All about covalent bonds. The main provisions of the theory of hybridization

The covalent bond is carried out due to the socialization of electrons belonging to both atoms participating in the interaction. The electronegativities of non-metals are large enough that electron transfer does not occur.

Electrons in overlapping electron orbitals are shared. In this case, a situation is created in which the outer electronic levels of atoms are filled, that is, an 8- or 2-electron outer shell is formed.

The state in which the electron shell is completely filled is characterized by the lowest energy and, accordingly, the maximum stability.

There are two mechanisms of education:

1. donor-acceptor;
2. exchange.

In the first case, one of the atoms provides its pair of electrons, and the second - a free electron orbital.

In the second, one electron from each participant in the interaction comes to the common pair.

Depending on what type they are- atomic or molecular, compounds with a similar type of bond can vary significantly in physicochemical characteristics.

molecular substances most often gases, liquids or solids with low temperatures melting and boiling, non-conductive, with low strength. These include: hydrogen (H 2), oxygen (O 2), nitrogen (N 2), chlorine (Cl 2), bromine (Br 2), rhombic sulfur (S 8), white phosphorus(P 4) and other simple substances; carbon dioxide (CO 2), sulfur dioxide (SO 2), nitric oxide V (N 2 O 5), water (H 2 O), hydrogen chloride (HCl), hydrogen fluoride (HF), ammonia (NH 3), methane (CH 4), ethyl alcohol (C 2 H 5 OH), organic polymers and others.

Substances atomic exist in the form of solid crystals having high temperatures boiling and melting, insoluble in water and other solvents, many do not conduct electricity. An example is a diamond, which has exceptional strength. This is due to the fact that diamond is a crystal consisting of carbon atoms connected by covalent bonds. There are no individual molecules in a diamond. Substances such as graphite, silicon (Si), silicon dioxide (SiO 2), silicon carbide (SiC) and others also have an atomic structure.

Covalent bonds can be not only single (as in the Cl2 chlorine molecule), but also double, as in the O2 oxygen molecule, or triple, as, for example, in the N2 nitrogen molecule. At the same time, triple ones have more energy and are more durable than double and single ones.

The covalent bond can be formed both between two atoms of the same element (non-polar), and between atoms of different chemical elements(polar).

It is not difficult to indicate the formula of a compound with a covalent polar bond if we compare the values ​​of the electronegativity that make up the molecules of atoms. The absence of a difference in electronegativity will determine non-polarity. If there is a difference, then the molecule will be polar.

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Covalent non-polar chemical bond

Characteristic for simple substances non-metals. The electrons belong to the atoms equally, and there is no displacement of the electron density.

The following molecules are examples:

H2, O2, O3, N2, F2, Cl2.

Exceptions are inert gases. Their outer energy level is completely filled, and the formation of molecules is not energetically beneficial for them, and therefore they exist in the form of individual atoms.

Also, an example of substances with a non-polar covalent bond would be, for example, PH3. Even though the material is various elements, the values ​​of the electronegativity of the elements do not actually differ, which means that there will be no displacement of the electron pair.

Covalent polar chemical bond

Considering the covalent polar bond, there are many examples: HCl, H2O, H2S, NH3, CH4, CO2, SO3, CCl4, SiO2, CO.

formed between atoms of non-metals with different electronegativity. In this case, the nucleus of an element with greater electronegativity attracts common electrons closer to itself.

Scheme of the formation of a covalent polar bond

Depending on the mechanism of formation, common can become electrons of one or both atoms.

The picture clearly shows the interaction in the hydrochloric acid molecule.

A pair of electrons belongs to both one atom and the second, both of them, so the outer levels are filled. But more electronegative chlorine attracts a pair of electrons a little closer to itself (while it remains common). The difference in electronegativity is not large enough for a pair of electrons to pass to one of the atoms completely. The result is a partial negative charge for chlorine and a partial positive charge for hydrogen. The HCl molecule is a polar molecule.

Physical and chemical properties of the bond

Communication can be characterized by the following properties: directivity, polarity, polarizability and saturation.

In which one of the atoms donated an electron and became a cation, and the other atom accepted an electron and became an anion.

The characteristic properties of a covalent bond - directionality, saturation, polarity, polarizability - determine the chemical and physical properties connections.

The direction of the bond is due to the molecular structure of the substance and geometric shape their molecules. The angles between two bonds are called bond angles.

Saturation - the ability of atoms to form a limited number of covalent bonds. The number of bonds formed by an atom is limited by the number of its outer atomic orbitals.

The polarity of the bond is due to the uneven distribution of the electron density due to differences in the electronegativity of the atoms. On this basis, covalent bonds are divided into non-polar and polar (non-polar - a diatomic molecule consists of identical atoms (H 2, Cl 2, N 2) and the electron clouds of each atom are distributed symmetrically with respect to these atoms; polar - a diatomic molecule consists of atoms of different chemical elements , and the general electron cloud shifts towards one of the atoms, thereby forming an asymmetry in the distribution of the electric charge in the molecule, generating a dipole moment of the molecule).

The polarizability of a bond is expressed in the displacement of the bond electrons under the influence of an external electric field, including another reacting particle. Polarizability is determined by electron mobility. The polarity and polarizability of covalent bonds determine the reactivity of molecules with respect to polar reagents.

However, twice winner Nobel Prize L. Pauling pointed out that "in some molecules there are covalent bonds due to one or three electrons instead of a common pair." A single-electron chemical bond is realized in the molecular ion hydrogen H 2 + .

The molecular hydrogen ion H 2 + contains two protons and one electron. The single electron of the molecular system compensates for the electrostatic repulsion of two protons and keeps them at a distance of 1.06 Å (the length of the H 2 + chemical bond). The center of the electron density of the electron cloud of the molecular system is equidistant from both protons by the Bohr radius α 0 =0.53 A and is the center of symmetry of the molecular hydrogen ion H 2 + .

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  A covalent bond is formed by a pair of electrons shared between two atoms, and these electrons must occupy two stable orbitals, one from each atom.

  A + B → A: B

  As a result of socialization, electrons form a filled energy level. A bond is formed if their total energy at this level is less than in the initial state (and the difference in energy will be nothing more than the bond energy).

  According to the theory of molecular orbitals, the overlap of two atomic orbitals leads in the simplest case to the formation of two molecular orbitals (MOs): binding MO and antibonding (loosening) MO. Shared electrons are located on a lower energy binding MO.

  Formation of a bond during the recombination of atoms

  However, the mechanism of interatomic interaction long time remained unknown. Only in 1930, F. London introduced the concept of dispersion attraction - the interaction between instantaneous and induced (induced) dipoles. At present, the attractive forces due to the interaction between fluctuating electric dipoles of atoms and molecules are called "London forces".

  The energy of such an interaction is directly proportional to the square of the electronic polarizability α and inversely proportional to the sixth power of the distance between two atoms or molecules.

  Bond formation by the donor-acceptor mechanism

  In addition to the homogeneous mechanism of covalent bond formation described in the previous section, there is a heterogeneous mechanism - the interaction of oppositely charged ions - the H + proton and negative ion hydrogen H - called hydride ion:

  H + + H - → H 2

  When the ions approach, the two-electron cloud (electron pair) of the hydride ion is attracted to the proton and eventually becomes common to both hydrogen nuclei, that is, it turns into a binding electron pair. The particle that supplies an electron pair is called a donor, and the particle that accepts this electron pair is called an acceptor. Such a mechanism for the formation of a covalent bond is called donor-acceptor.

  H + + H 2 O → H 3 O +

  The proton attacks the lone electron pair of the water molecule and forms a stable cation that exists in aqueous solutions acids.

  Similarly, a proton is attached to an ammonia molecule with the formation of a complex ammonium cation:

  NH 3 + H + → NH 4 +

  In this way (according to the donor-acceptor mechanism for the formation of a covalent bond) one obtains big class onium compounds, which includes ammonium, oxonium, phosphonium, sulfonium and other compounds.

  A hydrogen molecule can act as an electron pair donor, which, upon contact with a proton, leads to the formation of a molecular hydrogen ion H 3 + :

  H 2 + H + → H 3 +

  The binding electron pair of the molecular hydrogen ion H 3 + belongs simultaneously to three protons.

  Types of covalent bond

  There are three types of covalent chemical bonds that differ in the mechanism of formation:

  1. Simple covalent bond. For its formation, each of the atoms provides one unpaired electron. When a simple covalent bond is formed, the formal charges of the atoms remain unchanged.

  • If the atoms that form a simple covalent bond are the same, then the true charges of the atoms in the molecule are also the same, since the atoms that form the bond equally own a shared electron pair. Such a connection is called non-polar covalent bond. Simple substances have such a connection, for example: 2, 2, 2. But not only non-metals of the same type can form a covalent non-polar bond. Non-metal elements whose electronegativity is of equal value can also form a covalent non-polar bond, for example, in the PH 3 molecule, the bond is covalent non-polar, since the EO of hydrogen is equal to the EO of phosphorus.
  • If the atoms are different, then the degree of ownership of a socialized pair of electrons is determined by the difference in the electronegativity of the atoms. An atom with greater electronegativity attracts a pair of bond electrons to itself more strongly, and its true charge becomes negative. An atom with less electronegativity acquires, respectively, the same positive charge. If a compound is formed between two different non-metals, then such a compound is called polar covalent bond.

  In the ethylene molecule C 2 H 4 there is a double bond CH 2 \u003d CH 2, its electronic formula: H: C:: C: H. The nuclei of all ethylene atoms are located in the same plane. Three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of about 120°). The cloud of the fourth valence electron of the carbon atom is located above and below the plane of the molecule. Such electron clouds of both carbon atoms, partially overlapping above and below the plane of the molecule, form a second bond between carbon atoms. The first, stronger covalent bond between carbon atoms is called a σ-bond; the second, weaker covalent bond is called π (\displaystyle \pi )-communication.

  In a linear acetylene molecule

  H-S≡S-N (N: S::: S: N)

  there are σ-bonds between carbon and hydrogen atoms, one σ-bond between two carbon atoms and two π (\displaystyle \pi ) bonds between the same carbon atoms. Two π (\displaystyle \pi )-bonds are located above the sphere of action of the σ-bond in two mutually perpendicular planes.

  All six carbon atoms of the C 6 H 6 cyclic benzene molecule lie in the same plane. σ-bonds act between carbon atoms in the plane of the ring; the same bonds exist for each carbon atom with hydrogen atoms. Each carbon atom spends three electrons to make these bonds. Clouds of the fourth valence electrons of carbon atoms, having the shape of eights, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In the benzene molecule, not three separate π (\displaystyle \pi )-connections, but a single π (\displaystyle \pi ) dielectrics or semiconductors. Typical examples of atomic crystals (the atoms in which are interconnected by covalent (atomic) bonds) are

  Substances of a molecular structure are formed using a special type of relationship. A covalent bond in a molecule, both polar and non-polar, is also called an atomic bond. This name comes from the Latin "co" - "together" and "vales" - "having force". With this method of formation of compounds, a pair of electrons is divided between two atoms.

  What is a covalent polar and non-polar bond? If a new compound is formed in this way, thensocialization of electron pairs. Typically, such substances have a molecular structure: H 2, O 3, HCl, HF, CH 4.

  There are also non-molecular substances, in which the atoms are connected in this way. These are the so-called atomic crystals: diamond, silicon dioxide, silicon carbide. In them, each particle is connected to four others, resulting in a very strong crystal. Crystals with a molecular structure usually do not have high strength.

  Properties of this method of formation of compounds:

  • multiplicity;
  • orientation;
  • degree of polarity;
  • polarizability;
  • conjugation.

  The multiplicity is the number of shared electron pairs. They can be from one to three. Oxygen lacks two electrons before the shell is filled, so it will be double. For nitrogen in the N 2 molecule, it is triple.

  Polarizability - the possibility of the formation of a covalent polar bond and non-polar. Moreover, it can be more or less polar, closer to ionic, or vice versa - this is the property of the degree of polarity.

  Directionality means that atoms tend to connect in such a way that there is as much electron density between them as possible. It makes sense to talk about directivity when p or d orbitals connect. S-orbitals are spherically symmetrical, for them all directions are equivalent. The p-orbitals have a non-polar or polar covalent bond directed along their axis, so that the two "eights" overlap at the vertices. This is a σ-bond. There are also less strong π-bonds. In the case of p-orbitals, the "eights" overlap with their sides outside the axis of the molecule. In the double or triple case, p-orbitals form one σ-bond, and the rest will be of the π type.

  Conjugation is the alternation of primes and multiples, making the molecule more stable. This property is characteristic of complex organic compounds.

  

  Types and methods of formation of chemical bonds

  Polarity

  Important! How to determine whether substances with a non-polar covalent or polar bond are in front of us? It's very simple: the first always occurs between identical atoms, and the second - between different, having unequal electronegativity.

  Examples of a covalent non-polar bond - simple substances:

  • hydrogen H 2 ;
  • nitrogen N 2 ;
  • oxygen O 2 ;
  • chlorine Cl 2 .

  The scheme for the formation of a covalent non-polar bond shows that, by combining an electron pair, atoms tend to complete the outer shell to 8 or 2 electrons. For example, fluorine is one electron short of an eight-electron shell. After the formation of a shared electron pair, it will be filled. A common formula for a substance with a covalent non-polar bond is a diatomic molecule.

  

  Polarity is usually associated only:

  • H 2 O;
  • CH4.

  But there are exceptions, such as AlCl 3 . Aluminum has the property of being amphoteric, that is, in some compounds it behaves like a metal, and in others it behaves like a non-metal. The difference in electronegativity in this compound is small, so aluminum combines with chlorine in this way, and not according to the ionic type.

  In this case, the molecule is formed different elements, but the difference in electronegativity is not so great that an electron completely passes from one atom to another, as in substances of an ionic structure.

  Schemes for the formation of a covalent structure of this type show that the electron density shifts to a more electronegative atom, that is, the shared electron pair is closer to one of them than to the second. The parts of the molecule acquire a charge, which is denoted by the Greek letter delta. In hydrogen chloride, for example, chlorine becomes more negatively charged and hydrogen more positively. The charge will be partial, not whole, like ions.

  Important! The polarity of the bond and the polarity of the molecule should not be confused. In methane CH4, for example, the atoms are polarly bonded, while the molecule itself is nonpolar.

  Useful video: polar and non-polar covalent bond

  Mechanism of education

  The formation of new substances can take place according to the exchange or donor-acceptor mechanism. This combines atomic orbitals. One or more molecular orbitals are formed. They differ in that they cover both atoms. As on an atomic one, no more than two electrons can be on it, and their spins must also be in different directions.

  How to determine which mechanism is involved? This can be done by the number of electrons in outer orbitals.

  Exchange

  In this case, an electron pair in a molecular orbital is formed from two unpaired electrons, each of which belongs to its own atom. Each of them tends to fill its outer electron shell, to make it stable eight- or two-electron. In this way, substances with a non-polar structure are usually formed.

  For example, consider hydrochloric acid HCl. Hydrogen has one electron in its outer level. Chlorine has seven. Having drawn the schemes for the formation of a covalent structure for it, we will see that each of them lacks one electron to fill the outer shell. By sharing an electron pair with each other, they can complete the outer shell. By the same principle, diatomic molecules of simple substances are formed, for example, hydrogen, oxygen, chlorine, nitrogen and other non-metals.

  

  Mechanism of education

  Donor-acceptor

  In the second case, both electrons are a lone pair and belong to the same atom (donor). The other (acceptor) has a free orbital.

  The formula of a substance with a covalent polar bond formed in this way, for example, the ammonium ion NH 4 +. It is formed from a hydrogen ion, which has a free orbital, and ammonia NH3, which contains one "extra" electron. The electron pair from ammonia is socialized.

  Hybridization

  When an electron pair is shared between orbitals various shapes, for example, s and p, a hybrid electron cloud sp is formed. Such orbitals overlap more, so they bind more strongly.

  This is how the molecules of methane and ammonia are arranged. In the CH 4 methane molecule, three bonds should have been formed in p-orbitals and one in s. Instead, the orbital hybridizes with three p orbitals, resulting in three hybrid sp3 orbitals in the form of elongated droplets. This is because the 2s and 2p electrons have similar energies, they interact with each other when they combine with another atom. Then you can form a hybrid orbital. The resulting molecule has the shape of a tetrahedron, hydrogen is located at its vertices.

  Other examples of substances with hybridization:

  • acetylene;
  • benzene;
  • diamond;
  • water.

  Carbon is characterized by sp3 hybridization, so it is often found in organic compounds.

  Useful video: covalent polar bond

  Conclusion

  A covalent bond, polar or non-polar, is characteristic of substances of a molecular structure. Atoms of the same element are nonpolarly bonded, and polarly bonded are different, but with slightly different electronegativity. Usually, non-metal elements are connected in this way, but there are exceptions, such as aluminum.

  Covalent, ionic, and metallic are the three main types of chemical bonds.

  Let's get to know more about covalent chemical bond. Let's consider the mechanism of its occurrence. Let's take the formation of a hydrogen molecule as an example:

  A spherically symmetric cloud formed by a 1s electron surrounds the nucleus of a free hydrogen atom. When atoms approach each other up to a certain distance, their orbitals partially overlap (see Fig.), as a result, a molecular two-electron cloud appears between the centers of both nuclei, which has a maximum electron density in the space between the nuclei. With an increase in the density of the negative charge, there is a strong increase in the forces of attraction between the molecular cloud and the nuclei.

  So, we see that a covalent bond is formed by overlapping electron clouds of atoms, which is accompanied by the release of energy. If the distance between the nuclei of the atoms approaching to touch is 0.106 nm, then after the overlap of the electron clouds it will be 0.074 nm. The greater the overlap of electron orbitals, the stronger the chemical bond.

  covalent called chemical bonding carried out by electron pairs. Compounds with a covalent bond are called homeopolar or atomic.

  Exist two types of covalent bond: polar and non-polar.

  With non-polar covalent bond formed by a common pair of electrons, the electron cloud is distributed symmetrically with respect to the nuclei of both atoms. An example can be diatomic molecules that consist of one element: Cl 2, N 2, H 2, F 2, O 2 and others, in which the electron pair belongs to both atoms equally.

  At polar In a covalent bond, the electron cloud is displaced towards the atom with a higher relative electronegativity. For example, volatile molecules inorganic compounds such as H 2 S, HCl, H 2 O and others.

  The formation of the HCl molecule can be represented as follows:

  

  Because the relative electronegativity of the chlorine atom (2.83) is greater than that of the hydrogen atom (2.1), the electron pair shifts towards the chlorine atom.

  In addition to the exchange mechanism for the formation of a covalent bond - due to overlap, there is also donor-acceptor the mechanism of its formation. This is a mechanism in which the formation of a covalent bond occurs due to a two-electron cloud of one atom (donor) and a free orbital of another atom (acceptor). Let's look at an example of the mechanism for the formation of ammonium NH 4 +. In the ammonia molecule, the nitrogen atom has a two-electron cloud:

  The hydrogen ion has a free 1s orbital, let's denote it as .

  In the process of ammonium ion formation, the two-electron cloud of nitrogen becomes common for nitrogen and hydrogen atoms, which means it is converted into a molecular electron cloud. Therefore, a fourth covalent bond appears. The process of ammonium formation can be represented as follows:

  The charge of the hydrogen ion is dispersed among all atoms, and the two-electron cloud that belongs to nitrogen becomes common with hydrogen.

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  chemical bond - electrostatic interaction between electrons and nuclei, leading to the formation of molecules.

  A chemical bond is formed by valence electrons. For s- and p-elements, the electrons of the outer layer are valence, for d-elements, the s-electrons of the outer layer and the d-electrons of the pre-outer layer. When a chemical bond is formed, the atoms complete their outer electron shell to the shell of the corresponding noble gas.

  Link length is the average distance between the nuclei of two chemically bonded atoms.

  Chemical bond energy- the amount of energy required to break the bond and throw the fragments of the molecule to an infinitely long distance.

  Valence angle is the angle between lines connecting chemically bonded atoms.

  The following main types of chemical bond are known: covalent (polar and non-polar), ionic, metallic and hydrogen.

  covalent called a chemical bond formed by the formation of a common electron pair.

  If the bond is formed by a pair of common electrons, equally belonging to both connecting atoms, then it is called covalent non-polar bond. This bond exists, for example, in the molecules H 2 , N 2 , O 2 , F 2 , Cl 2 , Br 2 , I 2 . A covalent non-polar bond occurs between identical atoms, and the electron cloud connecting them is evenly distributed between them.

  In molecules between two atoms can form different number covalent bonds (for example, one in the halogen molecules F 2, Cl 2, Br 2, I 2, three in the nitrogen molecule N 2).

  covalent polar bond occurs between atoms with different electronegativity. The electron pair that forms it shifts towards the more electronegative atom, but remains bound to both nuclei. Examples of compounds with a covalent polar bond: HBr, HI, H 2 S, N 2 O, etc.

  Ionic called the limiting case of a polar bond, in which the electron pair completely passes from one atom to another and the bound particles turn into ions.

  Strictly speaking, only compounds for which the difference in electronegativity is greater than 3 can be classified as ionic compounds, but very few such compounds are known. These include fluorides of alkali and alkaline earth metals. It is conventionally believed that an ionic bond occurs between atoms of elements whose electronegativity difference is greater than 1.7 on the Pauling scale. Examples of compounds with an ionic bond: NaCl, KBr, Na 2 O. More details about the Pauling scale will be discussed in the next lesson.

  metal called the chemical bond between positive ions in metal crystals, which is carried out as a result of the attraction of electrons freely moving through the metal crystal.

  Metal atoms turn into cations, forming a metal crystal lattice. In this lattice, they are held by electrons common to the entire metal (electron gas).

  Training tasks

  1. Each of the substances is formed by a covalent non-polar bond, the formulas of which are

  1) O 2, H 2, N 2
  2) Al, O 3 , H 2 SO 4
  3) Na, H 2 , NaBr
  4) H 2 O, O 3, Li 2 SO 4

  2. Each of the substances is formed by a covalent polar bond, the formulas of which are

  1) O 2, H 2 SO 4, N 2
  2) H 2 SO 4, H 2 O, HNO 3
  3) NaBr, H 3 PO 4, HCl
  4) H 2 O, O 3, Li 2 SO 4

  3. Each of the substances is formed only by ionic bond, the formulas of which

  1) CaO, H 2 SO 4, N 2
  2) BaSO 4 , BaCl 2 , BaNO 3
  3) NaBr, K 3 PO 4, HCl
  4) RbCl, Na 2 S, LiF

  4. The metallic bond is specific to list items

  1) Ba, Rb, Se
  2) Cr, Ba, Si
  3) Na, P, Mg
  4) Rb, Na, Cs

  5. Compounds with only ionic and only covalent polar bonds are, respectively,

  1) HCl and Na 2 S
  2) Cr and Al (OH) 3
  3) NaBr and P 2 O 5
  4) P 2 O 5 and CO 2

  6. An ionic bond is formed between elements

  1) chlorine and bromine
  2) bromine and sulfur
  3) cesium and bromine
  4) phosphorus and oxygen

  7. A polar covalent bond is formed between elements

  1) oxygen and potassium
  2) sulfur and fluorine
  3) bromine and calcium
  4) rubidium and chlorine

  8. In volatile hydrogen compounds elements of VA group of the 3rd period chemical bond

  1) covalent polar
  2) covalent non-polar
  3) ionic
  4) metal

  9. In higher oxides of elements of the 3rd period, the type of chemical bond changes with an increase in the ordinal number of the element

  1) from ionic bond to covalent polar bond
  2) from metallic to covalent non-polar
  3) from covalent polar bond to ionic bond
  4) from a covalent polar bond to a metallic bond

  10. The length of the chemical bond E–N increases in a number of substances

  1) HI - PH 3 - HCl
  2) PH 3 - HCl - H 2 S
  3) HI - HCl - H 2 S
  4) HCl - H 2 S - PH 3

  11. The length of the chemical bond E–N decreases in a number of substances

  1) NH 3 - H 2 O - HF
  2) PH 3 - HCl - H 2 S
  3) HF - H 2 O - HCl
  4) HCl - H 2 S - HBr

  12. The number of electrons that participate in the formation of chemical bonds in the hydrogen chloride molecule is

  1) 4
  2) 2
  3) 6
  4) 8

  13. The number of electrons that participate in the formation of chemical bonds in the P 2 O 5 molecule, -

  1) 4
  2) 20
  3) 6
  4) 12

  14. In phosphorus(V) chloride, the chemical bond

  1) ionic
  2) covalent polar
  3) covalent non-polar
  4) metal

  15. The most polar chemical bond in a molecule

  1) hydrogen fluoride
  2) hydrogen chloride
  3) water
  4) hydrogen sulfide

  16. Least polar chemical bond in a molecule

  1) hydrogen chloride
  2) hydrogen bromide
  3) water
  4) hydrogen sulfide

  17. Due to the common electron pair, a bond is formed in a substance

  1) Mg
  2) H2
  3) NaCl
  4) CaCl2

  18. A covalent bond is formed between elements whose serial numbers

  1) 3 and 9
  2) 11 and 35
  3) 16 and 17
  4) 20 and 9

  19. An ionic bond is formed between elements whose serial numbers

  1) 13 and 9
  2) 18 and 8
  3) 6 and 8
  4) 7 and 17

  20. In the list of substances whose formulas are compounds with only ionic bonds, these are

  1) NaF, CaF2
  2) NaNO 3 , N 2
  3) O2, SO3
  4) Ca(NO 3) 2, AlCl 3