Chemical Bonding - Practice Questions & MCQ

From the small number of elements, there are millions of known compounds, and many more are discovered daily. This is because of the fact that there are so many possible ways in which atoms can combine to form molecules. Atoms combine with each other on the basis of electrons they have. In this universe, most of the substances are found in cluster forms or aggregates of atoms. These types of aggregation, in which atoms are held together and which is electrically neutral is called a molecule. The molecules are made of two or more atoms joined together by some force acting between them. This force acting between the atoms is known as a chemical bond. Thus, a chemical bond is defined as a force that acts between two or more atoms to hold them together as a stable molecule or as a force that holds a group of two or more atoms together and makes them function as a unit. For example, in water, the fundamental unit is the H-O-H molecule, which is described as being held together by the two O-H bonds.

Cause of Chemical Combination

Atoms combine with each other on the basis of the following reasons:

(i) Decrease in energy: All systems in the universe tend to lose potential energy and achieve more stability. It is an observed fact that a bonded state is more stable than unbonded state because bonded state has lower potential energy than unbonded state. Thus when two atoms approach each other, they combine only under the condition that there is a decrease in potential energy. When two atoms approach each other, new kinds of forces of attraction and repulsion starts acting. These forces are:

1. Electrons and nuclei attract each other: Attractive forces are always energetically favourable, thus an electron attracted to a nucleus is of lower energy and therefore more stable than a free electron. 

2. Electrons repel each other: Because of this repulsion, the energy is raised and the stability reduces.

3. Nuclei repel each other: The repulsion exists between the nuclei also and this also reduces the stability.

Among all these above forces, If the net result is the attraction, then the total potential energy of the system decreases and a chemical bond formation takes place. No chemical bonding is possible if the net result is repulsion.

(ii) Lewis Octet Rule: The atoms of all elements during the bond formation try to attain the stable noble gas configuration, i.e., they try to obtain either 2 electrons (when only one energy shell) or 8 electrons in their outermost energy level which is of maximum stability and hence of minimum energy. Thus, the tendency of atoms to achieve eight electrons in their outermost shell is known as the Lewis octet rule. The octet rule is the basis of the electronic theory of valency. All the noble gases like helium, neon, etc. are not active towards the bond formation because of their already filled outermost shell, in other words, their octet is already complete and thus these elements do not need to combine with other elements to complete its octet.

Lewis symbol of elements

For explaining the formation of bonds, the lewis symbol representation of atoms is necessary. To write Lewis symbol for an element, we write down its symbol surrounded by a number of dots that are equal to the number of valence electrons. Paired and unpaired valence electrons are also indicated. The Lewis symbols for some of the elements like Chlorine, Aluminium, and Argon are mentioned below:

A type of bonding that results from the mutual attraction of atoms for a shared pair of electrons. are known as covalent bonds. Covalent bonds are formed between two atoms when both have similar tendencies to attract electrons to themselves. For example, two hydrogen atoms bond covalently to form an H₂ molecule; each hydrogen atom in the H₂ molecule has two electrons stabilizing it, giving each atom the same number of valence electrons as the noble gas He. Compounds that contain covalent bonds exhibit different physical properties than ionic compounds. Because the attraction between molecules, which are electrically neutral, is weaker than that between electrically charged ions, covalent compounds generally have much lower melting and boiling points than ionic compounds. In fact, many covalent compounds are liquids or gases at room temperature, and, in their solid states, they are typically much softer than ionic solids. Furthermore, whereas ionic compounds are good conductors of electricity when dissolved in water, most covalent compounds are insoluble in water; since they are electrically neutral, they are poor conductors of electricity in any state.

Formation of Covalent Bonds

Nonmetal atoms frequently form covalent bonds with other nonmetal atoms. For example - the hydrogen molecule, H₂, contains a covalent bond between its two hydrogen atoms. The figure given below shows the explanation of this bond. Starting on the far right, we have two separate hydrogen atoms with a particular potential energy, indicated by the red line. Along the x-axis is the distance between the two atoms. As the two atoms approach each other their valence orbitals (1s) begin to overlap. The single electrons on each hydrogen atom then interact with both atomic nuclei, occupying the space around both atoms. The strong attraction of each shared electron to both nuclei stabilizes the system, and the potential energy decreases as the bond distance decreases. If the atoms continue to approach each other, the positive charges in the two nuclei begin to repel each other, and the potential energy increases. The bond length is determined by the distance at which the lowest potential energy is achieved.

The potential energy of two separate hydrogen atoms (right) decreases as they approach each other, and the single electrons on each atom are shared to form a covalent bond. The bond length is the internuclear distance at which the lowest potential energy is achieved.

Coordinate Bonds

It is a special type of covalent bond in which both the shared electrons are contributed by one atom only. Such a bond is also known as dative bond. A coordinate or a dative bond is established between two such types of atoms, out of which one has a complete octet and while the other is short of a pair of electrons. This bond is represented by “→”.

The atom which donates the electron pair is called the donor while the atom which accepts the electron pair is called acceptor. The compounds in which the coordinate bond exist are known as complex or coordination compounds. Some example include [Pt(en)₂]CO₃, [Ni(H₂O)₆]Cl₂, etc.

Characteristics of Coordination Compounds

The main properties of the coordination compounds are mentioned below:

• Melting and boiling points: The melting and boiling points of these compounds are higher than purely covalent compounds but lower than purely ionic compounds.

• Solubility: These compounds are sparingly soluble in polar solvents like water but readily soluble in non-polar solvents.

• Stability: The stability of these compounds are similar to the covalent compounds.

• Conductivity: Like covalent compounds, these are also bad conductors of electricity.

• Dielectric constant: The compounds containing coordinate bond have high values of the dielectric constants.