Q.Second period of periodic table contains elements :

Forming a pentahalide requires expanding the central atom's octet using d-orbitals; phosphorus (n=3) has vacant 3d orbitals available for this, while nitrogen (n=2) has no d-orbitals at all in its valence shell, so it is structurally incapable of forming $NCl_5$.

Octet expansion and $d$-orbital availability

To form $PCl_5$, the central phosphorus atom must bond to 5 chlorine atoms, requiring 5 hybrid orbitals ($sp^3d$, trigonal bipyramidal geometry) — i.e. an expanded octet of 10 electrons around P. This is possible because phosphorus is in period 3 and its valence shell ($n=3$) includes empty $3d$ orbitals that are energetically accessible enough to participate in hybridisation, accommodating the extra electron pairs.

Why nitrogen cannot do the same

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White phosphorus is molecular; in the vapour state it exists as P₄ molecules, with the four phosphorus atoms arranged at the corners of a regular tetrahedron.

White (yellow) phosphorus is the most reactive allotrope of phosphorus and consists of discrete P₄ tetrahedral units, both in the solid and in the vapour state. Each P atom is bonded to three other P atoms by single P–P bonds, with a P–P–P bond angle of about 60° (strained), which is one rea …

Second period = 8 elements (Li → Ne).

The second period corresponds to filling the n = 2 shell (2s and 2p orbitals), which hold a maximum of 8 electrons. The elements are Li, Be, B, C, N, O, F, Ne — a total o …

Group 1 elements are the alkali metals.

The elements of Group 1 (Li, Na, K, Rb, Cs, Fr) are highly reactive metals whose hydroxides are strong bases (alkalis) that dissolve readily in water — hence 'alkali metals'. Group 2 elements a …

Only the symmetric trigonal-planar BF₃ has μ = 0.

• BF₃: sp², trigonal planar, F at 120°; the three equal B→F dipoles cancel → μ = 0.
• NF₃: sp³ pyramidal with a lone pair → μ ≠ 0. …

Inert-pair effect makes Pb²⁺ the most stable +2 state.

For heavy p-block elements the ns² pair becomes reluctant to bond (inert-pair effect), stabilising the lower (+2) oxidation state. In group 14 this is strongest for lead, so Pb²⁺ is very stable while Pb⁴⁺ is oxidising. …

N2O is called laughing gas because inhaling it produces a mild euphoric/intoxicating sensation, and it is used medically as a weak anaesthetic (e.g., in dentistry).

Laughing gas is the common name for nitrous oxide, $\text{N}_2\text{O}$, an oxide of nitrogen in which nitrogen is in the +1 oxidation state.

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