Class 11th

4.5.  Kössel and Lewis developed an important theory of chemical combination between atoms known as electronic theory of chemical bonding. According to this rule, atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells. This is known as octet rule.

Significance: It helps to explain why different atoms combine with each other to form ionic compounds or covalent compounds.

Limitations of Octet rule:

According to Octet rule, atoms take part in chemical combination to achieve the configuration of the nearest noble gas elements. However, some of noble gas elements like Xenon have formed compounds with fluorine and oxygen. For example: XeF₂, XeF₄  Therefore, validity of the octet rule has been challenged. This theory does not account for shape of molecules.

Stability of odd electron species: Molecules like NO and NO₂ do not satisfy the octet rule and yet they exist as stable molecules. The octet rule failed to explain the reason for this.

Expansion of octet: The rule could not explain the stability of compounds which have more than 8 valence electrons around the central atom. For eg SF₆, PF₅, IF₇

Electron deficient compounds: The octet rule could not explain the stability of electron deficient compounds like LiCl, AlCl₃, BH₃

4.4. 

The properties of molecules and compounds are determined by their molecular structure, which is the arrangement of the atoms in space. The molecular structure is determined by the type of bond and the number and arrangement of atoms in the molecule. The shape of the molecule affects its polarity, reactivity, and physical properties such as boiling point, melting point, and solubility.

The valence shell electron pair repulsion (VSEPR) theory is a model used to predict the molecular structure of covalent molecules based on the repulsion between valence electron pairs. This theory states that the electron pairs in the valence shell of an atom repel each other and try to occupy positions that minimise repulsion. The VSEPR theory is used to predict the geometry and shape of molecules, such as linear, trigonal planar, tetrahedral, and octahedral.

In summary, chemical bonding and molecular structure are important concepts in chemistry that help us understand the behaviour and properties of molecules and compounds. The type of bond and molecular structure determine the properties of a substance, including its reactivity, polarity, and physical properties. The VSEPR theory is a useful tool for predicting the molecular structure of covalent molecules based on the repulsion between valence electron pairs.

3.57. Ionisation enthalpy depends on the following factors:

1. Atomic size: The ionization enthalpy decreases with an increase in atomic size because with the increase in the atomic size, the number of electron shells increases. Therefore, the force that binds the electrons to the nucleus decreases.

2. Nuclear charge: The ionization enthalpy increases with the increase in the magnitude of the nuclear charge. This is because as the magnitude of the positive charge on the nucleus of an atom increases, the attraction with the electrons also increases.

3. Screening or shielding effect: Greater the magnitude of the screening effect less will be the value of ionization enthalpy.