Final answer:
Helium atoms have a full outer energy level with two electrons in the 1s subshell and require no additional electrons for stability. This meets its maximum capacity for the first and only shell, making helium very stable with an electron configuration of 1s².
Step-by-step explanation:
Helium needs no additional valence electrons to have a filled outermost energy level. In its stable form, helium naturally has two electrons. These two electrons occupy helium's first and only energy level, known as the 1s subshell. Since s subshells can hold up to two electrons, the electron configuration for helium is 1s². This configuration indicates a full valence shell, making helium a very stable element. Thus, under standard conditions, helium has a complete outer shell and does not need extra electrons to achieve stability. The octet rule, which applies to larger atoms with more than one shell, suggests that atoms are most stable with eight electrons in their outer shell, but this rule does not apply to helium, as its full capacity is two electrons.