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Home  / GENERAL CHEMISTRY Textbook / Chapter 6. Molecule structure / **Donor - acceptor Bond (DAB)

**Donor - acceptor Bond (DAB)

As indicated above, during the formation of the covalent bond, the bonding electrons enter the outermost shells of the atoms being bonded.

During covalent bond formation, both bonding electrons enter the outermost shells of both atoms being bonded. Respectively, the number of electrons in the outermost shells of the atoms to be bonded is increased by one electron.

In the case of double and triple bond formation between the atoms, the number of electrons in the outermost shells of the atoms to be bonded is increased by two and three respectively.

If one atom, say, carbon (C), forms four bonds with hydrogen and chlorine atoms, the number of electrons in the outermost shell of a carbon atom increases by four electrons.

That is, the increase of the number of electrons in the outermost layer during chemical bond formation, is not limited by the number of electrons that enter the outermost layer during bond formation, but is limited only by the maximal number of electrons that can be situated in the outermost layer.  

Recall that for elements of the 2nd and 3rd periods with fewer that 4 electrons, the number of covalent bonds, which the given element can form, is equal to the number of electrons in the outermost shell of the given element. Thus, for example, lithium (Li), beryllium (Be), and boron (B) can form 1, 2, and 3 covalent bonds respectively.

On the other hand, for elements of these periods with more than 4 electrons, the maximal number of electrons that can enter the outermost shell of the given element, when forming a covalent bond, defines the number of covalent bonds that the given element can form.

The total number of electrons that can be situated in the outermost shell of the elements of the 2nd and 3rd periods is equal to 8. Therefore, nitrogen (N), oxygen (O), fluorine (F), and neon (Ne) with 5, 6, 7, and 8 electrons in the outermost shell can form 3, 2, 1, 0 covalent chemical bonds respectively.

Atoms of sodium (Na), magnesium (Mg), and aluminum (Al), after forming the maximal number of covalent bonds, for example, after the formation of molecules of the NaF, MgF2, AlF3 type, contain 2, 4, and 6 electrons in the outermost shell respectively. This proves that the atoms' outermost shells contain fewer than 8 electrons, i.e., they remain non-saturated.

However, atoms of nitrogen (N), oxygen (O), and fluorine (F), after the formation of 3, 2, and 1 covalent bonds, contain 8 electrons in the outermost shell, of which the bonding ones are: nitrogen (N) - 6 electrons; oxygen (O) - 4 electrons; fluorine (F) - 2 electrons.

Atoms of nitrogen (N), oxygen (O), and fluorine (F), in compounds like ammonium  (NH3), water (H2O), and hydrogen fluoride (HF) contain 2, 4, and 6 electrons relatively in the outermost shells and do not take part in chemical bond formation, i.e., they are regarded as being free. Since in the formed compounds of NH3, H2O, and HF there are 8 electrons in the outermost shell of the N, O, and F atoms, the free, nonbonding electrons cannot take part in covalent bond formation because the outermost shell of these atoms is saturated with electrons.

These free (nonbonding) electrons do play the role of the electrons' donors in chemical bond formation. 

What can be a truly reliable helper for these electrons in the formation of such bonds?

First of all, of course - those atoms whose shells contain fewer than 8 electrons. The atoms in molecules, formed of elements of the 2nd and 3rd periods (with fewer than 4 electrons in the outermost layer), are of this kind; for example, recall the previously sighted Na, Mg, and Al in compounds NaF, MgF2, and AlF3. As already said, the outermost shells of Na, Mg, and Al in these molecules are unsaturated, i.e., capable of bonding electrons. Molecules like NaF, MgF2 and AlF3 are good partner-acceptors for such donor-electrons as atoms of N, O, and F in molecules that include these atoms.

The following stable compounds are well known:

H3B ← NH3;  H3B ← N(CH3)2;  F3B ← NH3;  F3B ← O(CH3)2

               Cl2Be ← O(C2H5)2;  Cl3Al ←NH3; etc.    

The arrows (←) here indicate the donor-acceptor bonds.

Donors (N and O atoms) offer two electrons in these examples. The acceptors are B, Be and Al.

When forming DABs the acceptor atoms increase the number of electrons in their outermost shells by 2.

According to DAB formation rules, inert gases can form DABs as donors of electronic pairs with the atoms having 6 electrons. Thus the synthesis of XeO, XeO2 and XeO3 is an experimental confirmation of this theoretical supposition.

It is not only neutral atoms and molecules that can play the role of electrons' donors and acceptors, but also - the positively and negatively charged atoms and molecules, i.e., cations and anions.

Thus, for example, with the help of ammonia (NH3) a hydrogen cation (H+) forms the cation (NH4+) where one atom is bonded to the nitrogen at the expense of the nitrogen's electrons. Anion (Cl-) is the donor of the electron pair in such ions  (ClO) -.

The bonding energy between Al and N in compound Cl3Al←NH3 comprises 165 kJ/mol; while the covalent bonding energy between Al and N is equal to about 400 kJ/mol.

A molecule of NaCl is formed of atoms of sodium and chlorine. Chlorine has 8 electrons in its outermost shell, while sodium has only 2. That is, the outermost shell of the sodium is unfilled. On the other hand, only 2 electrons from the filled chlorine shell take part in the formation of the heteropolar bond Na-Cl. Six electrons (3 pairs) in the outermost shell of Cl do not take part in bond formation. That is, in the case of molecule NaCl, the sodium can bond 6 more electrons while the chlorine can offer 6 electrons for bonding. Thus, one NaCl molecule can connect another NaCl molecule with the formation of a dimmer Na2Cl2.

The Na2Cl2 molecule was received experimentally, so was the reaction energy 2NaCl →Na2Cl2. This energy is equal to 186 kJ/mol (one DAB being equal to 93 kJ/mol). The bonding energy of a NaCl monomer is equal to 410 kJ/mol, while the bonding energies of Na - Na   and Cl - Cl comprise, relatively, 75 kJ/mol and 238 kJ/mol, i.e., the DAB is much weaker than the covalent bond. As a rule, the DAB is only half as strong as the covalent bond.

One of the main reasons for the comparatively weak DAB is as follows. When calculating the bonding energy, the initial atoms' electronic energy, taking part in bond formation, is subtracted from the molecule's calculated energy. The ionization energy of the two non-bonding chlorine electrons in NaCl exceed the sum of the FIEs of both sodium and chlorine atoms.

That is, the energy gain (difference between the electronic energies of the divided and bonded atoms via chemical bonding) in the case of the DAB is smaller at the expense of the energy increase of the divided atoms.

Chapter 6. Molecule structure >>   
Conclusions >>   
**Molecules formed of multi-electron atomS >>  
**Ionization energy of multi-electron atoms >>       
**Chemical Energy. FIEs of element and bonding energy >> 
**Chemical Bonding Energy >>
***Bond Lengths >>
Conclusions >>
Valence >>
Conclusions >>     
**Donor - acceptor Bond (DAB)  
Van der Waals Bond (VWB) >>    
Dynamic Bonds >>
Conclusions >>