IMO Practice Test — Chemical Bonding and Molecular Structure
14 Questions • 15 min • Olympiad level
15:00
Question 1 of 14
Which set of central-atom electron pairs gives a bent molecule?
2 bond pairs and 2 lone pairs
4 bond pairs
3 bond pairs
6 bond pairs
Explanation: 2 bond pairs + 2 lone pairs (as in H2O) give a bent shape.
Question 2 of 14
The decreasing order of bond angle CH4, NH3, H2O is due to:
increasing molar mass
increasing number of lone pairs
decreasing electronegativity
increasing bond order
Explanation: More lone pairs increase repulsion and compress the bond angle.
Question 3 of 14
If a diatomic species has Nb = 8 and Na = 4, its bond order is:
1
3
4
2
Explanation: Bond order = (8 - 4)/2 = 2.
Question 4 of 14
Which of the following is paramagnetic?
O2
N2
F2
H2
Explanation: O2 has two unpaired electrons in pi* orbitals, so it is paramagnetic.
Question 5 of 14
The species with the highest bond order among the following is:
O2
N2
H2
He2
Explanation: N2 has bond order 3, higher than O2 (2), H2 (1) and He2 (0).
Question 6 of 14
Removing one electron from N2 to form N2+ changes the bond order to:
3
2.5
2
1.5
Explanation: Losing a bonding electron gives bond order = (9 - 4)/2 = 2.5.
Question 7 of 14
Which molecule has the largest dipole moment?
CO2
CCl4
BF3
H2O
Explanation: Bent H2O has non-cancelling dipoles; the other three are symmetric (mu = 0).
Question 8 of 14
The hybridisation and geometry of SF6 are:
sp3d2, octahedral
sp3, tetrahedral
sp3d, trigonal bipyramidal
sp2, trigonal planar
Explanation: Six bond pairs need sp3d2 hybridisation, giving octahedral geometry.
Question 9 of 14
A bond order of zero implies that:
the molecule is very stable
the molecule does not exist
the molecule is paramagnetic
the bond is ionic
Explanation: Zero bond order (e.g. He2) means there is no net bond, so the molecule is not stable.
Question 10 of 14
Which compound shows the maximum covalent character (Fajans' rules)?
NaCl
MgCl2
AlCl3
KCl
Explanation: The small, highly charged Al3+ has the greatest polarising power, giving AlCl3 the most covalent character.
Question 11 of 14
Abnormally high boiling point of water compared with H2S is due to:
greater molar mass
intermolecular hydrogen bonding
ionic bonding
higher bond order
Explanation: Strong O-H...O hydrogen bonds in water raise its boiling point well above H2S.
Question 12 of 14
A triple bond consists of:
3 sigma bonds
2 sigma and 1 pi bond
3 pi bonds
1 sigma and 2 pi bonds
Explanation: A triple bond is one sigma bond plus two pi bonds.
Question 13 of 14
Among C-C, C=C and C≡C, the shortest bond is:
C-C
C=C
C≡C
all equal
Explanation: Higher bond order (triple) shortens the bond, so C-triple-bond-C is shortest.
Question 14 of 14
The number of unpaired electrons in O2 according to MO theory is:
2
0
1
3
Explanation: Two electrons singly occupy the degenerate pi* orbitals, giving 2 unpaired electrons.