Why are noble gases, like helium and krypton, unlikely to form a bond? A. They do not have valence electrons and therefore cannot form bonds by sharing or trading
electrons.
B. They always have an equal number of protons and electrons.
C. They have full valence
orbitals and already have low energy.
D. They are nonmetals and gases and therefore their ionization energy is too high to form a bond.

The valence electrons are the only parts of an atom that make chemical bonds. Noble gases don't form chemical bonds because they have a full outer shell (8 electrons).

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.