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Periodic trends and bonding models
5 ECAT Chemistry questions with answers and explanations.
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Which ionic compound has the highest lattice energy?
- 1 NaCl
- 2 KBr
- 3 MgO
- 4 NaBr
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Correct answer: MgO
Lattice energy is greatest for ions with high charges and small radii. MgO has 2+ and 2- ions.
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What is the bond order of the O2 molecule according to molecular orbital theory?
- 1 1
- 2 2
- 3 3
- 4 2.5
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Correct answer: 2
O2 has 8 bonding and 4 antibonding electrons in the valence orbitals, so bond order = (8 - 4) / 2 = 2.
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What is the shape of the ammonium ion, NH4+?
- 1 Trigonal pyramidal
- 2 Square planar
- 3 Tetrahedral
- 4 Linear
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Correct answer: Tetrahedral
Nitrogen has four bond pairs and no lone pairs, giving a tetrahedral shape.
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Which of these molecules has zero dipole moment?
- 1 CHCl3
- 2 CCl4
- 3 NH3
- 4 H2O
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Correct answer: CCl4
In CCl4 the four polar bonds are arranged symmetrically in a tetrahedron, so the dipoles cancel.
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How many lone pairs of electrons are on the oxygen atom in a water molecule?
- 1 1
- 2 2
- 3 3
- 4 4
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Correct answer: 2
Oxygen has six valence electrons. Two are used in bonds and the remaining four form two lone pairs.