Fundamentals of General Chemistry. Terms and Problems in Tests In 2 parts. P.1. Terms and Examples in Tasks. Study guide

The valence-shell electron-pair repulsion (VSEPR) model is used to predict the shapes of molecular species formed from the p-block elements, It is based on the assumption that electron pairs adopt arrangements that minimize repulsions between them. Table 4.1 summarizes all the possible molecular shapes that result from valence shells containing two to six electron pair along with their bond angles.

Table 4.1

Electron pair arrangements and the structure of AXnEm molecules

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Еnd of table 4.1

n + m, total number of electron pairs; n, number of bonding pairs;

m, number of lone pairs;

the central atom (А);

ligand (В); lone pair (Е);

-bond (bonding pair).

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4 . 2 . W o r k e d e x a m p l e s

Example 1. Using VSEPR theory, predict the shape of an BF3 molecule. Has the BF3 molecular dipole moment and is this molecule polar or not?

Answer

The valence electronic configurations of boron and fluorine atoms are shown in figure:

The boron atom is the central atom of an BF3 molecule and uses all of its valence electrons to form three B–F single bonds by unpaired electrons from three fluorine atoms. Then there are three bond pairs of electrons in the valence shell of the boron atom in BF3, and by the VSEPR model, the molecular shape of BF3 is trigonal planar (see table 4.1):

F

В

F F

For polyatomic species, the net molecular dipole moment depends upon the magnitudes and relative directions of all the bond dipole moments in the moleculе. In the molecule BF3 еach B–F bond is polar ( (F) > (В)) and possesses an electric dipole moment. The three bond moments (each a vector of equivalent magnitude) oppose and cancel one another. Therefore, the BF3 molecule does not possess a molecular dipole moment and, therefore, is non-polar.

F

В

F F

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Example 2. Using VSEPR theory, predict the molecular shape of NH3. Explain the experimental value of the H–N–H bond angle is equal to 107.5о.

Answer

The valence electronic configurations of a nitrogen and hydrogen atoms are shown in Figure:

The nitrogen atom has five valence electrons. Three of them with unpaired electrons of three hydrogen atoms form three bonding pairs of electrons, one lone pair remains. Around the N centre there are three bonding pairs and one lone pair of electrons. It has a tetrahedral arrangement of electron pairs (fig. a) but not a tetrahedral arrangement of atoms (fig. b). The molecular shape of ammonia is a trigonal pyramidal (fig. c).

N

N H H

N H H H

H

According to the VSEPR model, if one or more lone pairs of electrons are present, electron – electron repulsion decrease in the sequence: lone pair – lone pair > lone pair – bonding pair > bonding pair – bonding pair. The bond angle between bonding pairs decreases as the number of lone pairs increases. The lone pair takes up more space around the central atom than a bonding pair and tend to compress the angles between the bonding pairs. The H–N–H band angle in NH3 is, therefore, smaller than the bond angle in tetrahedron (see table 4.1).

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6. According to the VSEPR model the molecule with the smallest bond angle is:

А) H2O; B) SO2; C) NH3; D) ВCl3.

7. According to the VSEPR model the molecular shape of CCl4 is:

8. According to the VSEPR model the molecular shape of NF3 is:

10. According to the VSEPR model the molecular shape of SO3 is:

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11. According to the VSEPR model molecular shape of H2O is:

13. Find a relationship between the chemical formula of a molecule and its molecular shape:

F)octahedral.

14.Find a relationship between the chemical formula of a molecule and its molecular shape:

D)linear;

E)disphenoidal;

F)T-shaped.

15.Find a relationship between the chemical formula of a molecule and its molecular shape:

F)octahedral.

16.According to the VSEPR model the structure of SbF−6 is:

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17. Find a relationship between the chemical formula of a molecule and its molecular shape:

D)linear;

E)tetrahedral;

F)octahedral.

18.Find a relationship between the chemical formula of a molecule and its

D)linear;

E)tetrahedral;

F)square planar.

19.Find a relationship between the chemical formula of a molecule and its

F)octahedral.

20.According to the VSEPR model the structure of BeF24− is:

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4 . 4 . З а д а н и я д л я с а м о к о н т р о л я

1.Согласно модели ОВЭП геометрия молекулы SF4: А) тетраэдрическая; Б) искаженно-тетраэдрическая (дисфеноидальная); В) октаэдрическая; Г) треугольная.

2.Наибольший валентный угол в молекуле:

4. Согласно модели ОВЭП геометрия молекулы аммиака: А) треугольная; Б) угловая; В) тетраэдрическая; Г) тригонально-пирамидальная.

10. Согласно модели ОВЭП геометрия молекулы SO3: А) линейная; Б) треугольная; В) угловая; Г) тетраэдрическая.

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11. Согласно модели ОВЭП геометрия молекулы воды:

13. Установите соответствие между формулой и геометрией молекулы:

Д) тетраэдрическая; Е) октаэдрическая.

14. Установите соответствие между формулой и геометрией молекулы:

15. Установите соответствие между формулой и геометрией молекулы:

Д) тетраэдрическая; Е) октаэдрическая.

16. Согласно модели ОВЭП структура иона SbF−6 :

17. Установите соответствие между формулой и геометрией молекулы:

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18. Установите соответствие между формулой и геометрией молекулы:

Д) тетраэдрическая; Е) квадрат.

19. Установите соответствие между формулой и геометрией молекулы:

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