Class 10 FRANK Solutions Chemistry Chapter 2 - Chemical Bonding

Chemical bond: A chemical bond may be defined as the linkage that stands for the force which actually holds the atoms together within the molecule.
Chemical bonding: The phenomenon during which a chemical bond is formed is called chemical bonding.

Atoms combine to attain the electronic configuration of nearest inert gases as the atoms of inert gases are very stable having 8 electrons or duplet (or 2 electrons in case of helium atom) in their outermost shell.

Electrovalent compounds: The chemical compounds containing electrovalent bonds are called electrovalent or ionic compounds.
For example: Sodium chloride (NaCl).
Covalent compounds: The chemical compound, formed as a result of mutual sharing of electrons or electron pairs thereby establishing a covalent bond is called a covalent or molecular compound.
For example: Hydrogen molecule (H₂)

The conditions for the formation of an electrovalent bond are:
1) Low ionization energy of electropositive atom
2) High electron affinity of the electronegative atom.
3) Large electronegativity difference.
4) High lattice energy.
Concept Insight:  1) Lower is the ionization energy of atom, higher is its tendency to lose electron to form a cation and form ionic bond.
2) Higher the value of electron affinity of an atom, greater will be its tendency to form anion and form ionic bond.
3) If the electronegativity difference of two elements is higher, more easy will be the transfer of electrons and hence more chances of ionic bond formation.
4) Lattice energy is the energy released when positive and negatively charged atoms called ions come closer to form a crystal because the attractive forces among the oppositely charged ions tend to decrease the energy of the system. Higher is the lattice energy, greater will be the ease of formation of the compound.

Ioniccompounds	Covalent compounds
These contain electrovalent or ionic bond.	These contain covalent bond.
They are generally crystalline solids.	They are generally volatile liquids or gases.
They have high melting and boiling points.	They have low melting and boiling points.
They are bad conductors of electricity in solid state but are good conductors in the molten or aqueous states.	They do not conduct electricity.
They are generally soluble in water but insoluble in organic solvents.	They are generally soluble in organic solvents but insoluble in water.
They consist of ions. So, their reactions in solutions are fast.	They consist of molecules. So, their reactions in solutions are slow.

Hydrogen chloride has a polar covalent bond because in hydrogen chloride the higher electronegativity of chlorine atom attracts the shared electron pair towards itself. As a result, the chlorine atom gets a partial negative charge while the hydrogen atom gets a partial positive charge. Hence such a covalent bond with charge separation is called polar covalent bond.
While methane has a non polar covalent bond  because in case of methane molecule the shared electron pairs are at equal distance from the carbon and hydrogen atoms, because neither the carbon atom nor the hydrogen atom has enough electronegativity difference  between each other to attract the shared pairs of electrons towards itself. Hence no charge separation occurs in the covalent bond due to which it is called non polar covalent bond.  
Concept Insight:  When a covalent bond is formed between the atoms of the same elements of equal electronegativity then the electron pairs are shared equally between the atoms and the bond so formed is called non polar covalent bond. On the other hand, if the covalent bond is formed between atoms of different elements, with difference in electro negativity, the electrons are not shared equally between the atoms. The more electronegative atom pulls the bonded pair of electrons towards itself and acquires negative charge while the other less electro negative atom acquires positive charge and the bond becomes polar covalent bond.

In terms of electron transfer, oxidation is defined as the phenomenon in which an atom loses electron to form a positively charged cation while reduction is defined as the phenomenon in which an atom gains electron to form a negatively charged ion called anion.
During formation of ionic bond one atom undergoes oxidation while another atom undergoes reduction.

(i) (d) High lattice energy

(ii) (b) Low boiling and low melting points

(iii) (a) Equal electro negativities of combining atoms

(iv) (d) NH₄⁻

(v) (b) Methane

(a) Sodium chloride dissolves in water because it is an ionic compound and water is also a polar covalent compound. Water decreases the electrostatic forces of attraction among the sodium and chloride ions due to which these ions become free in water, hence sodium chloride dissolves.
On the other hand, carbon tetra chloride has non polar covalent bond and water has polar covalent bond. Hence, water is unable to break the non polar covalent bond of carbon tetra chloride. So it is insoluble in water
(b) Helium does not form He₂ molecule as it has its outermost shell complete i.e. two electrons in its valence shell. Due to this complete valence shell helium atom is very stable hence does not participate in chemical bonding to form He2 molecule.
(c) Pure water does not conduct electricity because it has a polar covalent molecule hence does not have ions in it which can conduct electricity.
On adding sodium chloride to pure water, sodium chloride breaks apart into sodium and chloride ions because water being polar decreases the strong forces of attraction among sodium and chloride ions. Now, pure water has ions present in it which can conduct electricity.
(d) Cl₂ is a non polar molecule because the bond is between same atoms that is chlorine with zero electronegativity difference among them. So the shared electron pair is attracted equally by the two chlorine atoms hence there is no separation of charges in the bond formed so the chlorine molecule is non polar.
In case of HCl the bond is formed between two different atoms that is hydrogen and chlorine with enough electro negativity difference so that the shared electron pair is attracted towards more electronegative chlorine atom which acquires partial negative charge while the hydrogen atom acquires partial positive charge hence HCl is a polar molecule.
(e) Metals have low ionization energy due to which they can lose their outermost electrons easily to form positive metallic ions hence metals are electropositive.
For example Sodium metal always form Na⁺ ions, Potassium forms K⁺ ions etc.

(a) (i) when the electro negativity difference between the two atoms is high then the bond formed will be purely ionic.
(ii) When the electro negativity difference between the two atoms is low then the bond formed will be polar covalent bond.
(iii) When the electro negativity difference between the two atoms is zero then the bond formed will be purely covalent.
(b) Ionic compounds = NO, NH₄Cl,  NH₄NO3

    Covalent compounds = N₂, NH₃, NO
    Polar compounds = NCl₃


Concept Insight:  Electro negativity difference between the bonded atoms determines the ease of transfer of electrons between the atoms. On the basis of extent of transfer of electrons between the two atoms the bond will be ionic, covalent or polar.

(a) MgCl₂, CaCl₂.
(b) Urea, Glucose.
(c) CH₄, benzene.
(d) SO₂, H₂S
(e) H₂, N₂

The necessary conditions for the formation of covalent molecule are:
Number of valence electrons: Both the participating atoms should have four or more valence electrons in their valence shell.
Equal electro negativities: The combining atoms should have equal electro negativities so that no transfer of electrons takes place.
Equal electron affinities: The combining atoms should also have equal electron affinities i.e. equal attraction for electrons.
Ionization energy: It should be high for both the atoms so that there is no chance of removal of electrons.
High nuclear charge and small inter nuclear distance: Both these conditions favor the formation of covalent bond because during the formation of a covalent bond the electron density gets concentrated between the nuclei of the combining atoms and this electronic charge is responsible for holding the two nuclei together.   
The properties of covalent compounds are:
Nature: They are generally volatile liquids or gases. Some may be gases like urea, sugar etc.
Low melting and boiling points: Since the intermolecular forces of attraction are weak, very small amount of heat energy is required to overcome these forces hence their melting and boiling points are low.
Electrical conductivity:  Since covalent compounds are made up of molecules and not ions, so they do not conduct electricity.
Solubility: These are insoluble in water but soluble in organic solvents.
Ionization in solution: These do not ionize when dissolved in water except some polar covalent compounds like HCl.
Molecular reactions:  These participate in reactions as a molecule so the reactions are called molecular reactions. These are slow reactions.

Coordinate bond: The bond formed between two atoms by a pair of electrons, provided entirely by one of the combining atoms, is called a coordinate bond or dative bond.
 Conditions for the formation of coordinate bond:
One of the two atoms must have at least one lone pair of electrons.
Another atom should be short of at least a lone pair of electrons.

Lone pair: A pair of electrons which is not shared with any other atom is known as the lone pair of electrons.
For example in NH₃, Nitrogen has a lone pair of electrons which is not shared with any hydrogen atom.
Shared pair: A pair of electrons which is shared with other atoms to form a bond is known as shared pair of electrons.
For example in HCl the pair of electrons responsible for bond formation between H and Cl is called shared pair.

(a) Structure of hydronium ion: Hydronium ions (H3O⁺) are formed when a hydrogen ion released from acid combines with water molecules.
Oxygen atom in water molecule attains an octet by forming two single covalent bonds with two hydrogen atoms but it still contains two lone pairs of electrons.
H₃O⁺ is formed when the oxygen atom in water molecule donates a lone pair of electrons to a hydrogen ion thus establishing a coordinate bond between the oxygen atom and the hydrogen ion.

(b) Structure of Ammonium ion: Ammonium ion (NH₄⁺) is formed when a hydrogen ion or proton combines with ammonia molecule. Nitrogen atom in ammonia has a lone pair of electrons on it which it shares with the hydrogen ion which tends to acquire the configuration of helium atom. Thus, a co-ordinate bond results between nitrogen atom and hydrogen ion in ammonium ion.

(a) The forces of attraction between the molecules of covalent compounds are weak because the molecules are neutral. So, they are generally gases or liquids or soft solids.
(b)Covalent compounds have low melting and boiling point because the intermolecular forces of attraction among the molecules of covalent compounds are weak. Hence very small amount of heat energy is required to overcome the attraction between the molecules.
(c)  On the basis of principle like dissolves like we can interpret the insolubility of non polar covalent compounds. Since water is a polar covalent compound that is it has positively and negatively charged ends but the non-polar covalent compounds do not have any kind of charge separation. So water molecules are unable to interact with the molecules of non polar compound and break apart the intermolecular forces of attraction among non-polar molecules making them soluble in water.
(d) Polar covalent compounds are good conductors of electricity because when these are dissolved in water, they ionize and act as electrolyte to produce ions which are responsible for conduction of electricity.
For example polar covalent compound HCl in water behaves as:

These hydronium and chloride ions produced on dissolution of HCl in water are responsible for conduction of electricity.

(a) The electrovalent bond or ionic bond is called heteropolar bond.

(b) When ionic compounds are dissolved in water, their constituent ions get separated; this phenomenon is called ionisation or dissociation.

(c) Ionic compounds generally are soluble in water and insoluble in organic solvents.

(d) Coordinate bond is also called dative bond.

(e) A coordinate bond is represented by an arrow pointing from donor to acceptor atom.

(f) Four single covalent bonds are formed between the carbon and chlorine atoms.

(i) (b)
(ii) (a)

ionises when dissolved in water

(i) solid
(ii) low

(i) Covalent bond.
(ii) Co-ordinate bond

 

Column A	Column B
(i) Sodium chloride (ii) Ammonium ion (iii) Carbon tetrachloride	Ionic bond Covalent and coordinate bond Covalent bond

 

 

HCl is a covalent compound formed by sharing one electron between chlorine and hydrogen. Because chlorine is more electronegative than hydrogen, the shared pair of electrons shifts towards the chlorine atom. So, a partial negative charge (δ-) develops on chlorine and a partial positive charge (δ+) develops on hydrogen. Hence, the covalent bond is polar in nature.

 

 

Carbon tetrachloride	Sodium chloride
It is insoluble in water but dissolves in organic solvents.	It is soluble in water but insoluble in organic solvents.
It is a non-conductor of electricity due to the absence of ions.	It does not conduct electricity in the solid state but conducts electricity in the fused or aqueous state.

 

 

Dative or coordinate bond

(c) insoluble in water

(b) Ammonium chloride

b. D

a. B

(a) Ions, Na⁺ and OH^-

(b) Ions, H⁺ and OH^-

(c) Molecules, C₁₂H₂₂O₁₁

(d) Ammonium chloride

Atom	Electronic configuration	Nearest noble gas	To attain stable electronic configuration of a nearest noble gas
Carbon	126C [2,4]	Neon [2,8]	Carbon needs four electrons to complete the octet.
Hydrogen	11H [1]	Helium [2]	Hydrogen needs one electron to complete the duplet.

 

One atom of carbon shares four electron pairs, one with each of the four atoms of hydrogen.

 

 

Before combination (4 [H] atoms and 1 [C] atom)	After combination (CH4 - Methane molecule)

 

 

(a) Y + 3e^-→ Y^3-

(b) 3X +Y₂ → X₃Y₂

(a) Covalent bonding

(b) Reduction

(a) 1. Electrovalent bond

2. Covalent bond

(b) 1. XZ

2. WX

(b) Mainly ions

(a) 17

 

(a) The compound that does not have a lone pair of electrons is carbon tetrachloride.

(c) Always soluble in water

(a) Polar covalent bond

(b) Ionic bond

(a)  

Formation of carbon tetrachloride  

 

 

 

 

 

(b)  

Electron dot structure of magnesium chloride: