Chemical Bonding and Molecular Structure

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Identify a molecule which does not exist. Answer: 1. He ₂ A molecule exists only if its bond order (from molecular orbital theory) is greater than zero. He₂ : 4 electrons, σ1s²σ^*1s²... Molecular orbital theory, bond order, bond length, bond energy 1 answer
A compound is formed by cation C and anion A. The anions form hexagonal close packed (hcp) lattice and the cations occupy 75% of octahedral voids. The formula of the compound is : Answer: 3. C ₃ A ₄ In hcp, if there are N anions then there are N octahedral voids. Cations occupy 75% of octahedral voids, so cations = 0.7... Ionic bonding and lattice enthalpy 1 answer
Which of the following diatomic molecular species has only π bonds according to Molecular Orbital Theory ? Answer: 3. C ₂ Write the MO configuration for each molecule. C₂ (12 electrons): σ_1s²σ_1s^(*2)σ_2s²σ_2s^(*2)π_2p⁴ . The 2p electrons occupy... Molecular orbital theory, bond order, bond length, bond energy 1 answer
Among CaH_2, BeH_2, BaH_2, the order of ionic character is Answer: 1. BeH ₂ < CaH ₂ < BaH ₂ Ionic character of a metal hydride increases with the electropositive (metallic) character of the metal, wh... Ionic bonding and lattice enthalpy 1 answer
Magnesium reacts with an element (X) to form an ionic compound. If the ground state electronic configuration of (X) is 1s^2 2s^2 2p^3, the simplest formula for this compound is Answer: 4. Mg ₃ X ₂ X has configuration 1s²2s²2p³ , so it is nitrogen and forms mathrmX³⁻ . Magnesium forms mathrmMg²⁺ . Balancing charges:... Kossel-Lewis approach, ionic and covalent bonds 1 answer
Iron exhibits bcc structure at room temperature. Above 900°C, it transforms to fcc structure. The ratio of density of iron at room temperature to that at 900°C (assuming molar mass and atomic radii of iron remains constant with temperature) is Answer: 2. 3√(3)/4√(2) Density =Z M/a³ N_A . Since M and atomic radius r are the same, compare Z/a³ . bcc: Z=2 , a=4r/sqrt3 . fcc: Z=4 , a=2... Kossel-Lewis approach, ionic and covalent bonds 1 answer
Consider the following species :CN^+, CN^-, NO and CNWhich one of these will have the highest bond order ? Answer: 3. CN ^- Count electrons and fill the molecular orbitals. CN^- : 14 electrons, bond order 3 (isoelectronic with N₂ , CO). CN: 13 ele... Molecular orbital theory, bond order, bond length, bond energy 1 answer
In the structure of ClF_3, the number of lone pairs of electrons on central atom 'Cl' is Answer: 3. two Count electron pairs on Cl in ClF₃ : Cl has 7 valence electrons, 3 are used for bonds with F, leaving 4 electrons = 2 lone pa... VSEPR theory and shapes of molecules 1 answer
Which is the incorrect statement ? Answer: 3. Frenkel defect is favoured in those ionic compounds in which sizes of cation and anions are almost equal. Key idea: Frenkel defec... Ionic bonding and lattice enthalpy 1 answer
Which one of the following pairs of species have the same bond order ? Answer: 2. CN^-, CO Key idea: compare bond orders from molecular orbital theory (isoelectronic species have equal bond orders). CN^- and CO... Molecular orbital theory, bond order, bond length, bond energy 1 answer
The species, having bond angles of 120° is : Answer: 3. BCl₃ Key idea: 120^° belongs to a trigonal planar molecule with no lone pair on the central atom. BCl₃ : B has 3 bond pairs, sp²... VSEPR theory and shapes of molecules 1 answer
Which of the following pairs of compounds is isoelectronic and isostructural ? Answer: 2. IBr₂^-, XeF₂ Key idea: isoelectronic means equal number of valence electrons, isostructural means same shape. XeF₂ : 8 + 14 = 22... VSEPR theory and shapes of molecules 1 answer
In calcium fluoride, having the fluorite structure, the coordination numbers for calcium ion (Ca^2+) and fluoride ion (F^-) are Answer: 2. 8 and 4 Key idea: in the fluorite structure, Ca²⁺ forms an fcc lattice and F^- occupies all tetrahedral holes. Each F^- is surrou... Ionic bonding and lattice enthalpy 1 answer
Which one of the following compounds shows the presence of intramolecular hydrogen bond? Answer: 2. Cellulose Key idea: the key gives cellulose; the stem is partly unreadable, so the answer follows the key. HCN, concentrated acet... Metallic bonding and hydrogen bonding 1 answer
The hybridizations of atomic orbitals of nitrogen in NO_2^+, NO_3^- and NH_4^+ respectively are Answer: 2. sp , sp² and sp³ Key idea: hybridisation = (bond pairs + lone pairs) on N. NO₂^+ : O=N=O, 2 bond pairs, no lone pair, so sp . NO₃... Valence bond theory, hybridization (s, p, d), resonance 1 answer
Among the following, which one is a wrong statement? Answer: 2. SeF₄ and CH₄ have same shape. Key idea: compare VSEPR shapes. SO₂ has pπ - dπ bonding: correct. SeF₄ is see-saw (4 bond pairs + 1... Valence bond theory, hybridization (s, p, d), resonance 1 answer
Which of the following pairs of ions is isoelectronic and isostructural? Answer: Dropped in the official answer key: full marks to every candidate Key idea: compare total electrons and shape. CO₃²⁻ and NO₃^- : 32... VSEPR theory and shapes of molecules 1 answer
The correct geometry and hybridization for XeF_4 are Answer: 3. square planar, sp³d² Key idea: Xe in XeF₄ has 4 bond pairs and 2 lone pairs. Steric number 6 gives sp³d² hybridisation and an oct... VSEPR theory and shapes of molecules 1 answer
The ionic radii of A^+ and B^- ions are 0.98×10^-10 m and 1.81×10^-10 m. The coordination number of each ion in AB is : Answer: 1. 6 Key idea: the radius ratio decides the coordination number. r_+/r_-=0.98/1.81=0.54 This lies in 0.414-0.732, which means octahe... Ionic bonding and lattice enthalpy 1 answer
Match the compounds given in column I with the hybridisation and shape given in column II and mark the correct option.Column I: (a) XeF_6; (b) XeO_3; (c) XeOF_4; (d) XeF_4Column II: (i) distorted octahedral; (ii) square planar; (iii) pyramidal; (iv)… Answer: 1. (i) (iii) (iv) (ii) Key idea: use VSEPR for each xenon compound. XeF₆ : sp³d³ , 6 bond pairs + 1 lone pair, distorted octahedral... Valence bond theory, hybridization (s, p, d), resonance 1 answer
Predict the correct order among the following : Answer: 1. lone pair - lone pair > lone pair - bond pair > bond pair - bond pair Key idea (VSEPR): a lone pair is held only by one nucleus,... VSEPR theory and shapes of molecules 1 answer
Consider the molecules CH_4, NH_3 and H_2O. Which of the given statements is false ? Answer: 2. The H-O-H bond angle in H₂O is larger than the H-C-H bond angle in CH₄ . Key idea: lone pairs repel more than bond pairs, so bond... VSEPR theory and shapes of molecules 1 answer

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