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p-Block Elements: Part II (Group 15-18) revision notes

Every idea in the p-Block Elements: Part II (Group 15-18) chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 24 and download the PDF.

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Trends and anomalies across groups 15–18

Oxidation states and the inert pair
5× asked, last 2026
15: −3,+3,+516: −2,+2,+4,+617: −1,+1,+3,+5,+7\text{15: }-3,+3,+5\qquad \text{16: }-2,+2,+4,+6\qquad \text{17: }-1,+1,+3,+5,+7
Inert pair: lower down, the state two below the maximum wins. Higher state is more covalent: SbClX5>SbClX3\ce{SbCl5}>\ce{SbCl3}.
-3-2-10+1+2+3+4+5+6+7group 15group 16group 17red ring: max − 2, the state the inert pair favours lower down
Common oxidation states (sheet); lower down the group the state two below the maximum wins
  • Not 'the +5 or +6 state gets more stable lower down'. Only BiFX5\ce{BiF5} is a well-characterised Bi(V) compound. Nor do all +3 states disproportionate: N (+1 to +4) and P do, but As, Sb, Bi(+3) grow more stable.
Why N, O and F are anomalous
13× asked, last 2026
N≡N vs PX4,O=O vs SX8,max covalency of N, O=4\ce{N#N}\ \text{vs}\ \ce{P4},\qquad \ce{O=O}\ \text{vs}\ \ce{S8},\qquad \text{max covalency of N, O}=4
Small, very electronegative, no dd orbitals: covalency ≤ 4 (no NClX5\ce{NCl5}). P, As form dπd\pi–dπd\pi bonds to transition metals.
  • N and O form pπp\pi–pπp\pi multiple bonds, so they are diatomic gases; P and S cannot, so they form single-bonded PX4\ce{P4} and SX8\ce{S8}. That atomicity explains the huge m.p./b.p. gap between OX2\ce{O2} and S.
  • Not 'smaller atom, stronger single bond'. N–N, O–O, F–F are weaker than P–P, S–S, Cl–Cl as lone pairs on the short bond repel. And O is not always −2: it is +2 in OFX2\ce{OF2}, with covalency up to 4.
Hydrides of groups 15–17
14× asked, last 2026
NHX3>PHX3>AsHX3>SbHX3>BiHX3 (stability, basicity, angle 107.8∘→91.3∘)b.p. (K): PHX3 185.5<AsHX3 210.6<NHX3 238.5<SbHX3 254.6<BiHX3 290b.p.: HCl 189<HBr 206<HI 238<HF 293;m.p.: HCl 159<HBr 185<HF 190<HI 222\begin{gathered}\ce{NH3}>\ce{PH3}>\ce{AsH3}>\ce{SbH3}>\ce{BiH3}\ (\text{stability, basicity, angle }107.8^\circ\to91.3^\circ)\\ \text{b.p. (K): }\ce{PH3}\ 185.5<\ce{AsH3}\ 210.6<\ce{NH3}\ 238.5<\ce{SbH3}\ 254.6<\ce{BiH3}\ 290\\ \text{b.p.: }\ce{HCl}\ 189<\ce{HBr}\ 206<\ce{HI}\ 238<\ce{HF}\ 293;\quad \text{m.p.: }\ce{HCl}\ 159<\ce{HBr}\ 185<\ce{HF}\ 190<\ce{HI}\ 222\end{gathered}
One cause: the E–H bond weakens down a group, so acidity (HX2Te\ce{H2Te}, HI top) and reducing power (BiHX3\ce{BiH3} top) rise.
238.5NHX3\ce{NH3}185.5PHX3\ce{PH3}210.6AsHX3\ce{AsH3}254.6SbHX3\ce{SbH3}290BiHX3\ce{BiH3}group 15293HF\ce{HF}189HCl\ce{HCl}206HBr\ce{HBr}238HI\ce{HI}group 17b.p. (K)
b.p.: the H-bonded first hydride (red) jumps above the next one; BiHX3\ce{BiH3} still tops group 15
273HX2O\ce{H2O}188HX2S\ce{H2S}208HX2Se\ce{H2Se}222HX2Te\ce{H2Te}group 16190HF\ce{HF}159HCl\ce{HCl}185HBr\ce{HBr}222HI\ce{HI}group 17m.p. (K)
m.p.: HX2O\ce{H2O} tops group 16, but HI (222) still melts above HF (190)
  • Not 'stronger bond, stronger acid'. A weak H–E bond releases HX+\ce{H+} easily, so HX2Te\ce{H2Te} and HI are the strongest acids and reducers. NHX3\ce{NH3} is most stable, least reducing, most basic (diffuse lone pair below).
  • Not 'lightest hydride boils lowest'. H-bonding lifts NHX3\ce{NH3}, HX2O\ce{H2O}, HF, so PHX3\ce{PH3}, HX2S\ce{H2S}, HCl boil lowest. HI still melts above HF; m.p. (K): HX2S\ce{H2S} 188 < HX2Se\ce{H2Se} 208 < HX2Te\ce{H2Te} 222 < HX2O\ce{H2O} 273.
Oxides: acidity down the group
5× asked, last 2026
NX2OX3, PX4OX6 acidic;AsX2OX3, SbX2OX3 amphoteric;BiX2OX3 basicacidity: EX2OX5>EX2OX3;SOX2 reducing, TeOX2 oxidising\begin{gathered}\ce{N2O3},\ \ce{P4O6}\ \text{acidic};\quad \ce{As2O3},\ \ce{Sb2O3}\ \text{amphoteric};\quad \ce{Bi2O3}\ \text{basic}\\ \text{acidity: }\ce{E2O5}>\ce{E2O3};\qquad \ce{SO2}\ \text{reducing},\ \ce{TeO2}\ \text{oxidising}\end{gathered}
Acidity falls down a group (metallic character rises) and rises with oxidation state. Group 16 EOX2\ce{EO2}, EOX3\ce{EO3} are acidic.
NX2OX3\ce{N2O3}acidicPX4OX6\ce{P4O6}acidicAsX2OX3\ce{As2O3}amphotericSbX2OX3\ce{Sb2O3}amphotericBiX2OX3\ce{Bi2O3}basic
The +3 oxides of group 15: acidity falls as metallic character rises
  • Not 'the higher oxide is less acidic'. More O on the same element pulls more electron density, so NX2OX5\ce{N2O5} beats NX2OX3\ce{N2O3}. SOX2\ce{SO2} (S +4 → +6) reduces, but TeOX2\ce{TeO2} oxidises: the inert pair keeps Te from rising to +6.
Also remember
  • Halides: NFX3\ce{NF3} is the only stable N trihalide; SFX6\ce{SF6} resists hydrolysis (six F shield S), unlike BFX3\ce{BF3}, SiClX4\ce{SiCl4}, PClX5\ce{PCl5}.

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Practise p-Block Elements: Part II (Group 15-18)

20 more concepts in 5 topics

Group 15: dinitrogen, ammonia and nitric acid; Group 15: phosphorus and its compounds; Group 16: ozone, sulphur, SO₂ and H₂SO₄; Group 17: halogens; and Group 18: noble gases. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Group 15: dinitrogen, ammonia and nitric acid

  • Dinitrogen: preparation and inertness7×
  • Ammonia: Haber process2×
  • Nitric acid: Ostwald process and reactions4×
  • Oxides of nitrogen12×

Group 15: phosphorus and its compounds

  • Allotropes of phosphorus3×
  • Phosphine4×
  • PCl₃ and PCl₅6×
  • Oxoacids of phosphorus11×

Group 16: ozone, sulphur, SO₂ and H₂SO₄

  • Ozone1×
  • Allotropes of sulphur5×
  • Sulphur dioxide3×
  • Sulphuric acid: contact process and action1×
  • Oxoacids of sulphur4×

Group 17: halogens

  • Halogen trends and anomalies3×
  • Halogens as oxidants; Cl₂ with alkali6×
  • Hydrogen chloride and HX acidity2×
  • Oxoacids of halogens4×
  • Interhalogen compounds3×
  • Also remember3

Group 18: noble gases

  • Occurrence, properties and uses2×
  • Xenon fluorides and oxides3×

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