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Hybridization atomic orbitals

Linus Carl Pauling

Hybridization of atomic orbitals - a change in the shape and energy of the orbitals of an atom during the formation of a covalent bond in order to achieve a more efficient overlap of orbitals.

Different orbitals with slightly different energies form a corresponding number of hybrid orbitals. The number of hybrid orbitals is equal to the number of atomic orbitals involved in hybridization. Hybrid orbitals are the same in the shape of the electron cloud and in energy.

Hybridization involves not only bonding electrons, but also unshared electron pairs.

Compared to atomic orbitals, hybrid orbitals are more elongated in the direction of formation chemical bonds and therefore cause a better overlap of electron clouds.

The hybrid orbital is more elongated on one side of the nucleus than on the other.

Coord. number Type of hybridization Spatial configuration of a molecule whose central atom undergoes hybridization Arrangement of atoms in a molecule Examples of compounds 2 sp Linear BeCl 2 , CO 2 , HCN 3 sp 2 Trigonal BF 3 , BCl 3 , NO 3 - , HgI 3 - , CdCl 3 - 4 sp 3 Tetrahedral CH 4 , CCl 4 , XeO 4 , HgI 4 - ,

sp hybridization is a hybridization in which the atomic orbitals of one s and one p electrons participate

During the hybridization process, 2 hybrid orbitals are formed, which are oriented to each other at an angle of 180 °

The concept of sp hybridization of orbitals can be applied to explain the linear form of the BeH 2 molecule, in which the beryllium atom is formed by hybrid sp orbitals.

Formation of a beryllium fluoride molecule. Each fluorine atom that is part of this molecule has one unpaired electron, which is involved in the formation of a covalent bond.

The beryllium atom in the unexcited state does not have unpaired electrons: Therefore, in order to participate in the formation of chemical bonds, the beryllium atom must go into an excited state:

with the expenditure of some energy, instead of the original s - and p-orbitals of the beryllium atom, two equivalent hybrid orbitals (sp - orbitals) can form.

Examples chemical compounds, which are characterized by sp-hybridization: BeCl 2, BeH 2, CO, CO 2, HCN, carbine, acetylenic hydrocarbons (alkynes).

sp 2 -hybridization - hybridization in which the atomic orbitals of one s - and two p-electrons participate

As a result of hybridization, three hybrid sp 2 orbitals are formed, located in the same plane at an angle of 120 ° to each other

This type of hybridization is observed in the BCl 3 molecule.

sp 2 - hybridization of the boron atom in the boron fluoride molecule. Here, instead of the original one s - and two p-orbitals of the excited boron atom

three equivalent sp 2 orbitals are formed. Therefore, the molecule is built in the form of a regular triangle, in the center of which there is a boron atom, and at the vertices there are fluorine atoms.

Examples of compounds in which sp 2 hybridization is observed: SO 3, BCl 3, BF 3, AlCl 3, CO 3 2-, NO 3 -, graphite, ethylene hydrocarbons (alkenes), carboxylic acids and aromatic hydrocarbons (arenes).

sp 3 - hybridization - hybridization, in which the atomic orbitals of one s - and three p-electrons participate

Four sp 3 hybrid orbitals are symmetrically oriented in space at an angle of 109°28"

not always the spatial configuration of a molecule corresponds to a tetrahedron, it depends on the number of atoms in the molecule. An example of this are the molecules of water and ammonia NH 3.

The valence of the nitrogen atom is III, its five electrons of the outer level occupy four orbitals, which means that the type of hybridization is sp 3, but only three orbitals take part in the formation of a chemical bond. A tetrahedron without one vertex turns into a pyramid. Therefore, the ammonia molecule has a pyramidal shape of the molecule, the bond angle is distorted to 107°30 ′.

oxygen in the water molecule is in the sp 3 hybrid state, and the shape of the molecule is angular, the bond angle is 104°27′.

Examples of compounds that are characterized by sp 3 hybridization: H 2 O, NH 3, POCl 3, SO 2 F 2, SOBr 2, NH 4+, H 3 O +, diamond, saturated hydrocarbons (alkanes, cycloalkanes).

Continuation. For the beginning, see № 15, 16/2004

Lesson 5
atomic orbitals of carbon

A covalent chemical bond is formed using common bonding electron pairs of the type:

Form a chemical bond, i.e. only unpaired electrons can create a common electron pair with a “foreign” electron from another atom. When writing electronic formulas, unpaired electrons are located one by one in the orbital cell.
atomic orbital is a function that describes the density of the electron cloud at each point in space around the nucleus of an atom. An electron cloud is a region of space in which an electron can be found with a high probability.
To harmonize the electronic structure of the carbon atom and the valency of this element, the concepts of excitation of the carbon atom are used. In the normal (unexcited) state, the carbon atom has two unpaired 2 R 2 electrons. In an excited state (when energy is absorbed) one of 2 s 2-electrons can pass to free R-orbital. Then four unpaired electrons appear in the carbon atom:

Recall that in electronic formula atom (for example, for carbon 6 C - 1 s 2 2s 2 2p 2) large numbers in front of the letters - 1, 2 - indicate the number of the energy level. Letters s And R indicate the shape of the electron cloud (orbitals), and the numbers to the right above the letters indicate the number of electrons in a given orbital. All s- spherical orbitals:

At the second energy level except 2 s-there are three orbitals 2 R-orbitals. These 2 R-orbitals have an ellipsoidal shape, similar to dumbbells, and are oriented in space at an angle of 90 ° to each other. 2 R-Orbitals denote 2 p x, 2r y and 2 pz according to the axes along which these orbitals are located.

When chemical bonds are formed, the electron orbitals acquire the same shape. Yes, in saturated hydrocarbons mixed one s-orbital and three R-orbitals of a carbon atom to form four identical (hybrid) sp 3-orbitals:

This - sp 3 - hybridization.
Hybridization– alignment (mixing) of atomic orbitals ( s And R) with the formation of new atomic orbitals, called hybrid orbitals.

Hybrid orbitals have an asymmetric shape, elongated towards the attached atom. Electron clouds repel each other and are located in space as far as possible from each other. At the same time, the axes of four sp 3-hybrid orbitals turn out to be directed to the vertices of the tetrahedron (regular triangular pyramid).
Accordingly, the angles between these orbitals are tetrahedral, equal to 109°28".
The tops of electron orbitals can overlap with the orbitals of other atoms. If electron clouds overlap along a line connecting the centers of atoms, then such a covalent bond is called sigma()-bond. For example, in a C 2 H 6 ethane molecule, a chemical bond is formed between two carbon atoms by overlapping two hybrid orbitals. This is a connection. In addition, each of the carbon atoms with its three sp 3-orbitals overlap with s-orbitals of three hydrogen atoms, forming three -bonds.

In total, three valence states with different types of hybridization are possible for a carbon atom. Except sp 3-hybridization exists sp 2 - and sp-hybridization.
sp 2 -Hybridization- mixing one s- and two R-orbitals. As a result, three hybrid sp 2 -orbitals. These sp 2 -orbitals are located in the same plane (with axes X, at) and are directed to the vertices of the triangle with an angle between the orbitals of 120°. unhybridized
R-orbital is perpendicular to the plane of the three hybrid sp 2 orbitals (oriented along the axis z). Upper half R-orbitals are above the plane, the lower half is below the plane.
Type sp 2-hybridization of carbon occurs in compounds with a double bond: C=C, C=O, C=N. Moreover, only one of the bonds between two atoms (for example, C=C) can be a bond. (The other bonding orbitals of the atom are directed in opposite directions.) The second bond is formed as a result of the overlap of non-hybrid R-orbitals on both sides of the line connecting the nuclei of atoms.

Covalent bond formed by lateral overlap R-orbitals of neighboring carbon atoms is called pi()-bond.

Education - communications

Due to less overlap of orbitals, the -bond is less strong than the -bond.
sp-Hybridization is a mixing (alignment in form and energy) of one s- and one
R-orbitals with the formation of two hybrid sp-orbitals. sp- Orbitals are located on the same line (at an angle of 180 °) and directed to opposite sides from the nucleus of a carbon atom. Two
R-orbitals remain unhybridized. They are placed perpendicular to each other.
directions - connections. On the image sp-orbitals are shown along the axis y, and the unhybridized two
R-orbitals - along the axes X And z.

The triple carbon-carbon bond CC consists of a -bond that occurs when overlapping
sp-hybrid orbitals, and two -bonds.
The relationship between such parameters of the carbon atom as the number of attached groups, the type of hybridization and the types of chemical bonds formed is shown in Table 4.

Table 4

Covalent bonds of carbon

Number of groups related with carbon	Type hybridization	Types participating chemical bonds	Examples of compound formulas
4	sp 3	Four - connections
3	sp 2	Three - connections and one is connection
2	sp	Two - connections and two connections	H-CC-H

Exercises.

1. What electrons of atoms (for example, carbon or nitrogen) are called unpaired?

2. What does the concept of "shared electron pairs" mean in compounds with a covalent bond (for example, CH 4 or H 2 S )?

3. What are the electronic states of atoms (for example, C or N ) are called basic, and which are excited?

4. What do the numbers and letters mean in the electronic formula of an atom (for example, C or N )?

5. What is an atomic orbital? How many orbitals are in the second energy level of a C atom and how do they differ?

6. What is the difference between hybrid orbitals and the original orbitals from which they were formed?

7. What types of hybridization are known for the carbon atom and what are they?

8. Draw a picture of the spatial arrangement of orbitals for one of the electronic states of the carbon atom.

9. What chemical bonds are called and what? Specify-And-connections in connections:

10. For the carbon atoms of the compounds below, indicate: a) the type of hybridization; b) types of its chemical bonds; c) bond angles.

Answers to exercises for topic 1

Lesson 5

1. Electrons that are one per orbital are called unpaired electrons. For example, in the electron diffraction formula of an excited carbon atom, there are four unpaired electrons, and the nitrogen atom has three:

2. Two electrons participating in the formation of one chemical bond are called common electron pair. Usually, before the formation of a chemical bond, one of the electrons of this pair belonged to one atom, and the other electron belonged to another atom:

3. The electronic state of the atom, in which the order of filling of electronic orbitals is observed: 1 s 2 , 2s 2 , 2p 2 , 3s 2 , 3p 2 , 4s 2 , 3d 2 , 4p 2 etc. are called main state. IN excited state one of the valence electrons of an atom occupies a free orbital with more high energy, such a transition is accompanied by the separation of paired electrons. Schematically it is written like this:

Whereas in the ground state there were only two valence unpaired electrons, in the excited state there are four such electrons.

5. An atomic orbital is a function that describes the density of an electron cloud at each point in space around the nucleus of a given atom. There are four orbitals on the second energy level of the carbon atom - 2 s, 2p x, 2r y, 2pz. These orbitals are:
a) the shape of the electron cloud ( s- ball, R- dumbbell);
b) R-orbitals have different orientations in space - along mutually perpendicular axes x, y And z, they are denoted p x, r y, pz.

6. Hybrid orbitals differ from the original (non-hybrid) orbitals in shape and energy. For example, s-orbital - the shape of a sphere, R- symmetrical figure eight, sp-hybrid orbital - asymmetric figure eight.
Energy Differences: E(s) < E(sp) < E(R). Thus, sp-orbital - an orbital averaged in shape and energy, obtained by mixing the initial s- And p-orbitals.

7. Three types of hybridization are known for the carbon atom: sp 3 , sp 2 and sp (see the text of lesson 5).

9. -bond - a covalent bond formed by frontal overlapping of orbitals along a line connecting the centers of atoms.
-bond - a covalent bond formed by lateral overlap R-orbitals on either side of the line connecting the centers of atoms.
- Bonds are shown by the second and third lines between the connected atoms.

Chemistry lesson on the topic:

Hybridization of electron orbitals. Geometry of molecules

This lesson is designed for 11th grade students.sastudents studying chemistry under the program Gabrielyan O.S. according to the textbook “Chemistry. Grade 11”, authors O.S. Gabrielyan et al. “Drofa” publishing house, 2006”.

The versatility of this development lies in the fact that it can be successfully used by teachers working on the programs of other authors in general education and specialized classes.

The work presented includes: technological map chemistry lesson in grade 11 with applications and electronic presentation. The originality of the work is determined by interactive inserts in the presentation, the use of information from the Internet, and at the same time independence from the Internet during the lesson. Illustrations included from various sources, their combination and method of presentation make it possible to fully implement interdisciplinary connections in the lesson, form a scientific worldview, and instill in students a love of beauty.

Development can be used as Toolkit. It is designed to help a novice chemistry teacher, as well as a teacher introducing information Technology in teaching chemistry.

Lesson objectives:

To reveal the universal nature of the hybridization process for organic, complex inorganic substances and allotropic modifications of carbon.

Show the dependence of the geometry of molecules on the type of hybridization of electron orbitals, and the properties of substances on the geometry of molecules.

To draw students' attention to the influence of the fundamental laws of nature and structural features of molecules on the existing order and beauty in the world.

Equipment: PC, multimedia projector, screen, electronic presentation. Ball-and-stick models of molecules of methane, pentane, graphite, diamond, ethylene, acetylene, models of molecules made from balloons, geometric models of a tetrahedron and a triangular pyramid. Demonstration table "Allotropic modifications of carbon", photographs depicting molecules and crystals, students' reports, a portrait of L. Pauling.

Lesson plan

I. Essence of hybridization of electron orbitals, its mechanism.

II. From the history of the issue. Pauling L. - the great chemist of the twentieth century, his merits in the study and description of the structures of molecules.

III. The geometry of molecules of organic and inorganic substances, due to:

sp 3 __ hybridization;

sp 2 __ hybridization;

sp - hybridization.

Task for the lesson: repeat the hybridization of the electron orbitals of the carbon atom, the properties of the chemical bond. 1 student is preparing an electronic presentation "The life and work of L. Pauling."

Board decoration

Lesson progress

I. Organizing time . Slide number 1.

II. Homework conversation (6 min). Slide number 2, formulas of substances on the board.

What properties of a covalent bond did we study in the last lesson? (length, E, strength, saturation)

What is bond length and what does it depend on? (depending on the size of the atom and the multiplicity of bonds)

What is bond energy and what does it depend on? (the amount of energy needed to break the bond; depends on the strength of the bond)

What is bond strength and what does it depend on? (on which connection is ?, or ?, and which clouds overlap - hybrid or non-hybrid)

How are the properties of a covalent bond related? (the longer the length, the lower the strength and energy)

How does the bond length change in hydrogen halide molecules (see on the board - 1st column) and why? (increases as the size of the atom increases)

Which of the given connections (on the board) is the strongest? (HF)

Hydrogen halides are dissolved in water to form acids. Which of these acids is the strongest and why? (HJ, because acidity is the ability to release H + , HJ has the weakest bond)

Which acid is the weakest? (HF - hydrofluoric acid, dissolves glass)
Conclusion : The properties of a substance depend on the size of the atoms that form them.

How does the bond strength change in a series of hydrocarbons (see the 2nd column on the board) and what does it depend on? (from top to bottom, the bond strength increases, because the multiplicity increases and the length decreases)

How does this affect the properties of these substances? (for alkanes having only?-bonds, substitution reactions are characteristic, for alkenes with?-bonds - additions, and for alkynes - addition reactions and substitution reactions of hydrogen atoms in the triple bond)

Using the example of molecules of simple substances chlorine, oxygen, nitrogen (see on the board - 3rd column), explain how the structure of their molecules affects their properties. (free chlorine is not found - a single bond, oxygen in the air 21% - a double bond, nitrogen in the air 78%, an inert substance - a triple bond)
Conclusion : The properties of organic and inorganic substances depend on the multiplicity of bonds.

How bond saturation affects the properties of substances (see on the board - 4th column) (methane does not have unsaturated bonds, ammonia and water have unsaturated bonds, therefore they are dipoles).
Conclusion : The properties of substances depend on the properties of the covalent bond.

II. Studying new topic

№ pp

Conclusion . We should be proud of the fact that we live in Russia, where wonderful scientists and chemists of world renown lived and worked. These are Lomonosov M.V. - encyclopedic scientist, Mendeleev D.I. - creator of the Periodic Law, Borodin A.P. - chemist and composer, Butlerov A.M. - creator of the theory of the structure of organic compounds, Lebedev S.V. - creator 1 artificial rubber in Russia and many others who have made a great contribution to the development of chemical science. But we also great respect should refer to scientists from other countries, and among them is Linus Pauling, who is a world-famous scientist, and every educated person should know about him.

heuristic conversation. Using the example of the structure of molecules of organic substances (hydrocarbons) and inorganic substances (compounds of silicon, nitrogen, oxygen, boron, beryllium; allotropic modifications of carbon), the teacher shows the universality of the concept of “hybridization” and the dependence of the geometry of molecules on hybridization, and the properties of substances on the geometry of molecules. During the conversation, students get acquainted with the geometry of the molecules of inorganic substances and the influence of unshared electron pairs on their properties.

IV. Homework : §7, notes in a notebook, prepare for testing (see. ).

List of sources used :

Gabrielyan O.S. etc. Handbook of the teacher. Chemistry. Grade 11: At 2 o'clock - M .: Bustard, 2003.

Ilchenko V.R. Crossroads of physics, chemistry and biology. – M.: Enlightenment, 1986.

CD" virtual school Cyril and Methodius” Biology lessons. Animals.

CD “Virtual School of Cyril and Methodius” Biology lessons. General biology.

CD “Virtual School of Cyril and Methodius” Chemistry lessons. 10-11 grades.

To explain the facts when an atom forms more bonds than the number of unpaired electrons in its ground state (for example, a carbon atom), the postulate of hybridization of atomic orbitals close in energy is used. AO hybridization occurs upon the formation of a covalent bond, if more efficient orbital overlap is achieved. Hybridization of the carbon atom is accompanied by its excitation and electron transfer from 2s- to 2p-AO:

AOs with a large difference in energy (for example, 1s and 2p) do not enter into hybridization. Depending on the number of p-AOs involved in hybridization, the following types of hybridization are possible: for carbon and nitrogen atoms, sp3, sp2, and sp; for the oxygen atom - sp3, sp2; for halogens - sp3.

The axes of sp3-hybrid orbitals are directed to the vertices of a regular tetrahedron. The tetrahedral angle between them is 109°28", which corresponds to the lowest electron repulsion energy.

Sp2 hybridization (trigonal planar) One s- and two p-orbitals mix to form three equivalent sp2 hybrid orbitals located in the same plane at an angle of 120°. They can form three s-bonds. The third p-orbital remains unhybridized and is oriented perpendicular to the plane of the hybrid orbitals. This p-AO is involved in the formation of the p-bond.

This lesson will help you get an idea about the topic “Geometry of molecules. The concept of the theory of hybridization. The universal nature of the hybridization process for organic, complex inorganic substances and allotropic modifications of carbon will be disclosed. You will learn about the dependence of the geometry of molecules on the type of hybridization of electron orbitals and the properties of substances on the geometry of molecules.

Topic: Introduction to organic chemistry

Lesson: Geometry of molecules. The concept of the theory of hybridization

on the example of molecules with single bonds

External level carbon atom in the ground (unexcited) state is described by the formula 2s 2 2p 2 or by the scheme:

2 s

This building contains the preconditions for a peculiar symmetry There are exactly 4 orbitals for four electrons. Back in the middle of the 19th century, the German scientist Friedrich Kekule rightly suggested that in organic compounds the valency of carbon is four.

From the point of view of the electronic structure of the atom, this can be explained as follows:

One electron from the 2s-orbital "jumps" to the 2p-orbital, while the carbon atom goes into the so-called excited state:

Excited state of an atom carbon 2s 1 2p 3:

2 s

allows a carbon atom to form 4 covalent bonds through the exchange mechanism.

Three p-orbitals are traditionally depicted in the form of "dumbbells" mutually perpendicular to each other, and the s-orbital is in the form of a ball. The three p-electron bonds must be at 90° to each other and are significantly longer than the s-electron bond. But methane CH 4 is a symmetrical tetrahedron.

Back in 1874, many years before it became possible direct definition structure of molecules, Jacob Henrik van't Hoff (1852-1911), being a student at the University of Utrecht, suggested that the carbon atom in compounds has a tetrahedral structure. The structure of the methane molecule CH 4 - regular tetrahedron with a carbon atom in the center. Valence angles H-C-H bonds are equal to 109 about 28 '.

Simplified explanation: all orbitals of the outer level of carbon align in energy and shape, mix, i.e. "hybridize" to form identical hybrid orbitals. See fig. 1.

Rice. 1. Hybridization is the mixing of electron clouds during the formation of chemical bonds

Mixing one s-orbitals and three p-orbitals gives four sp 3-hybrid orbitals elongated at the corners of a tetrahedron with a C atom in the center. Carbon in methane is in a state of sp 3 hybridization. Rice. 2.

Rice. 2. The structure of methane

The structure of ammonia

In the same way, four orbitals of the nitrogen atom hybridize in ammonia molecule NH 3: The nitrogen atom has 5 electrons in its outer level. Therefore, on one sp 3 orbital there is a lone pair of electrons, and on the other three - electron pairs N-H bonds. All four electron pairs are located at the corners of a distorted tetrahedron (the electron cloud of the lone pair is larger than that of the bonding pair). Rice. 3

Rice. 3. The structure of ammonia

The structure of water

An oxygen atom has 6 electrons in its outer level. Therefore, lone pairs of electrons are located on two sp 3 orbitals, and electron pairs are located on the other two O-H bonds. The molecule has an angular structure. Rice. 4.

Rice. 4. The structure of water

In such an analysis of the structure of molecules, it is important not to confuse the geometry of the arrangement of electron pairs in space and the geometry of chemical bonds. We see that in ammonia and water, not all electron pairs participate in the formation of chemical bonds.

The geometry of molecules or chemical bonds considers precisely the arrangement of atoms in space, without describing the arrangement of unshared electron pairs. The electron clouds of hybrid orbitals try to push each other as far as possible. If there are four clouds, then they will diverge at the corners of the tetrahedron, three - they will be located in a plane at an angle of 120 °.

The structure of the moleculeBF 3

The outer level of the boron atom has 3 electrons. When bonds are formed, boron, like carbon, goes into an excited state. One s- and two p-orbitals, which have electrons, hybridize, forming three identical sp 2 hybrid orbitals, located at the corners of an equilateral triangle with a boron atom in the center. Rice. 5

Rice. 5. Structure of three boron fluorides

Conclusion: The geometry of molecules considers the arrangement of atoms in space without describing the arrangement of lone electron pairs. So, the structure of a water molecule, consisting of three atoms, is not tetrahedral, but angular.

Summing up the lesson

You got an idea about the topic “Geometry of molecules. The concept of the theory of hybridization. The universal nature of the hybridization process for organic, complex inorganic substances and allotropic modifications of carbon was revealed. You learned about the dependence of the geometry of molecules on the type of hybridization of electron orbitals and the properties of substances on the geometry of molecules.

Bibliography

1. Rudzitis G.E. Chemistry. Basics general chemistry. Grade 10: textbook for educational institutions: a basic level of/ G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Chemistry. Grade 10. Profile level: studies. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2008. - 463 p.

3. Chemistry. Grade 11. Profile level: textbook. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2010. - 462 p.

4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering the universities. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.

Homework

1. Nos. 1-3 (p. 22) Rudzitis G.E. , Feldman F.G. Chemistry: Organic chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Why, having the same type of hybridization (which one?), do methane and ammonia molecules have different spatial structures?

3. What is the difference between the ground state of the carbon atom and the excited state?