Questions on shapes of molecules and ions

Question 1

Explain how electron-pair repulsion theory accounts for the bond angles in CH₄, NH₃ and H₂O.

Accepted Answer

Indicative content

each central atom has four pairs of electrons, which repel and get as far apart as possible in a tetrahedral arrangement
methane has four bonding pairs, all repelling equally, so its angle is 109.5°
lone pairs repel more strongly than bonding pairs, because they are held by one nucleus and sit closer to it
ammonia has three bonding pairs and one lone pair, so its bonding pairs are pushed together to 107°
water has two bonding pairs and two lone pairs, so its angle is closed further to 104.5°
the shapes are tetrahedral, pyramidal and bent, named from the positions of the atoms

Levels

Level 3 (5–6 marks) all three molecules are explained from four electron pairs in a tetrahedral arrangement, the greater repulsion of lone pairs is stated with the reason, and the three angles and three shape names are correct
Level 2 (3–4 marks) two or three angles are correct and lone pairs are said to repel more; the reason lone pairs repel more, the tetrahedral starting point, or a shape name is absent
Level 1 (1–2 marks) one angle or shape is right; the usual error is to call NH₃ or H₂O tetrahedral, or to say the hydrogen atoms repel each other

Question 2

Explain the shape of an ammonia molecule and why its bond angle is 107°.

Accepted Answer

B1 nitrogen has three bonding pairs and one lone pair B1 the four pairs repel and get as far apart as possible, in a roughly tetrahedral arrangement B1 the lone pair repels more strongly than the bonding pairs, pushing them closer together than 109.5° B1 the shape of the atoms is pyramidal with an angle of 107°

Question 3

Explain the shape of a PCl₅ molecule, giving its bond angles.

Accepted Answer

B1 phosphorus has five outer electrons and five chlorine atoms add one each: ten electrons, five pairs B1 all five are bonding pairs, and they repel to be as far apart as possible B1 the shape is trigonal bipyramidal B1 angles of 120° between the equatorial bonds and 90° between axial and equatorial bonds

Question 4

Compare the shapes of CO₂ and SO₂, explaining the difference.

Accepted Answer

B1 both molecules have two double bonds to oxygen, each counting as one region of electron density B1 carbon uses all four outer electrons in bonding and has no lone pair, so CO₂ is linear, 180° B1 sulfur uses four of its six outer electrons in bonding and keeps one lone pair, giving three regions B1 so SO₂ is bent, with an angle of about 119°

Question 5

Outline how the number of electron pairs round the iodine atom in the ion IF₄⁺ is found, and use it to deduce the shape of the ion.

Accepted Answer

M1 iodine has seven outer electrons; four fluorine atoms add one each; subtract one for the positive charge: 7 + 4 − 1 = 10 M1 10 ÷ 2 = 5 pairs: four bonding pairs, one for each bond, and one lone pair B1 five pairs are based on a trigonal bipyramid, and the lone pair takes an equatorial position, where it has fewest neighbours at 90° B1 the ion is a seesaw, a distorted tetrahedron, with angles a little under 120° and 180°; reject tetrahedral

Question 6

Give the shape and bond angle of each of the molecules A, B and C in the diagram.

Accepted Answer

B1 BeCl₂ is linear, 180° B1 BF₃ is trigonal planar, 120° B1 SF₆ is octahedral, 90°

Question 7

Explain why the H–N–H bond angle in NH₄⁺ is larger than in NH₃.

Accepted Answer

B1 NH₄⁺ has four bonding pairs and no lone pair, so it is tetrahedral, 109.5° B1 NH₃ has three bonding pairs and one lone pair B1 the lone pair repels more than a bonding pair, closing the angle in NH₃ to 107°

Question 8

Explain why XeF₄ is square planar.

Accepted Answer

B1 xenon has eight outer electrons and four fluorine atoms add four: twelve electrons, six pairs, four bonding and two lone B1 six pairs are based on an octahedron B1 the two lone pairs repel most strongly so they take positions opposite each other, at 180°, leaving the four fluorine atoms in a square plane at 90°

Question 9

Explain why the ClF₃ molecule is T-shaped, with F–Cl–F angles a little under 90°.

Accepted Answer

B1 chlorine has seven outer electrons and three fluorine atoms add three: ten electrons, five pairs, three bonding and two lone B1 five pairs are based on a trigonal bipyramid, and the lone pairs take equatorial positions, where they have fewest neighbours at 90° B1 the three fluorine atoms, two axial and one equatorial, form a T-shape with angles a little under 90°

Question 10

Describe how a methane molecule is drawn to show its three-dimensional shape.

Accepted Answer

B1 two C–H bonds are drawn as plain lines in the plane of the paper B1 one C–H bond is drawn as a solid wedge, coming towards the viewer, and one as a hatched wedge or dashed line, going away B1 a bond angle of 109.5° is labelled between two bonds

Question 11

Suggest the shape of a BrF₅ molecule and estimate its bond angle.

Accepted Answer

B1 bromine has seven outer electrons and five fluorine atoms add five: twelve electrons, six pairs, five bonding and one lone B1 six pairs are based on an octahedron with one position taken by the lone pair, so the shape is square pyramidal B1 the lone pair repels the bonding pairs more strongly, so the angles are a little under 90°

Question 12

Suggest the shape of the triiodide ion, I₃⁻, and give its bond angle.

Accepted Answer

B1 the central iodine has seven outer electrons, two iodine atoms add two, and the charge adds one: ten electrons, five pairs B1 two are bonding pairs and three are lone pairs, which take the three equatorial positions of a trigonal bipyramid B1 the ion is linear, with an angle of 180°

Question 13

Evaluate a student's prediction that XeF₂ is bent, like water, because both have two bonded atoms and lone pairs on the central atom.

Accepted Answer

B1 xenon has eight outer electrons plus two from the fluorine atoms: ten electrons, five pairs, two bonding and three lone B1 the three lone pairs take the equatorial positions of a trigonal bipyramid, leaving the two fluorine atoms axial B1 XeF₂ is therefore linear, 180°, so the prediction is wrong: water has four pairs, not five, and only two lone pairs

Question 14

Suggest the shape of the amide ion, NH₂⁻, and estimate its bond angle.

Accepted Answer

B1 nitrogen has five outer electrons, the two hydrogen atoms add two and the charge adds one: eight electrons, four pairs B1 two are bonding pairs and two are lone pairs, which repel more strongly B1 the ion is bent, with an angle of about 104.5°, like the isoelectronic water molecule

Question 15

Explain how the shape round the boron atom changes when ammonia and boron trifluoride join to form H₃N→BF₃.

Accepted Answer

B1 in BF₃ boron has three bonding pairs and no lone pair, so it is trigonal planar with angles of 120° B1 the lone pair on nitrogen forms a dative bond to boron, giving boron four bonding pairs B1 four bonding pairs repel equally and get as far apart as possible, so the bonds round boron become tetrahedral, about 109.5°

Question 16

Explain why benzene, C₆H₆, is a planar molecule with bond angles of 120°.

Accepted Answer

B1 each carbon atom is bonded to two carbon atoms and one hydrogen atom and has no lone pair, so it has three regions of electron density B1 the three regions repel and get as far apart as possible, so each carbon is trigonal planar with angles of 120° B1 six trigonal planar carbons joined in a ring form a flat regular hexagon, with all twelve atoms in one plane

Question 17

State the principle of electron-pair repulsion theory.

Accepted Answer

B1 pairs of electrons round a central atom repel each other B1 they arrange themselves as far apart as possible, which fixes the shape

Question 18

Give the shape of the oxonium ion, H₃O⁺, and its bond angle.

Accepted Answer

B1 pyramidal, with three bonding pairs and one lone pair on oxygen B1 a bond angle of about 107°

Question 19

Suggest the shape of a molecule of sulfur dichloride, SCl₂, and estimate its bond angle.

Accepted Answer

B1 sulfur has six outer electrons and the two chlorine atoms add one each: eight electrons, four pairs, two bonding and two lone B1 the molecule is bent, with a Cl–S–Cl angle of about 104.5°; allow 100° to 105°

Question 20

Suggest the shape of the nitrate ion, NO₃⁻, and its bond angle, given that it is isoelectronic with BF₃.

Accepted Answer

B1 the nitrogen has three regions of electron density and no lone pair, as boron does in BF₃ B1 the ion is trigonal planar, with O–N–O angles of 120°

Question 21

State the order of strength of the three kinds of repulsion between electron pairs.

Accepted Answer

B1 lone pair–lone pair repulsion is greater than lone pair–bonding pair, which is greater than bonding pair–bonding pair