Unit 3: Chemical Bonding — Long Questions

Formation of Ionic Bond
In sodium chloride (NaCl) crystal, the ions arrange themselves in specific three - dimensional structure known as face-centered cubic lattice and rock salt structure .This compound is formed when the elements sodium and chlorine react chemically. The electronic configurations of these elements are as
1st shell 2nd shell 3rd shell
11Na 2 8 1
17Cl 2 8 7

Transfer of electrons
An electron from the outermost shell of sodium atom is transferred to the outermost shell of chlorine atom and in doing so, both these atoms acquire the electronic configurations of their nearest noble gases.
In this way, ionic bond is formed between Na⁺ ion and Cl⁻ ion.

Na Na⁺ + e⁻
e⁻ + Cl Cl⁻
Na + Cl NaCl

[Diagram showing sodium atom bonding with chlorine atom to form sodium ion and chloride ion]

Formation of Covalent bond Definition
A covalent bond is formed by the mutual sharing of an electron pair provided by the bonded atoms. This is called a covalent bond.
Types of Covalent bonds:
It is classified into three types
i. Single Covalent bond (-):
In some compounds the atoms share one electron each to form a single covalent bond. A single covalent bond is represented by a single line (-)
For Example:

[Diagrams showing H-H bonding and Cl-Cl bonding]

ii. Double Covalent bond (=):
In some compounds, the atoms share two electrons each to form a double covalent bond. A double covalent bond is represented by two lines (=)
For Example:

[Diagram showing O=O bonding]

iii. Triple Covalent bond (≡):
In some compound, the atoms share three electrons each to form a triple covalent bond. A triple covalent bond is represented by three lines (≡).
For Example:

[Diagram showing N≡N bonding]

In sodium chloride (NaCl) crystal, the ions arrange themselves in specific three - dimensional structure known as face-centered cubic lattice and rock salt structure. This compound is formed when the elements sodium and chlorine react chemically. The electronic configurations of these elements are as
1st shell 2nd shell 3rd shell
11Na 2 8 1
17Cl 2 8 7

Transfer of electrons
An electron from the outermost shell of sodium atom is transferred to the outermost shell of chlorine atom and in doing so, both these atoms acquire the electronic configurations of their nearest noble gases.

Na Na⁺ + e⁻
e⁻ + Cl Cl⁻
Na + Cl NaCl

[Diagrams showing crystal lattice of NaCl and crystal lattice of CaCl2]

Metals have several distinct properties that can be explained by the nature of metallic bonds.

i. Electrical Conductivity: The free-moving electrons in metals allow them to conduct electricity easily. When a voltage is applied, these electrons can flow, carrying electric current through the metal.

ii. Thermal Conductivity: Metals are also good conductors of heat. The free electrons can transfer thermal energy quickly throughout the metal, making it efficient for heat transfer.

iii. Malleability and Ductility: Metals can be hammered or rolled into thin sheets (malleability) and drawn into wires (ductility) without breaking. When stress is applied this is because the layers of atoms in a metal can slide over each other while maintaining the metallic bond due to the presence of the sea of electrons, which allows the structure to remain intact.

iv. Metallic Luster: Metals have a shiny appearance, known as luster. This is due to the ability of the free electrons to reflect light, giving metals their characteristic shine.

v. High Melting and Boiling Points: The strong metallic bonds between the atoms require a significant amount of energy to break, resulting in high melting and boiling points for most metals.

Ionic Compounds Covalent Compounds

i. Nature:
In ionic compounds oppositely charged ions are properly arranged to give a crystalline structure. As a whole the compound is neutral. There exists a strong electrostatic force between their ions.

i. Nature:
Covalent compounds mostly exist as discrete neutral molecules. There exists a strong electrostatic attraction between the two nuclei and the shared electrons.

ii. Melting and Boiling Points:
Ionic compounds are usually solids having high melting and boiling points.
e.g:
The melting point of sodium chloride, is 801°C because it is difficult to break the strong electrostatic forces of attraction between the oppositely charged ions.

ii. Melting and Boiling Points:
Covalent compounds are made of two or more non-metals. Lower molecular mass covalent compounds are gases or low boiling liquids. High molecular mass covalent compounds exist as solids. Generally, they have lower melting and boiling points.

iii. Solubility:
Ionic compounds are generally soluble in polar solvent like water

iii. Solubility:
They are usually insoluble in water but soluble in non-polar solvents like ether, benzene and acetone.

iv. Conductivity:
They are usually good conductor of electricity in molten state or in aqueous solution form. Their conductance is due to the presence of free ions.

iv. Conductivity:
They are usually bad conductor of electricity

Graphite is an allotropic form of carbon that exhibits electrical conductivity which can be explained by its unique structure and bonding.

Arrangement In graphite, each carbon atom is bonded to three other carbon atoms in a planar hexagonal arrangement forming layers of graphene. The fourth electron from each carbon atom is not involved in bonding and is free to move within the layer.

Electrical Conductivity When it comes to electrical conductivity, the free-moving electrons within each layer can carry an electric current. However, graphite's conductivity is primarily in the plane of the layers which means that it conducts electricity well along the layers but not as effectively perpendicular to them.

Delocalized Electron/ Electrode formation The electrical conductivity of graphite is due to the presence of delocalized electrons that can move freely within the layers of carbon atoms, allowing it to conduct electricity efficiently along those layers.

The strength of a metallic bond depends upon two factors:
i. The number of positive charges present on the positive ions
ii. The number of mobile electrons set free by each atom

Explanation Each sodium atom sets free only one electron. The metallic bond in sodium metal is therefore, not very strong. In magnesium metal, each magnesium atom releases two electrons to acquire two positive charges. The metallic bond in magnesium metal will evidently be stronger than that in sodium metal. This explains why the magnesium metal melts at a higher temperature than sodium metal.

Conductivity of Metals The presence of freely moving electrons in metals make them good conductor of heat and electricity. Moreover, in metals, the atoms are strongly held and arranged in the form of rows one above the other. This arrangement makes them hard and heavy.

In the vapour phase, aluminum chloride AlCl₃ exists as a dimer, represented by the formula Al2Cl6. This means that two AlCl₃ molecules combine to form a larger molecule. The bonding between the two AlCl₃ units in this dimeric form involves coordinate covalent bonds, resulting a stable structure (achieve a stable electronic configuration.

i. Lewis Structure: In the AlCl₃ molecule, the aluminum atom has three valence electrons and forms three covalent bonds with three chlorine atoms. However, aluminum is electron-deficient because it has only six electrons in its outer shell, which is less than the stable octet configuration.

ii. Dimer Formation: To achieve a more stable configuration, two AlCl₃ molecules can come together. One aluminum atom from one AlCl₃ donates an empty p-orbital to accept a pair of electrons from the chlorine atom of the other AlCl₃. This forms a coordinate covalent bond, where one atom provides both electrons for the bond.

iii. Resulting Structure: The dimer Al2Cl6 consists of two aluminum atoms and six chlorine atoms. Each aluminum atom is surrounded by four chlorine atoms: three from its own AlCl₃ unit and one that is shared with the other aluminum atom. This sharing of electrons stabilizes the dimer.

iv. Bonding Characteristics: The dimeric structure allows Al2Cl6 to achieve a more stable configuration than individual AlCl₃ molecules. This dimerization is favoured in the vapour phase due to the electron-deficient nature of aluminum and the ability of chlorine to donate electron pairs.

Silicon dioxide commonly known as sand ( SiO₂ ) has a unique structure that is essential to its properties. The structure of SiO₂ can be described as a three-dimensional network solid which means that it consists of a continuous framework of silicon and oxygen atoms.

i. Basic Units: The fundamental unit of SiO₂ is the silica tetrahedron. Each silicon atom is covalently bonded to four oxygen atoms, forming a tetrahedral shape. The silicon atom is at the center and the four oxygen atoms are located at the corners of the tetrahedron.

ii. Tetrahedral Arrangement: In the solid state, these tetrahedral are linked together in a three-dimensional network. Each oxygen atom in a tetrahedron is shared with neighboring silicon atoms in adjacent tetrahedra. This sharing creates a strong and stable structure.

iii. Bonding: The covalent bonds between silicon and oxygen are strong, which contributes to the hardness and high melting point of sand. The arrangement of these tetrahedral creates a rigid framework that is characteristic of network solids.

iv. Properties: The three-dimensional network structure of SiO₂ gives sand its characteristic properties, such as high strength, chemical stability, and resistance to weathering. These properties make it an important material in construction and various industrial applications.

Definition Coordinate covalent bond is a type of covalent bond in which the shared electron pair is donated by one atom only. This bond is formed when a species has an electron pair to donate to another species.

Donor The species which donates the electron pair, is called a donor.

Acceptor This species that accepts the electron pair is called an acceptor.

Representation An arrow head (→) pointing towards the acceptor represents this type of bond.

Example

i. Hydronium ion (H3O⁺)
Acids provide protons (H⁺) when dissolved in water. This proton has an empty outer shell and can accept one of the two pairs of electrons present on the oxygen atom in water molecule. As a result of this, a hydronium ion (H3O⁺) is formed.

:O: + H⁺ → [:O → H]⁺
H H H
Hydronium ion

The positive charge covers whole of the hydronium ion. After the formation of hydronium ion, there does not remain any difference between a coordinate covalent bond and a covalent bond. All the three bonds of oxygen behave exactly alike.

(i) Water
A water molecule is formed when two hydrogen atoms share their electrons separately with the electrons of one oxygen atom.

[Diagrams showing water molecule formation]

(ii) Carbon dioxide
A carbon dioxide molecule is formed when an atom of carbon shares its four electrons with two oxygen atoms. Each oxygen atom also shares two electrons.

[Diagrams showing carbon dioxide molecule formation]

[Note: This answer continues beyond page 7]