Model Test – Solved ഉത്തരങ്ങൾ

Differences: C–S 2.58 − 2.55 = 0.03; N–H 3.04 − 2.20 = 0.84; Li–Cl 3.16 − 0.98 = 2.18; P–Cl 3.16 − 2.19 = 0.97. Only Li–Cl is 1.7 or more → ionic (അയോണിക ബന്ധനം). Lithium is a metal and chlorine a non-metal.

Gaining electrons gives more electrons than protons → a negative ion, an anion. Atoms that lose electrons become positive ions, cations.

Na⁺ (11 − 1 = 10 electrons), Mg²⁺ (12 − 2 = 10) and F⁻ (9 + 1 = 10) all have 2, 8. K⁺ has 19 − 1 = 18 electrons: 2, 8, 8 – the configuration of argon.

Hydrogen and sulphur are both non-metals; the difference is 2.58 − 2.20 = 0.38, less than 1.7 → covalent bond. (MgO: 3.44 − 1.31 = 2.13 → ionic.)

Sulphur is 2, 8, 6. It needs 8 − 6 = 2 more electrons for an octet, so its valency (സംയോജകത) is 2.

KCl (3.16 − 0.82 = 2.34), CaO (3.44 − 1.00 = 2.44) and LiF (3.98 − 0.98 = 3.00) are ionic – a metal with a non-metal, difference ≥ 1.7. PCl₃ is made of two non-metals: 3.16 − 2.19 = 0.97 < 1.7 → covalent.

Ions: Ca²⁺ and NO₃⁻. Write Ca NO₃, swap the charge numbers → Ca₁(NO₃)₂. Omit the 1 and keep the brackets because the radical is taken twice: Ca(NO₃)₂. Check: (+2) + 2 × (−1) = 0.

Covalent compounds are generally insoluble in water but soluble in organic solvents (kerosene, carbon tetrachloride, benzene); they have low melting points and do not conduct. The other three are properties of ionic compounds.

(a) Potassium (2, 8, 8, 1) has 1 outermost electron and gives it away: K → K⁺ + 1e⁻. Sulphur (2, 8, 6) needs 2 electrons: S + 2e⁻ → S²⁻.

(b) K⁺: 19 − 1 = 18 electrons → 2, 8, 8; S²⁻: 16 + 2 = 18 electrons → 2, 8, 8. Both have the octet of argon (അഷ്ടകവിന്യാസം). The protons do not change: 19 and 16.

Sulphur (2, 8, 6) has 6 outermost electrons (×) and needs 2; each hydrogen has 1 (•) and needs 1. S shares one pair with each H.

Sulphur now has 8 electrons around it (2 shared pairs + 2 lone pairs) and each hydrogen 2 (duplet). Two single bonds: H–S–H.

(a) Li–O: 3.44 − 0.98 = 2.46. 2.46 ≥ 1.7 → ionic.

(b) S–O: 3.44 − 2.58 = 0.86. 0.86 < 1.7 → covalent (a polar covalent bond; oxygen carries δ−).

(a) Al: 2, 8, 3 → gives 3 electrons → valency 3. S: 2, 8, 6 → takes 2 → valency 2.

(b) Write Al S (less electronegative first), swap the valencies as subscripts: Al₂S₃. 2 and 3 have no common factor → Al₂S₃ (aluminium sulphide).

(a) An ionic crystal (ക്രിസ്റ്റലുകൾ) is held together by the strong electrostatic force of attraction between oppositely charged ions (Na⁺ and Cl⁻) on all sides. A lot of energy is needed to pull them apart, so the melting point (ഉരുകൽനില) is very high.

(b) Covalent compounds generally have low melting and boiling points: the forces between their molecules are weak, so naphthalene slowly turns into vapour at room temperature – it is volatile. (Ionic compounds are non-volatile.)

(a) Ca: 2, 8, 8, 2; Cl: 2, 8, 7.

(b) Calcium gives its 2 outermost electrons (•), one to each of two chlorine atoms (7 × each):

Ca²⁺ (2, 8, 8) and two Cl⁻ (2, 8, 8) → CaCl₂, ionic (3.16 − 1.00 = 2.16).

(c) Calcium ion: 20 − 2 = 18 electrons, 20 protons (unchanged) → charge 20 − 18 = +2.

(a) C (2, 4) has 4 outermost electrons (•) and needs 4; each O (2, 6) has 6 (×) and needs 2.

(b) Carbon shares 2 pairs with each oxygen → two double bonds (ദ്വിബന്ധനം): O=C=O (4 shared pairs in all). Every atom now has 8.

(c) The molecule is straight (linear, ജ്യാമിതീയ ആകൃതി): the two oxygens pull the shared electrons equally in opposite directions, so the pulls cancel and the molecule as a whole is non-polar.

Positive ion first, swap the charge numbers, simplify by the common factor, omit 1, and put a radical in brackets when it is taken more than once. Check: the total charge is zero – e.g. Al₂(SO₄)₃: 2 × (+3) + 3 × (−2) = 0; MgCO₃: Mg₂(CO₃)₂ ÷ 2.

(a) Polar (പോളാർ തന്മാത്രകൾ): H₂O, NH₃, HCl. Cl₂ has two identical atoms (equal pull); CO₂ has polar bonds but its shape cancels them.

(b) 3.04 − 2.20 = 0.84 → polar covalent. Nitrogen attracts the shared pair more: δ+H–Nδ−.

(c) Hydrogen bonding (ഹൈഡ്രജൻ ബന്ധനം): when hydrogen is covalently bonded to a highly electronegative atom (F, O or N), it gets a δ+ charge and is attracted by an electronegative atom of the same or another molecule. It is an intermolecular force (അന്തർതന്മാത്രാബലങ്ങൾ).

(a) A 2, 1; D 2, 5; E 2, 7; G 2, 8, 8; J 2, 8, 8, 2. G already has 8 electrons in its outermost shell (octet) – it is stable, a noble gas, and does not usually react.

(b) J (2, 8, 8, 2) loses its 2 outermost electrons: J → J²⁺ + 2e⁻ (J²⁺ 2, 8, 8).

(c) Valency of J = 2 (gives 2); D (2, 5) takes 3 → valency 3. Criss-cross: J₃D₂.

(d) A gives its one electron to E: A⁺ (2, like helium) and E⁻ (2, 8). Difference 3.98 − 0.98 = 3.00 ≥ 1.7 → ionic bond; formula AE.

(e) 3.98 − 3.04 = 0.94 < 1.7 → covalent (polar). E, the more electronegative atom, carries δ−; D carries δ+.

Real elements: A Li, D N, E F, G Ar, J Ca – J₃D₂ is calcium nitride, AE lithium fluoride, DE₃ nitrogen trifluoride.

(a) Salt is ionic. In the solution its ions (Na⁺, Cl⁻, K⁺) are free to move and carry the current. In the dry crystal the ions are held fixed in place – ionic compounds conduct only when molten or dissolved.

(b) K → K⁺ + 1e⁻ (2, 8, 8, 1 → 2, 8, 8); Cl + 1e⁻ → Cl⁻ (2, 8, 7 → 2, 8, 8); K⁺ + Cl⁻ → KCl.

(c) K–Cl: 3.16 − 0.82 = 2.34 ≥ 1.7 → ionic. C–H: 2.55 − 2.20 = 0.35 < 1.7 → covalent. Glucose is a covalent compound; it dissolves as whole molecules and forms no ions, so there is nothing to carry the current.

(d) Ionic compounds are generally soluble in polar solvents (പോളാർ ലായകങ്ങൾ) like water, not in oils. Grease is a covalent substance; covalent compounds dissolve in organic solvents such as kerosene (മണ്ണെണ്ണ) but not in water.

(e) Water is a polar molecule: O (3.44) pulls the shared pairs from H (2.20), so O is δ− and each H is δ+. The δ− oxygen attracts Na⁺ and the δ+ hydrogens attract Cl⁻, pulling the ions out of the crystal. (Hydrogen bonding between water molecules also explains its other special properties.)