Q.(a) Why bond angle of Phosphine (PH3) is less than Ammonia (NH3) ?

Oxygen normally shows a covalency of 2 (as in H2O), but because it has only 2s and 2p orbitals (no d-orbitals) it can use its lone pairs to form coordinate bonds too, giving a maximum covalency of 4 — a value not offered by options a–c.

Oxygen's ground-state configuration is $1s^2 2s^2 2p^4$, with 2 unpaired electrons, so it normally forms 2 covalent bonds (covalency 2, e.g. H2O). Like nitrogen, oxygen has no d-orbitals in its valence shell, so it cannot expand its octet beyond 4 total electron pairs (bonding + coordinate). By donating its lone pairs as coordinate bonds (e.g. in oxonium-type/complex species such as $[Be(OH_2)_4]^{2+}$), …

Neon is the noble gas used in botanical gardens and greenhouses to protect plants from the adverse effects of frost/severe cold.

Noble gases (Group 18) are chemically inert but have several practical uses that exploit their physical properties rather than their (near-absent) reactivity. Neon, filled into discharge tubes, produces a distinctive glow when electrically excited and gives off gentle warmth along with light; neon-filled tube arrangements are used in greenhouses and botanical gardens to keep the ambient temperature above freezing on c …

The noble gases occupy Group 18 of the modern periodic table.

The elements He, Ne, Ar, Kr, Xe and Rn form Group 18 of the modern periodic table (they were called 'Group 0' in the older, pre-IUPAC numbering). Except for helium (which has a $1s^2$ configuration), all members have a completely filled outermost shell with the general valence-shell configuration $ns^2np^6$. This closed-shell, most stable electronic configuratio …

Nitrogen is the one Group 15 element that does not show allotropy; the heavier members (P, As, Sb, Bi) all exist in multiple structural forms.

Among the Group 15 (nitrogen family) elements:

• Nitrogen exists only as the stable diatomic molecule N≡N (triple bond), and does not form other structural forms under ordinary conditions — it does not show allotropy.
• Phosphorus shows allotropy: white, red, and black phosphorus.
• Arsenic and Antimony also exist in yellow and grey (metallic) allotropic forms.
• Bismuth likewise has more than one crystalline form. …