Covalent bonding

Introduction to covalent bonding 

Salt like sodium chloride is crystalline in nature and they have a high melting point. Compounds such as hydrogen chloride and water have different properties. Under the room temperature, HCl is a gas while water is a liquid. Their atoms don’t give or accept electrons easily. In this case, they fight for a/an electron(s) until they decide to share the given number of electrons. The sharing of electrons by atoms leads to the formation of a  bond known as a covalent bond. Let us consider a simple molecule such as hydrogen molecule. A hydrogen atom consists of one valence electron in its outermost energy level similarly, a diatomic hydrogen molecule is formed as a result of the sharing of electrons between a pair of hydrogen atoms.

 

In the hydrogen molecule formed above, each H atom attains a stable configuration of two valence electrons. In this case, a single covalent bond is formed because the two hydrogen atoms have shared a pair of electrons. We should note that when writing a chemical formula to represent covalent bonds, a dash is used to represent a pair of electrons. In chemistry, such notations are known as Structural formulas. The dashes used to represent a pair of electrons in structural formulas are not used to show ionic bonds. As you can see from the hydrogen molecule, the chemical formulas of ionic compounds are used to describe formula units while those for covalent compounds describe molecules. Combinations of atoms of the non-metallic elements in Group 4A, 5A, 6A and 7A are likely to form covalent bonds. Lewis tried to summarize this tendency by formulating the octet rule for covalent bonding. Group 7A members form covalent in their diatomic molecules. For instance, the chlorine atom has seven valence electrons and requires only a single valence electron to attain stability. Through the process of electron sharing and formation of covalent bonds, two chloride atoms each attain the electron configuration of Ne. 

 

In the chlorine molecule, each chloride atom contributes one electron to complete the octet. You notice that the two chloride atoms share only one pair of valence electrons. The other valence electrons are not shared and are therefore referred to as lone pair electrons. So, the electron dot formula for ammonia molecule can be written as follows:

 

 

Lewis Structure

The valence electron of an atom is very imprtant due to its participation in chemical bonding. For this reason, we must study the valence electron(s) for atoms and compounds in detail. Dots and dashes are commonly used to represent valence electron and chemical bonds respectively. This notation is what we refer to as Lewis Electron-dot Structure. It is important that you understand how to interpret and write the lewis electron-dot structure since it helps you understand the arrangement of electrons in different molecules, their rearrangement during chemical reactions and how they contribute to determining the shape of molecules.

 

Electron-Dot Structure for different atoms

According to Lewis, the correct chemical symbol for the element represents the nucleus of the atom along with its inner electrons. Dots are placed around the symbol to represent the valence electrons. In order for you to write the correct electron dot structure for an atom, always infer from the periodic table so that you would find the right number of valence electrons. For instance, the chlorine atom has 7 valence electrons. First, you need to write the symbol for the chlorine atom Cl, and then place the dots around the symbol. Ensure that the number of electrons placed on any side of the chemical symbol of the atom doesn’t exceed 2.

 

The following guidelines can help you in writing electron-dot structures of molecules:

1. Get the sum of all the valence electrons for individual atom in the molecule. The periodic table can be used as a reference to help you determine the number of valence electrons for each atom. Ie. NH3 has a total of eight valence electrons( 5+1+1+1=8).

2. Write correct chemical symbols of the atoms present in the molecule. The central atom in most cases is the first atom in the formula and it is surrounded by other atoms present in the molecule ie.

NH3 =H  N  H

                H

3. Draw the dash between each pair of atoms covalently bonded together.

4. For each dash, you draw, subtract two from your total number of valence electrons. Then draw the remaining electrons as dots around the atoms. Arrange the atoms so that most atoms have eight valence electrons. In our case, NHhas three covalent bonds and therefore subtract (2)from the eight valence electrons. That will leave two valence electrons which will be added to the structure as lone pair electrons.

5. In case the electrons are not enough to give the atoms eight electrons, always shift unbounded electrons as necessary or change the single bonds to form double bonds or even triple bonds.

 

Octet rule

The octet rule or the electronic theory of bonding was developed by Kossel and Lewis. According to this rule, atoms can combine either by transfer of valence electrons from one atom to another or by sharing their valence electrons in order to attain the nearest noble gas configuration by having an octet in the valence shell. The octet rule successfully explained the formation of chemical bonds depending upon the nature of the element.

 

Limitations

1. This rule cannot be applied to those compounds in which the number of electrons surrounding the central atom is less than eight. For example, LiCl, BeHAlCl3, etc. do not obey the octet rule.

2. The octet rule is not satisfied for all atoms in a molecule having an odd number of electrons. For example, NO and NO2 don’t satisfy the octet rule.

3. The octet rule cannot be applied to the elements in and beyond the third period of the periodic table. The elements present in these periods have more than eight valence electrons around the central atom. For example PFand SF6.

4. It is based upon the inert nature of noble gases. However, some noble gases like Xe and Kr from compounds such as XeFand KrF2.





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