Why are the valence electrons of an atom the only electrons likely to be involved in bonding to other atoms?

Because valence electrons are on the outermost energy level; meaning they are farthest away from their nucleus. The protons in the nucleus try to hold onto the electrons in the energy levels, but the valence electrons have a weaker grip to the nucleus than the others due to the distance between them, and are thus the easiest to be involved in bonding to other atoms.

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