Here's why:

* Valence Electrons: Electrons in the outermost energy level of an atom, called valence electrons, are the ones primarily involved in chemical bonding. They are the "loose" electrons that can be shared or transferred to form bonds.

* Core Electrons: Electrons in inner energy levels are more tightly bound to the nucleus and generally don't participate in bonding. They are shielded from interaction with other atoms.

Example:

* Carbon (C): Carbon has 6 electrons (2 in the first shell, 4 in the second). The 4 electrons in the second shell are valence electrons and can form bonds. The 2 electrons in the first shell are core electrons and are not involved in bonding.

Exceptions:

* Transition Metals: Transition metals have a more complex electron configuration, and some of their inner shell electrons can participate in bonding under certain circumstances.

In summary: Only the valence electrons of an atom are typically available for bonding. Core electrons remain tightly bound to the nucleus and are not involved in chemical interactions.