Atomic Foundations of Matter
Laws that govern chemical reactions, how atoms bond together, and the language of chemical formulae — all explained simply.
- Water can come from rivers, borewells, and the ocean. Are all these samples of water chemically the same?
- Oxygen is sometimes written as O and sometimes as O₂. What is the difference between these two symbols?
- Why does salt dissolved in water conduct electricity, but sugar dissolved in water does not?
You will find the answers as you read through this chapter!
Introduction
From the previous chapter, you already know that atoms are made of electrons, protons, and neutrons. Atoms become stable when they have 8 electrons in their outermost shell (called an octet). To achieve this, atoms can lose, gain, or share electrons.
Here is something interesting: when hydrogen (a gas) and oxygen (another gas) combine, they form water — a liquid. Water doesn't burn, doesn't support burning, and even puts out fire. Yet the total mass of water formed always equals the sum of the masses of hydrogen and oxygen used. This amazing observation leads us to the first great law of chemistry.
9.1 Law of Conservation of Mass
⚖️ Law of Conservation of Mass
Matter can neither be created nor destroyed during a chemical reaction. The total mass of all reactants (substances that react) always equals the total mass of all products (substances formed).
Mathematically: Mass of Reactants = Mass of Products
Known as the Father of Modern Chemistry, Lavoisier proposed this law in 1789. He carefully weighed substances before and after chemical reactions and showed that the total mass never changes. His key statement: "In every operation, an equal quantity of matter exists both before and after the operation."
Why does mass seem to "disappear" sometimes?
When baking soda reacts with vinegar in an open container, the carbon dioxide gas produced escapes into the air. If you only weigh the liquid left behind, it looks like mass disappeared — but it hasn't! The gas is still matter. When you do the same reaction in a closed system (where gas cannot escape), the total mass before and after is exactly equal.
The Law of Conservation of Mass applies to all physical and chemical changes. Even when a gas is produced and "seems" to vanish, the total mass is conserved — you just need to account for every product, including gases.
Understanding Through Numbers
Consider: 4.0 g of calcium carbonate reacts with 2.92 g of hydrochloric acid. The products measured are 1.76 g of carbon dioxide, 0.72 g of water, and 4.44 g of calcium chloride.
Total products = 1.76 + 0.72 + 4.44 = 6.92 g
✔ Mass of reactants = Mass of products → Law obeyed!
Another example: 12 g of carbon combines with 32 g of oxygen to give 44 g of carbon dioxide. So if 2.4 g of carbon reacts:
Q: A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left. Does this mean the Law of Conservation of Mass is violated?
A: No! The ethanol burns to produce carbon dioxide and water vapour, which escape into the air. If you could collect all the gases, the total mass would still equal 10 g. The law is not violated — the products are just invisible gases.
Q: When 20 g of hydrogen reacts completely with 160 g of oxygen, how much water is formed?
A: Mass of water = 20 + 160 = 180 g (by the Law of Conservation of Mass).
9.2 Law of Constant Proportions
📐 Law of Constant Proportions (Definite Proportions / Proust's Law)
In any compound, the elements are always present in a fixed ratio by mass, no matter where the compound comes from or how it was made.
Example: Pure water always contains hydrogen and oxygen in the mass ratio of 1 : 8. So 9 g of water always gives 1 g of hydrogen and 8 g of oxygen.
A careful French chemist who studied many compounds, including copper carbonate. He showed that no matter how copper carbonate was prepared or where it was found, it always had copper, carbon, and oxygen in the same mass ratio. His work became one of the pillars of modern chemistry.
🪨 Threads of Curiosity: Cinnabar
Cinnabar (called hingula in ancient India) is a red mineral used as a pigment for thousands of years. Heating cinnabar always produces mercury and sulfur in the ratio of approximately 86.22% : 13.78% by mass — perfectly demonstrating the Law of Constant Proportions across different civilisations and centuries!
Working With the Law — Formula
If you know the mass ratio of elements in a compound, you can find how much of one element combines with a given amount of another:
If 46 g of Na reacts → Cl needed = (35.5 ÷ 23) × 46 = 71 g
Q3. A compound has 40% sulfur and 60% oxygen by mass. If a sample contains 20 g of sulfur, what mass of oxygen is present?
A: Ratio S : O = 40 : 60 = 2 : 3. So oxygen = (60 ÷ 40) × 20 = 30 g
Q4. Carbon monoxide (CO) has C : O = 3 : 4. How much oxygen combines with 9 g of carbon?
A: Oxygen = (4 ÷ 3) × 9 = 12 g
Q5. The Law of Definite Proportions holds for compounds but not mixtures. Why?
A: In a compound, elements are chemically combined in a fixed ratio. In a mixture, substances are simply mixed together in any proportion without any chemical bonding, so the ratio can vary.
Q6. Students X and Y prepared copper oxide in ratios 4:1 and 8:2. Do their results justify the Law of Constant Proportions?
A: Yes! 4:1 and 8:2 are the same ratio (8:2 = 4:1 when simplified). Both samples have copper and oxygen in the same proportion, which confirms the law.
9.3 Dalton's Atomic Theory
The two laws above raised a big question: Why do elements always combine in fixed ratios? Why is mass never lost? John Dalton answered this by proposing a theory about the tiny particles called atoms.
Born in England, Dalton moved to Manchester in 1793 to teach mathematics and science. In 1808, he presented his atomic theory — a turning point in the study of matter. A postulate is a fundamental assumption accepted as truth from which further ideas are built.
Dalton's Six Postulates
- All matter is made of very tiny particles called atoms, which take part in chemical reactions.
- Atoms are indivisible — they cannot be created or destroyed in a chemical reaction.
- All 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 ratios of simple whole numbers to form compounds.
- The relative number and kinds of atoms in a given compound are always constant.
How Dalton's Theory Explains the Two Laws
✅ Law of Conservation of Mass: Since atoms cannot be created or destroyed, only rearranged, the total number (and therefore mass) of atoms stays the same before and after a reaction.
✅ Law of Constant Proportions: Since atoms of each element have a fixed mass and combine in fixed whole-number ratios, every sample of a compound always has the same mass ratio of elements.
What if atoms could combine in any ratio and not a fixed one? Every time you made water, it might be a different substance with different properties! Chemistry as we know it would be impossible. Fixed ratios are what make the world around us predictable and reliable.
Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18 g of water.
Reason (R): According to Dalton's Atomic Theory, atoms combine in a simple whole number ratio by mass to form compounds.
Answer: (ii) Both A and R are true, but R is NOT the correct explanation of A.
Why? The assertion is true (2+16=18, conservation of mass). The reason is also true (whole number ratios). However, the reason explains why elements combine in fixed ratios, not why mass is conserved. The correct explanation for A is the Law of Conservation of Mass, not the whole-number ratio postulate.
9.4 How Atoms Combine?
A molecule is an electrically neutral particle made of two or more atoms bonded together, which can exist independently and shows all the properties of that substance.
Atoms combine to reach a stable electronic configuration — 8 electrons in the outermost shell (octet rule), or 2 electrons if only the K-shell exists (duplet rule for hydrogen and helium).
There are two main ways atoms combine:
- Sharing of electrons → forms a Covalent Bond
- Transfer of electrons → forms an Ionic Bond
When atoms bond, the total energy of the system drops — making the combination more stable than the individual atoms.
9.4.1 Covalent Bond — Bonding by Sharing Electrons
When atoms share electrons with each other, the shared pair of electrons is attracted by the nuclei of both atoms, holding them together. This is called a covalent bond.
A. Molecules of Elements (same atoms bonding)
Hydrogen Molecule (H₂) — Single Bond
Each hydrogen atom has 1 electron and needs 1 more to complete its K-shell (duplet). Two hydrogen atoms each share their 1 electron to form H₂.
Fig: Formation of a Hydrogen Molecule (H—H, single bond)
Chlorine Molecule (Cl₂) — Single Bond
Chlorine has 7 valence electrons and needs 1 more to complete its octet. Two chlorine atoms each share 1 electron to form Cl₂.
Fig: Formation of a Chlorine Molecule (Cl—Cl, single covalent bond)
Oxygen Molecule (O₂) — Double Bond
Oxygen has 6 valence electrons and needs 2 more. Two oxygen atoms each share 2 electrons, forming a double bond (O=O).
Fig: Formation of an Oxygen Molecule (double covalent bond)
Q8. Nitrogen has 5 valence electrons. Draw the structure of N₂.
A: Nitrogen needs 3 more electrons to complete its octet. Two nitrogen atoms share 3 electrons each, forming a triple bond (N≡N). The structure has 3 shared pairs between the two nitrogen nuclei, plus 1 lone pair on each nitrogen.
Q9. Fluorine has atomic number 9. Explain formation of F₂.
A: Fluorine's configuration is 2, 7 — it has 7 valence electrons and needs 1 more. Two fluorine atoms each share 1 electron, forming a single covalent bond (F—F). Each fluorine atom then has 8 electrons in its outer shell, completing the octet.
B. Molecules of Compounds (different atoms bonding)
Hydrogen Chloride (HCl)
H needs 1 electron; Cl needs 1 electron. They share 1 electron each → single covalent bond → H—Cl.
Water Molecule (H₂O)
Oxygen needs 2 electrons; each hydrogen needs 1 electron. Two hydrogen atoms each share 1 electron with oxygen → two single bonds → H—O—H (or H₂O).
Show the formation of:
(i) CO₂ (Carbon dioxide): Carbon has 4 valence electrons; needs 4 more. Each oxygen has 6 valence electrons; needs 2 more. Carbon shares 2 electrons with each of the 2 oxygen atoms → two double bonds → O=C=O.
(ii) H₂S (Hydrogen sulfide): Sulfur has 6 valence electrons; needs 2 more. Each of 2 hydrogen atoms shares 1 electron with sulfur → two single bonds → H—S—H.
(iii) NH₃ (Ammonia): Nitrogen has 5 valence electrons; needs 3 more. Three hydrogen atoms each share 1 electron with nitrogen → three single bonds → H—N—H with H below.
Q11. Neon (atomic number 10) neither transfers nor shares electrons. Explain.
A: Neon's electronic configuration is 2, 8. Its outermost shell already has 8 electrons — a complete octet. It is already stable. There is no need to gain, lose, or share electrons, so neon does not form chemical bonds.
C. Naming Covalent Compounds
Rules:
- The first element keeps its regular name.
- The second element's name ends in -ide.
- Prefixes show the number of atoms: mono-(1), di-(2), tri-(3), tetra-(4), penta-(5), hexa-(6).
- Mono- is usually not used for the first element but is used for the second.
- Drop the final vowel of prefix before a vowel: monoxide (not monooxide), pentoxide.
- When H is the first element, no prefix is used before it: H₂S = hydrogen sulfide (not dihydrogen sulfide).
| Formula | Name | Reason |
|---|---|---|
| CO | Carbon monoxide | 1 carbon, 1 oxygen (mono- for O) |
| CO₂ | Carbon dioxide | 1 carbon, 2 oxygens (di-) |
| CS₂ | Carbon disulfide | 2 sulfur atoms (di-) |
| PCl₃ | Phosphorus trichloride | 3 chlorine atoms (tri-) |
| SF₆ | Sulfur hexafluoride | 6 fluorine atoms (hexa-) |
| N₂O₄ | Dinitrogen tetroxide | 2 nitrogen (di-), 4 oxygen (tetr-) |
| N₂O₅ | Dinitrogen pentoxide | 2 nitrogen (di-), 5 oxygen (penta-) |
| H₂S | Hydrogen sulfide | No prefix before H |
| H₂O | Water (common name) | Technically: hydrogen monoxide |
| NH₃ | Ammonia (common name) | Technically: nitrogen trihydride |
9.4.2 Ionic Bond — Bonding by Transfer of Electrons
When an atom has fewer than 4 valence electrons, it usually donates those electrons to another atom. The atom that receives electrons usually has more than 4 valence electrons. After transfer, both atoms become charged particles called ions, held together by attraction between opposite charges. This attraction is the ionic bond.
- An atom that loses electrons becomes a positively charged cation (e.g. Na⁺).
- An atom that gains electrons becomes a negatively charged anion (e.g. Cl⁻).
- Cations and anions are collectively called ions.
- The electrostatic force of attraction between oppositely charged ions is the ionic bond.
Formation of Sodium Chloride (NaCl)
Fig: Formation of NaCl — Na loses 1 electron → Na⁺; Cl gains 1 electron → Cl⁻; they attract each other (ionic bond)
Crystal Structure of Ionic Compounds
Ionic compounds do not exist as individual pairs of ions. Instead, they form three-dimensional crystals. In NaCl, each Na⁺ ion is surrounded by 6 Cl⁻ ions, and each Cl⁻ is surrounded by 6 Na⁺ ions. This repeating pattern is called a crystal lattice.
🔍 Threads of Curiosity: Sulfide Ion
Sulfur has 6 valence electrons and needs 2 more to complete its octet. When sulfur gains 2 electrons, it acquires a charge of 2− and is written as S²⁻. This is an example of how non-metals with 6 valence electrons behave.
Q12. What kind of ion will oxygen form?
A: Oxygen has 6 valence electrons and needs 2 more to complete its octet. So oxygen gains 2 electrons and forms O²⁻ (oxide anion), a negatively charged ion with a valency of 2.
Q13. Fill in the blanks — magnesium and chlorine forming magnesium chloride:
A: Chlorine can take only one electron to become Cl⁻. Now, one (1) ion of magnesium and two (2) ions of chlorine combine to give magnesium chloride (MgCl₂).
Q14. Show the formation of cations of K and Ca and their chlorides.
K: Potassium (atomic number 19, config: 2,8,8,1) loses 1 electron → K⁺. K⁺ + Cl⁻ → KCl
Ca: Calcium (atomic number 20, config: 2,8,8,2) loses 2 electrons → Ca²⁺. Ca²⁺ + 2Cl⁻ → CaCl₂
Q15. Illustrate how sodium sulfide (Na₂S) is formed.
A: Sulfur needs 2 electrons → S²⁻. Each sodium loses 1 electron → Na⁺. Two Na⁺ ions combine with one S²⁻ ion to form Na₂S. The charges balance: 2×(+1) + 1×(−2) = 0.
A. Naming Ionic Compounds
In naming ionic compounds: write the cation name first, then the anion name (ending in -ide for simple anions). Metals form cations; non-metals form anions. Polyatomic ions (made of multiple atoms) generally do not end in -ide.
Common Monoatomic Ions
| Name of Ion | Formula | Valency |
|---|---|---|
| Sodium | Na⁺ | 1 |
| Lithium | Li⁺ | 1 |
| Potassium | K⁺ | 1 |
| Silver | Ag⁺ | 1 |
| Calcium | Ca²⁺ | 2 |
| Barium | Ba²⁺ | 2 |
| Iron (Ferrous) | Fe²⁺ | 2 |
| Iron (Ferric) | Fe³⁺ | 3 |
| Copper (Cuprous) | Cu⁺ | 1 |
| Copper (Cupric) | Cu²⁺ | 2 |
| Magnesium | Mg²⁺ | 2 |
| Zinc | Zn²⁺ | 2 |
| Aluminium | Al³⁺ | 3 |
| Fluoride | F⁻ | 1 |
| Chloride | Cl⁻ | 1 |
| Bromide | Br⁻ | 1 |
| Iodide | I⁻ | 1 |
| Oxide | O²⁻ | 2 |
| Sulfide | S²⁻ | 2 |
Common Polyatomic Ions
| Name of Ion | Formula | Valency |
|---|---|---|
| Hydroxide | OH⁻ | 1 |
| Nitrate | NO₃⁻ | 1 |
| Hydrogencarbonate | HCO₃⁻ | 1 |
| Carbonate | CO₃²⁻ | 2 |
| Sulfate | SO₄²⁻ | 2 |
| Ammonium | NH₄⁺ | 1 |
9.5 Writing Chemical Formulae
There is a quick and reliable method to write chemical formulae using valencies: the criss-cross method.
9.5.1 Covalent Compounds — Criss-Cross Method
Write the symbols of both elements, note their valencies below, then swap (criss-cross) the valency numbers as subscripts. If both valencies are the same, simplify to the smallest whole number.
(Subscript 1 is not written)
9.5.2 Ionic Compounds — Criss-Cross Method
Write the cation first (with its charge number below), then the anion (with its charge number below). Criss-cross the charge numbers as subscripts. Simplify if both subscripts have a common factor. Use brackets for polyatomic ions when subscript > 1.
Charge balance: 1×(+2) + 2×(−1) = 0 ✔
Brackets used because 2 polyatomic OH groups.
NOT AlOH₃
Q16. Name the following:
(i) CO₂ → Carbon dioxide | (ii) NO₂ → Nitrogen dioxide | (iii) SF₆ → Sulfur hexafluoride | (iv) PCl₃ → Phosphorus trichloride
Q17. Write the formulae:
(i) Sodium hydrogencarbonate → Na (1+), HCO₃ (1−) → NaHCO₃
(ii) Sulfur dioxide → S (4 valency, as in SO₂ one S, 2 O) → SO₂
(iii) Ferric chloride → Fe³⁺, Cl⁻ → cross: Fe¹Cl³ → FeCl₃
(iv) Cuprous oxide → Cu⁺ (cuprous = 1+), O²⁻ → cross: Cu²O¹ → Cu₂O
Q18. Formulae from ion pairs:
(i) Fe³⁺ and OH⁻ → cross: Fe¹(OH)³ → Fe(OH)₃
(ii) K⁺ and CO₃²⁻ → cross: K²(CO₃)¹ → simplify polyatomic: K₂CO₃
9.6 Properties of Ionic and Covalent Compounds
| Property | Ionic Compounds | Covalent Compounds |
|---|---|---|
| Solubility in water | Generally soluble | Generally insoluble (exceptions: sugar) |
| Solubility in kerosene/petrol | Generally insoluble | Generally soluble |
| Electrical conductivity (solid) | ❌ Does not conduct (ions are fixed) | ❌ Does not conduct |
| Electrical conductivity (dissolved in water) | ✅ Conducts (ions are free to move) | ❌ Does not conduct (no ions in solution) |
| Melting & boiling points | High (strong inter-ionic attractions) | Low (weak intermolecular forces) |
Sugar is a covalent compound. Even though it dissolves in water, it does not break into ions. Electrical conduction requires free-moving charged particles (ions or electrons). Since sugar solution has no ions, it cannot conduct electricity.
In solid ionic compounds, ions are locked in fixed positions in the crystal lattice by strong electrostatic forces. They cannot move. Only when the compound is dissolved in water (or melted) do the ions become free to move and carry electric current.
Q19. What type of chemical bond is in a solid compound that doesn't conduct electricity but conducts when dissolved in water?
A: Ionic bond. Ionic compounds do not conduct in the solid state (ions are fixed) but do conduct when dissolved in water because the ions become free to move.
Q20. Metal M has 2 electrons in its valence M-shell. It reacts with oxygen. Predict:
Identification: M-shell is the 3rd shell. Having 2 electrons in the M-shell → Magnesium (2,8,2). It loses 2 electrons → Mg²⁺. Oxygen gains 2 electrons → O²⁻.
(i) Formula: Mg²⁺ + O²⁻ → same valency → MgO
(ii) Type of bond: Ionic bond (metal + non-metal)
(iii) Electrical conductivity of aqueous solution: MgO is slightly soluble. Where it dissolves, it provides Mg²⁺ and O²⁻ ions → the solution conducts electricity to some extent.
9.7 Molecular Mass of Covalent Compounds
The molecular mass of a covalent compound is found by adding the atomic masses of all the atoms in one molecule.
Molecular mass of H₂O:
H = 1 u, O = 16 u
Molecular mass = (1 × 2) + (16 × 1) = 2 + 16 = 18 u
Molecular mass of CO₂:
C = 12 u, O = 16 u
Molecular mass = (12 × 1) + (16 × 2) = 12 + 32 = 44 u
Ionic compounds do NOT form discrete molecules — they form continuous 3D crystal lattices. So we do NOT calculate molecular mass for ionic compounds. Instead we use formula unit mass.
Q21. Molecular mass of nitric acid (HNO₃):
H=1u, N=14u, O=16u
= (1×1) + (1×14) + (3×16) = 1 + 14 + 48 = 63 u
Q22. Molecular mass of methane (CH₄):
C=12u, H=1u
= (1×12) + (4×1) = 12 + 4 = 16 u
9.8 Formula Unit Mass of Ionic Compounds
A formula unit is the simplest whole-number ratio of ions in an ionic compound. The formula unit mass is the sum of atomic masses of all atoms in that formula unit.
Formula unit mass of Na₂O (Sodium Oxide):
Na=23u, O=16u
= (23×2) + (16×1) = 46 + 16 = 62 u
Formula unit mass of Ca(NO₃)₂ (Calcium Nitrate):
Ca=40u, N=14u, O=16u
= (40×1) + [(14×1) + (16×3)] × 2
= 40 + [14 + 48] × 2
= 40 + 62×2 = 40 + 124 = 164 u
Q23. Formula unit mass of KCl:
K=39u, Cl=35.5u → 39 + 35.5 = 74.5 u
Q24. Formula unit mass of Mg(OH)₂:
Mg=24u, O=16u, H=1u
= 24 + [(16+1)×2] = 24 + 34 = 58 u
📝 Revise, Reflect, Refine — Answers
(i) A (config: 2,8,1) tends to give away 1 electron to become stable.
(ii) A forms a cation (positive ion) — specifically A⁺ (like Na⁺).
(iii) B (config: 2,6) needs 2 more electrons — tends to take 2 electrons.
(iv) B forms an anion (negative ion) — specifically B²⁻ (like O²⁻).
(v) A gives electrons to B → Ionic bond forms.
(vi) A (1+) + B (2−): using criss-cross → formula = A₂B
(i) X has 6 valence electrons and needs 2 more to complete its octet. It cannot achieve this alone, so two X atoms share electrons with each other to form a diatomic molecule, becoming stable.
(ii) Since two atoms of the same element share electrons → Covalent bond (specifically a double bond, since each shares 2 electrons).
(iii) X≡? No — 6 valence electrons need 2 more → each shares 2 → double bond → structure: X=X (like O=O).
(iv) Element Y has 2 electrons in its second shell (config: 2,2), meaning Y is carbon-like OR it could mean 2 electrons in second shell = config 2,2. But more likely Y has 2 valence electrons and is in Period 2 (like Be or C). If Y is the element with 2 valence electrons in the second shell (like carbon, 2,4)... However, a more straightforward reading: Y has two electrons in its second shell only → config 2,2 → valency 2. X (6 valence) needs 2 more; Y (2 valence) shares 2. So 1 X bonds with 1 Y: X=Y (a double bond compound). If X is like sulfur and Y is like carbon: CS₂-type: Y bonds with 2 X atoms → X=Y=X.
Check each option:
(i) 2 Al³⁺ = +6; 3 Cl⁻ = −3 → Total = +6 − 3 = not balanced ✗
(ii) 3 Mg²⁺ = +6; 1 PO₄³⁻ = −3 → not balanced ✗
(iii) 2 Fe³⁺ = +6; 3 O²⁻ = −6 → +6 − 6 = 0 ✔ Balanced!
(iv) 3 Ca²⁺ = +6; 2 SO₄²⁻ = −4 → not balanced ✗
Answer: (iii) 2 Fe³⁺ and 3 O²⁻
(i) FALSE. Correction: Elements (not compounds) are made up of atoms. Compounds are made of molecules (or formula units).
(ii) FALSE. Correction: A molecule of a compound is made up of two or more atoms of different kinds (e.g., HCl = H + Cl). A molecule of an element has atoms of the same kind (e.g., O₂).
(iii) FALSE. Correction: One molecule of nitrogen gas (N₂) contains two nitrogen atoms, not three.
(iv) TRUE. Water (H₂O) is made of two hydrogen atoms covalently bonded with one oxygen atom. ✔
(i) Aluminium nitrate: Al³⁺ and NO₃⁻ → criss-cross: Al¹(NO₃)³ → Al(NO₃)₃
(ii) Calcium oxide: Ca²⁺ and O²⁻ → same valency → simplify → CaO
(iii) Ferric oxide: Fe³⁺ and O²⁻ → criss-cross: Fe²O³ → Fe₂O₃
(i) Ca²⁺ and Br⁻ → criss-cross: Ca¹Br² → CaBr₂
(ii) Al³⁺ and CO₃²⁻ → criss-cross: Al²(CO₃)³ → Al₂(CO₃)₃
(iii) K⁺ and SO₄²⁻ → criss-cross: K²(SO₄)¹ → K₂SO₄
(iv) NH₄⁺ and Cl⁻ → criss-cross: (NH₄)¹Cl¹ → NH₄Cl
Chlorine (atomic number 17) has the configuration 2, 8, 7. When it gains 1 electron to form Cl⁻, its configuration becomes 2, 8, 8 — 17 protons but 18 electrons.
The correct figure should show: 3 shells, with 2 electrons in the first, 8 in the second, and 8 electrons in the third/outer shell, with a negative sign (−) indicating the ion.
Answer: Option (ii) — the one with 2, 8, 8 configuration and a negative charge symbol.
(i) Ammonium nitrate NH₄NO₃:
N=14u, H=1u, O=16u
= (14×1) + (1×4) + (14×1) + (16×3) = 14 + 4 + 14 + 48 = 80 u
(ii) Phosphoric acid H₃PO₄:
H=1u, P=31u, O=16u
= (1×3) + (31×1) + (16×4) = 3 + 31 + 64 = 98 u
(iii) Sodium hydrogencarbonate NaHCO₃:
Na=23u, H=1u, C=12u, O=16u
= 23 + 1 + 12 + (16×3) = 23 + 1 + 12 + 48 = 84 u
(i) Magnesium and nitrogen: Mg²⁺ and N³⁻ → criss-cross: Mg³N² → Mg₃N₂
(ii) Lithium and nitrogen: Li⁺ and N³⁻ → criss-cross: Li³N¹ → Li₃N
(iii) Sodium and sulfur: Na⁺ and S²⁻ → criss-cross: Na²S¹ → Na₂S
(iv) Aluminium and oxygen: Al³⁺ and O²⁻ → criss-cross: Al²O³ → Al₂O₃
| NO₃⁻ | SO₄²⁻ | PO₄³⁻ | |
|---|---|---|---|
| NH₄⁺ | NH₄NO₃ | (NH₄)₂SO₄ | (NH₄)₃PO₄ |
| Li⁺ | LiNO₃ ✓ | Li₂SO₄ | Li₃PO₄ |
| Al³⁺ | Al(NO₃)₃ | Al₂(SO₄)₃ | AlPO₄ |
| Cu²⁺ | Cu(NO₃)₂ | CuSO₄ | Cu₃(PO₄)₂ |
Total mass of reactants = 5.3 + 6.0 = 11.3 g
Total mass of products = 2.2 + 0.9 + 8.2 = 11.3 g
Mass of reactants = Mass of products = 11.3 g
✅ The Law of Conservation of Mass is valid.
(i) Atomic number: = number of protons = 11. Mass number = protons + neutrons = 11 + 12 = 23.
(ii) Charge: Protons = 11, Electrons = 10 → more protons → it is a cation (positive ion) with charge +1 (written as Na⁺).
(iii) Electronic configuration: 10 electrons → 2, 8
(iv) Name: Atomic number 11 = Sodium. This is the sodium cation, Na⁺.
The Journey Beyond
(i) Which is more reactive?
A has 5 valence electrons (needs 3 more) — it is a non-metal (like phosphorus). B has 7 valence electrons (needs only 1 more) — it is more electronegative and reactive (like chlorine). B is more reactive because it needs only 1 electron to complete its octet.
(ii) Ionic or covalent?
Both A and B are non-metals (both have >4 valence electrons). Non-metals tend to share electrons rather than transfer them. So A and B will form a covalent bond.
(iii) Formula of the compound:
A needs 3 electrons; B needs 1 electron. So 1 atom of A shares with 3 atoms of B: formula = AB₃ (like PCl₃ — phosphorus trichloride).
Answer: (iii) A is true, but R is false.
Explanation: The assertion is correct — CuSO₄ does not conduct in solid state but does in molten state. However, the reason is stated backwards. The correct explanation is: In solid state, ions are fixed in the lattice (cannot move → cannot conduct). In molten state, ions are free to move (→ can conduct electricity). The reason as written swaps solid and molten, making it false.
| Species | Protons | Mass No. | Electrons | Neutrons |
|---|---|---|---|---|
| ²⁷Al (neutral) | 13 | 27 | 13 (neutral) | 27 − 13 = 14 |
| ⁸⁰Br⁻ (gained 1e) | 35 | 80 | 35 + 1 = 36 | 80 − 35 = 45 |
| ²⁰¹Hg²⁺ (lost 2e) | 80 | 201 | 80 − 2 = 78 | 201 − 80 = 121 |
🔬 Science and Society: Nuclear Energy
Atoms can release enormous amounts of energy when their nuclei split (fission) or combine (fusion) to form new elements. This is called atomic or nuclear energy. In nuclear power plants, the heat from nuclear reactions creates steam that spins turbines and generates electricity — a cleaner alternative to burning fossil fuels. Nuclear energy is also used in medicine (cancer treatment with radiation) and space exploration.
In India, Raja Ramanna — often called the Father of the Indian Nuclear Programme — made landmark contributions to developing India's nuclear energy programme and promoting its peaceful use for national development.
Are there any chemical changes that do NOT obey the Law of Conservation of Mass?
In everyday chemistry — no, the law always holds. However, in nuclear reactions (like those in nuclear power plants or the Sun), a tiny amount of mass is actually converted into a huge amount of energy, following Einstein's famous equation E = mc². In this sense, nuclear reactions appear to involve a small "loss" of mass — but this mass converts to energy, so the total mass-energy is still conserved. This is a topic you will explore in higher classes!