and needs one more to complete its octet. Chlorine gains the electron released by sodium.
• After gaining one electron, chlorine becomes a chloride anion (Cl−) with 17 protons and 18 electrons, carrying a charge of $-1$.
• The ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions are held together by the strong electrostatic force of attraction between opposite charges – this is the ionic bond, and the compound formed is sodium chloride (NaCl).

Crystal structure of NaCl: Ionic compounds like NaCl usually do not remain as single units. They form three-dimensional (3-D) crystals in which ions are arranged in a repeating pattern. In NaCl, each sodium ion (${\text{Na}}^{+}$) is surrounded by six chloride ions (${\text{Cl}}^{-}$), and each chloride ion is surrounded by six sodium ions. These oppositely charged ions are arranged in a regular, repeating 3-D pattern known as a crystal lattice.

Chemical formula of Calcium Chloride (CaCl2) using criss-cross method:

Calcium ion: ${\text{Ca}}^{2+}$ (charge = 2+)  |  Chloride ion: ${\text{Cl}}^{-}$ (charge = 1-)

Criss-crossing the numbers of the charges: the subscript of Ca becomes 1 and the subscript of Cl becomes 2.$$\text{Formula}={\text{CaCl}}_2$$

Chemical formula of Aluminium Oxide (Al2O3) using criss-cross method:

Aluminium ion: ${\text{Al}}^{3+}$ (charge = 3+)  |  Oxide ion: ${\text{O}}^{2-}$ (charge = 2-)

Criss-crossing: subscript of Al becomes 2 and subscript of O becomes 3.$$\text{Formula}={\text{Al}}_2{\text{O}}_3$$
Ques 5: (i) What is molecular mass? Calculate the molecular mass of water (H2O) and carbon dioxide (CO2). (Given: H = 1 u, O = 16 u, C = 12 u)
(ii) What is formula unit mass? Why is it used for ionic compounds? Calculate the formula unit mass of calcium nitrate Ca(NO3)2. (Given: Ca = 40 u, N = 14 u, O = 16 u)

Ans: (i) Molecular Mass:

Molecular mass is the total mass of a molecule, calculated by adding the atomic masses of all the atoms present in it. It is expressed in atomic mass units (u).

Molecular mass of water (H2O):$$\text{Molecular mass of }{\text{H}}_2\text{O}=(1\text{ u}\times 2)+(16\text{ u}\times 1)=2+16=18\text{ u}$$

Molecular mass of carbon dioxide (CO2):$$\text{Molecular mass of }{\text{CO}}_2=(12\text{ u}\times 1)+(16\text{ u}\times 2)=12+32=44\text{ u}$$

(ii) Formula Unit Mass:

In ionic compounds, the ions form three-dimensional crystals and do not form discrete molecules. Therefore, the term “molecular mass” cannot be used for ionic compounds. Instead, the collection of the simplest whole number ratio of ions is called a formula unit, and the mass of this formula unit is called the formula unit mass.

Formula unit mass of calcium nitrate Ca(NO3)2:

Atoms present: 1 Ca, 2 N, 6 O$$\text{Formula unit mass}=(40\text{ u}\times 1)+[(14\text{ u}\times 1)+(16\text{ u}\times 3)]\times 2$$$$=40+[14+48]\times 2$$$$=40+62\times 2=40+124=164\text{ u}$$

Therefore, the formula unit mass of ${\text{Ca(NO}}_3{)}_2$ is 164 u.