Periodic Properties, s- and p-Block Elements — Practice

Question 1

The golden-yellow flame colour is characteristic of:
(a) sodium (b) potassium (c) lithium (d) calcium

Accepted Answer

(a) sodium.

Sodium gives the golden-yellow flame (the 589 nm D-line of sodium street lamps). Potassium is lilac, lithium crimson and calcium brick-red.

Question 2

On burning in excess air, lithium forms mainly:
(a) Li₂O (normal oxide) (b) Li₂O₂ (peroxide) (c) LiO₂ (superoxide) (d) LiH

Accepted Answer

(a) Li₂O (normal oxide).

Small Li forms the normal oxide, unlike sodium (peroxide Na₂O₂) and potassium (superoxide KO₂). This is part of lithium's anomalous behaviour.

Question 3

Lithium shows a diagonal relationship with:
(a) magnesium (b) sodium (c) beryllium (d) aluminium

Accepted Answer

(a) magnesium.

Li resembles Mg (both form normal oxides and nitrides, and covalent hydrated compounds) because increasing charge across the period balances increasing size down it. Be resembles Al by the same diagonal rule.

Question 4

Which oxide is amphoteric?
(a) BeO (b) MgO (c) CaO (d) Na₂O

Accepted Answer

(a) BeO.

Beryllium's small size and high charge density make BeO covalent and amphoteric (reacting with both acids and bases). MgO, CaO and Na₂O are basic.

Question 5

The correct order of Lewis acid strength of boron halides is: (a) BF₃ < BCl₃ < BBr₃ (b) BF₃ > BCl₃ > BBr₃ (c) BCl₃ < BF₃ < BBr₃ (d) all equal

Accepted Answer

(a) BF₃ < BCl₃ < BBr₃.

Back-bonding from the halogen lone pair into boron's empty p orbital is strongest in BF₃, partly satisfying boron and making it the weakest Lewis acid; it is weakest in BBr₃, the strongest Lewis acid.

Question 6

The bridging bonds in diborane (B₂H₆) are:
(a) 3-centre-2-electron bonds (b) normal covalent bonds (c) ionic bonds (d) hydrogen bonds

Accepted Answer

(a) 3-centre-2-electron bonds.

Diborane is electron-deficient; each B–H–B bridge is a banana-shaped 3-centre-2-electron bond, and there is no direct B–B bond.

Question 7

Which is the more stable oxidation state of lead?
(a) +2 (b) +4 (c) both equally (d) +6

Accepted Answer

(a) +2.

Due to the inert-pair effect, the +2 state (Pb²⁺) is more stable than +4 for lead. PbO₂ (Pb⁴⁺) is therefore a strong oxidising agent.

Question 8

CO₂ is a gas while SiO₂ is a high-melting solid because:
(a) carbon forms pπ–pπ double bonds giving discrete molecules (b) SiO₂ is ionic (c) CO₂ is a network solid (d) silicon is more electronegative

Accepted Answer

(a) carbon forms pπ–pπ double bonds giving discrete molecules.

Carbon makes strong O=C=O double bonds, giving small molecules held by weak forces (gas). Silicon is too large for pπ–pπ bonding, so SiO₂ is a giant covalent network of Si–O single bonds (solid).

Question 9

Nitrogen does not form NCl₅ but phosphorus forms PCl₅ because:
(a) nitrogen lacks vacant d orbitals (b) nitrogen is less electronegative (c) chlorine is too large (d) phosphorus is a gas

Accepted Answer

(a) nitrogen lacks vacant d orbitals.

Being a period-2 element, nitrogen cannot expand its octet beyond four bonds. Phosphorus (period 3) has accessible 3d orbitals, enabling sp³d hybridisation and five bonds in PCl₅.

Question 10

Which allotrope of phosphorus is stored under water because it is highly reactive and catches fire in air?
(a) white phosphorus (b) red phosphorus (c) black phosphorus (d) violet phosphorus

Accepted Answer

(a) white phosphorus.

White phosphorus (P₄) is poisonous and so reactive it ignites in air, so it is stored under water. Red and black phosphorus are far more stable.

Question 11

The strongest oxidising agent among the halogens is:
(a) F₂ (b) Cl₂ (c) Br₂ (d) I₂

Accepted Answer

(a) F₂.

Oxidising power decreases down group 17 (F₂ > Cl₂ > Br₂ > I₂), so fluorine is the strongest and can displace all the other halide ions from solution.

Question 12

The correct order of acid strength is: (a) HF < HCl < HBr < HI (b) HI < HBr < HCl < HF (c) HF > HCl > HBr > HI (d) all equal

Accepted Answer

(a) HF < HCl < HBr < HI.

Down the group the H–X bond lengthens and weakens, so the proton is released more easily and acidity increases. HF is the weakest acid (strong bond and hydrogen bonding), HI the strongest.

Question 13

Which hydride is the most acidic?
(a) H₂Te (b) H₂O (c) H₂S (d) H₂Se

Accepted Answer

(a) H₂Te.

Acidity increases down group 16 (H₂O < H₂S < H₂Se < H₂Te) as the H–X bond weakens, so H₂Te is the most acidic despite tellurium's low electronegativity.

Question 14

The shape of XeF₂ is:
(a) linear (b) bent (c) trigonal planar (d) tetrahedral

Accepted Answer

(a) linear.

Xe in XeF₂ has 2 bond pairs and 3 lone pairs (trigonal bipyramidal electron geometry); the three lone pairs occupy the equatorial positions, leaving the two F atoms axial → linear shape.

Question 15

Which noble gas most readily forms chemical compounds?
(a) xenon (b) helium (c) neon (d) argon

Accepted Answer

(a) xenon.

Xenon is large with a relatively low ionisation enthalpy, so it forms real compounds with the most electronegative elements — XeF₂, XeF₄, XeF₆ and XeO₃. The lighter noble gases remain effectively inert.

Practice — Periodic Properties, s- and p-Block Elements

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