Final answer:
Noble gases are stable because they have a full outer electron shell.
Step-by-step explanation:
Noble gases are stable due to their full outer electron shells. They have a complete set of valence electrons, which gives them high stability and makes them unreactive. The full electron shells prevent them from easily gaining or losing electrons, which is a characteristic of other elements. For example, helium (He) has 2 valence electrons, which is already a full shell, while neon (Ne) has 8 valence electrons, which is also a full shell.
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