Dipole-Dipole Forces: Occur between molecules with permanent dipoles (polar molecules).

Dipole-Induced Dipole Forces: Occur between a polar molecule and a non-polar molecule, where the polar molecule induces a temporary dipole in the non-polar one.

1.2 Driving Forces for Chemical Combination

Achieving Minimum Energy (Potential Energy Minimization):
- As two isolated atoms approach each other, a complex interplay of forces occurs:
  - Attractive forces: Between the nucleus of one atom and the electron cloud of the other atom. These forces lead to a decrease in potential energy.
  - Repulsive forces: Between the nuclei of both atoms (due to positive charges) and between the electron clouds of both atoms (due to negative charges). These forces lead to an increase in potential energy.
- A stable chemical bond forms *only if* the net attractive forces are stronger than the repulsive forces. This results in a net decrease in the potential energy of the system, indicating increased stability.
- The bond length of a stable molecule corresponds to the internuclear distance at which the total potential energy of the system is at its absolute minimum. At this optimal distance, the attractive and repulsive forces are balanced.
Achieving Noble Gas Configuration (The Octet Rule):
- This principle, primarily associated with Kossel and Lewis, posits that atoms tend to react in ways that allow them to attain the electron configuration of the nearest noble gas. This usually means having eight electrons in their outermost valence shell (an octet), which is a particularly stable configuration.
- For the lightest elements, specifically hydrogen (H) and lithium (Li), achieving a duplet ($1s^2$) configuration (like helium) is the goal.
- Crucially, only the valence electrons (electrons located in the outermost electron shell) are involved in chemical bonding. These are often visually represented using Lewis dot symbols.
- Atoms can achieve this stable configuration through:
  - Electron transfer: Leading to the formation of ionic bonds.
  - Electron sharing: Leading to the formation of covalent bonds.

2.0 VALENCY & OCTET RULE

2.1 Valency

Valency is a measure of the combining power of an element. It indicates the number of chemical bonds an atom can typically form.
Historical/Classical Concept: Initially defined by the number of hydrogen atoms an element could combine with (e.g., in $HCl$, Cl has a valency of 1; in $H_2O$, O has a valency of 2) or double the number of oxygen atoms. This concept was purely empirical.
Modern Concept (Electronic Valency): Based on the electronic configuration of atoms and their tendency to achieve noble gas configurations.
- For main group elements in Groups 1, 2, 13, and 14: Valency is generally e