In this post, we will discuss about the important orders in inorganic chemistry and then some exceptions with valid reasons in the last.
Important Orders in Inorganic Chemistry
Isotopes of Hydrogen
Three isotopes of hydrogen are Hydrogen (H), Deuterium (D) and Tritium (T).
| Relative Abundance | H > D > T |
| Atomic mass | H < D < T |
| Melting point | H < D < T |
| Boiling point | H < D < T |
| Density | H < D < T |
| Enthalpy of fusion | H < D |
| Enthalpy of vaporisation | H < D |
| Bond dissociation enthalpy | H < D |
| Internuclear distance | H = D |
Oxides of Isotopes of Hydrogen
| Melting point | H2O2 < H2O < D2O |
| Boiling point | H2O < D2O < H2O2 |
| Enthalpy of formation | H2O < D2O |
| Enthalpy of vaporisation | H2O < D2O |
| Temperature of maximum density | 4 °C (H2O) and 11 °C (D2O) |
| Density | H2O < D2O < H2O2 |
| Viscosity | H2O < D2O < H2O2 |
| Dielectric constant | H2O2 < D2O < H2O |
| Electrical conductivity | H2O2 < H2O |
Energy Released by Combustion of Fuels
| Per mole | H2(l) < H2(g) < CH4 < LPG < Octane |
| Per gram | Octane < LPG < CH4 < H2(l) < H2(g) |
| Per liter | H2(g) < CH4 < H2(l) < LPG < Octane |
Group 1 Elements
| Ionisation enthalpy | Li > Na > K > Rb > Cs |
| Hydration enthalpy | Li > Na > K > Rb > Cs |
| Metallic radius | Li < Na < K < Rb < Cs |
| Ionic radius | Li < Na < K < Rb < Cs |
| Melting point | Li > Na > K > Rb > Cs |
| Boiling point | Li > Na > K > Rb > Cs |
| Density | Li < K < Na < Rb < Cs |
| Standard reduction potential | Na > K < Cs > Rb > Li |
| Occurence in lithosphere | Na > K > Rb > Li > Cs |
Group 2 Elements
| Ionisation enthalpy | Be > Mg > Ca > Sr > Ra > Ba [IE(II)>IE(I)] |
| Hydration enthalpy | Be > Mg > Ca > Sr > Ba |
| Metallic radius | Be < Mg < Ca < Sr < Ba |
| Ionic radius | Be < Mg < Ca < Sr < Ba |
| Melting point | Mg < Ra < Ba < Sr < Ca < Be |
| Boiling point | Mg < Sr < Ca < Ba < Be |
| Density | Ca < Mg < Be < Sr < Ba < Ra |
| Standard reduction potential | Be > Mg > Ca > Sr > Ba = Ra |
| Occurence in lithosphere | Ca > Mg > Ba > Sr > Be |
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Group 13 Elements
| Atomic radius | B < Ga < Al < In < Tl |
| Ionic radius | B < Al < Ga < In < Tl |
| Ionisation enthalpy | IE1: In < Al < Ga < Tl < B IE2: Al < In < Tl < Ga < B IE3: In < Al < Tl < Ga < B |
| Electronegativity | Al < Ga < In < Tl < B |
| Density | B < Al < Ga < In < Tl |
| Melting point | Ga < In < Tl < Al < B |
| Boiling point | Tl < In < Ga < Al < B |
Group 14 Elements
| Atomic radius | C < Si < Ge < Sn < Pb |
| Ionic radius | Si < Ge < Sn < Pb |
| Ionisation enthalpy | IE1: C > Si > Ge > Pb > Sn IE2: C > Si > Ge > Pb > Sn IE3: C > Ge > Si > Pb > Sn IE4: C > Ge > Si > Pb > Sn |
| Electronegativity | C > Si = Ge = Sn < Pb (C > Pb) |
| Density | Graphite < Si < Diamond < Ge < Beta form of Sn < Pb |
| Melting point | C > Si > Ge > Pb > Sn |
| Boiling point | C > Ge > Sn > Pb |
| Standard reduction potential | C > Si = Ge > Pb > Sn |
Group 15 Elements
| Atomic radius | N < P < As < Sb < Bi |
| Ionic radius | Sb < Bi < N < P < As |
| Ionisation enthalpy | IE1: N > P > As > Sb > Bi IE2: N > P > As > Bi > Sb IE3: N > P > As > Bi > Sb |
| Electronegativity | N > P > As > Sb = Bi |
| Melting point | N < P < Bi < Sb < As |
| Boiling point | N < P < As < Bi < Sb |
| Density | N < P < As < Sb < Bi |
Hydrides of Group 15 Elements
| Melting point | PH3 < AsH3 < SbH3 < NH3 |
| Boiling point | PH3 < AsH3 < NH3 < SbH3 < BiH3 |
| Bond length | NH3 < PH3 < AsH3 < SbH3 |
| Bond angle | NH3 > PH3 > AsH3 > SbH3 |
| Enthalpy of formation | PH3 < NH3 < AsH3 < SbH3 < BiH3 |
| Enthalpy of dissociation | NH3 > PH3 > AsH3 > SbH3 |
Group 16 Elements
| Atomic radius | O < S < Se < Te < Po |
| Ionic radius | O < S < Se < Te < Po |
| Electron gain enthalpy | S > Se > Te > Po > O |
| Ionisation enthalpy | O > S > Se > Te > Po |
| Electronegativity | O > S > Se > Te > Po |
| Density | O < S < Se < Te |
| Melting point | O < S < Se < Te < Po |
| Boiling point | O < S < Se < Te < Po |
Hydrides of Group 16 Elements
| Melting point | H2S < H2Se < H2Te < H2O |
| Boiling point | H2S < H2Se < H2Te < H2O |
| Bond length | H2O < H2S < H2Se < H2Te |
| Bond angle | H2O > H2S > H2Se > H2Te |
| Enthalpy of formation | H2S < H2Se < H2Te < H2O |
| Enthalpy of dissociation | H2O > H2S > H2Se > H2Te |
| Dissociation constant | H2O < H2S < H2Se < H2Te |
Group 17 Elements
| Atomic radius | F < Cl < Br < I |
| Ionic radius | F < Cl <Br < I |
| Ionisation enthalpy | F > Cl > Br > I |
| Electron gain enthalpy | Cl > F > Br > I |
| Electronegativity | F > Cl > Br > I |
| Enthalpy of hydration | F > Cl > Br > I |
| Melting point | F2 < Cl2 < Br2 < I2 |
| Boiling point | F2 < Cl2 < Br2 < I2 |
| Density | F2 < Cl2 < Br2 < I2 |
| Bond length | F2 < Cl2 < Br2 < I2 |
| Bond dissociation enthalpy | I2 < F2 < Br2 < Cl2 |
| Standard reduction potential | F2 > Cl2 > Br2 > I2 |
Hydrogen Halides
| Melting point | HCl < HBr < HF < HI |
| Boiling point | HCl < HBr < HI < HF |
| Bond length | HF < HCl < HBr < HI |
| pKa value | HF > HCl > HBr > HI |
| Dissociation enthalpy | HF > HCl > HBr > HI |
Group 18 Elements
| Ionisation enthalpy | He > Ne > Ar > Kr > Xe > Rn |
| Electron gain enthalpy | He < Rn < Xe < Ar = Kr < Ne |
| Melting point | He < Ne < Ar < Kr < Xe < Rn |
| Boiling point | He < Ne < Ar < Kr < Xe < Rn |
| Density | He < Ne < Ar < Kr < Xe < Rn |
Most wanted inorganic chemistry
Exceptions in Inorganic Chemistry
Atomic Radius
Expected order: B < Al < Ga < In < Tl
Actual order: B < Ga < Al < In < Tl
Reason: Gallium has smaller size because of poor shielding of d-orbital.
Ionisation enthalpy
Expected order: B > Al > Ga > In > Tl
Actual order: B > Tl > Ga > Al > In
Reason: On moving down the group, the ionisation energy decreases, this is true for B and Al. The ionisation energy of Ga is unexpectedly higher than that of Al. This is due to poor shielding effect by completely filled d-orbitals in the inner shell of Ga, Hence, valence electrons of Ga are strongly held by nucleus, therefore the removal of electron from Ga is highly difficult compared to Al. A similar increase is observed from In to Tl due to presence of f-orbital electrons in the inner shell of Tl which have poor shielding effect.
Lewis Acid Strength
Expected order: BF3 > BCl3 > BBr3 > BI3
Actual order: BF3 < BCl3 < BBr3 < BI3
Reason: Back bonding
Acidic Strength
Acidic strength increases with increase in oxidation number. Examples are as follows:
HClO4 > HClO3 > HClO2 > HClO
H2SO4 > H2SO3
HNO3 > HNO2
But in case of oxoacids of phosphorus, it is the exceptional case, based on stability of conjugate base.
Expected order: H3PO4 > H3PO3 > H3PO2
Actual order: H3PO2 > H3PO3 > H3PO4
Reason: This can be explained on the basis of stability of conjugate base. More stable the conjugate base, more will the acidic strength. And cross conjugation leads to unstability of conjugate base.
Hybridisation
1. PCl5, when exists in gaseous state, exhibits sp3d hybridisation. But in solid state, it exists as [PCl4]+ and [PCl6]–. Here [PCl4]+ exhibits sp3 hybridisation while [PCl6]– exhibits sp3d2 hybridisation.
2. PBr5, when exists in gaseous state, exhibits sp3d hybridisation. But in solid state, it exists as [PBr4]+ and Br–. Here [PBr4]+ exhibits sp3 hybridisation.
Existence of compounds
| Compound | Existence | Compound | Existence |
|---|---|---|---|
| IF7 | Exist | IH7 | Not exist |
| PCl5 | Exist | PH5 | Not exist |
| SF4 | Exist | SH4 | Not exist |
| SF6 | Exist | SH6 | Not exist |
| XeF2 | Exist | XeH2 | Not exist |
| XeF4 | Exist | XeH4 | Not exist |
Electron affinity
Expected order: F > Cl > Br > I
Actual order: Cl > F > Br > I
Reason: Due to interelectronic repulsion.
Similarly the correct order for group 16 is S > Se > Te > Po > O
Bond Dissociation Enthalpy
Expected order: F2 > Cl2 > Br2 > I2
Actual order: Cl2 > Br2 > F2 > I2
Reason: Due to interelectronic repulsion between F-F, there is less amount of energy to break the bond.
Boiling point
Expected order: HF < HCl < HBr < HI
Actual order: HCl < HBr < HI < HF
Reason: Due to H-bonding in HF, it has highest boiling point.
Expected order: PH3 < AsH3 < SbH3 < BiH3 < NH3 (based on H-bonding)
Actual order: PH3 < AsH3 < NH3 < SbH3 < BiH3
Reason: Sb and Bi are heavier elements, so their boiling point are higher.
Expected order: H2O < H2S < H2Se < H2Te
Actual order: H2S < H2Se < H2Te < H2O
Reason: Hydrogen bonding
Alkali Metals
Alkali metals,. except Li, cannot react with nitrogen gas at room temperature.
Lithium easily reacts with nitrogen at room temperature to form Li3N.
Ionization enthalpy
Expected order: Li < Be < B < C < N < O < F < Ne
Actual order: Li < B < Be < C < O < N < F < Ne
Reason: B < Be (Penetration effect) and O < N (Half filled stability)
Bond Angle
Order: OCl2 (111o) > OH2 (104.5o) > OF2(103o)
Brown Ring Test
Complex formed in the brown ring test is [Fe(H2O)5NO]SO4. The oxidation state of NO is +1, so the oxidation state of Fe is +1.
Ionisation enthalpy of 3d-series
Sc < Ti > V < Cr < Mn < Fe > Co < Ni < Cu < Zn
Note: If there is any incorrect order, do comment the correct order. If any other exceptional case or important order you know, do comment down, that will be added into the post. Thanks.
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