Chemical Bonding: Why Atoms Stick Together
3.1. Why Do Atoms Form Chemical Bonds?
Tendency to Decrease Energy
Atoms have a natural tendency to decrease their energy. They can achieve this by combining with other atoms, a process that inherently increases their stability.
(Forms Bond)
↓It’s a fundamental principle: lower energy means greater stability. Atoms bond to achieve this more stable state.
Discovery of the Cause of Stability
Early chemists pondered how atoms achieve this lower energy state. The answer became clear with the discovery of the noble gases (He, Ne, Ar, Kr, Xe).
(Noble gases are unreactive due to their stable electron configuration.)
- Helium has two electrons in its outer shell.
- All other noble gases have eight electrons in their outermost shells.
- These gases do not combine with themselves or with other atoms, indicating their inherent stability.
Duplet and Octet Rule
This observation led to a fundamental principle of chemical bonding:
Duplet Rule
Having two electrons in the outermost shell (like Helium) implies stability.
Octet Rule
Having eight electrons in the outermost shell implies stability.
This principle was named as the Duplet or Octet Rule. Atoms form bonds to achieve this stable electron configuration, thereby lowering their energy.
Formation of Bonds Based on Energy Favorability
Atoms seek the energetically easier path to complete their duplet or octet.
Example: Sodium Atom (Na)
For Sodium, it’s easier to lose one electron to achieve a stable octet (2,8) than to gain seven electrons. Sodium chooses the energetically favorable path.
Example: Hydrogen Atom (H)
OR
For Hydrogen, it is energetically favorable to either lose one electron to become a proton (H⁺) or gain one electron to become a hydride ion (H⁻), completing its duplet.
Alkali and alkaline earth metals are therefore expected to be electropositive metals, which will form bonds with electronegative elements of 6th and 7th groups (due to their tendency to gain electrons).
Important Note: Although in the beginning, the octet rule played a significant role in understanding the nature of chemical bonds, further investigations found it to be less important (as exceptions exist).