AP BIOLOGY

Detailed explanation-1: -In general, atoms are most stable, least reactive, when their outermost electron shell is full. Most of the elements important in biology need eight electrons in their outermost shell in order to be stable, and this rule of thumb is known as the octet rule.

Detailed explanation-2: -The stability of atoms depends on whether or not their outer-most shell is filled with electrons. If the outer shell is filled, the atom is stable. Atoms with unfilled outer shells are unstable, and will usually form chemical bonds with other atoms to achieve stability.

Detailed explanation-3: -The properties of an element are determined by its outermost electrons, or those in the highest energy orbital. Atoms that do not have full outer shells will tend to gain or lose electrons, resulting in a full outer shell and, therefore, stability.

Detailed explanation-4: -Group 18 elements (helium, neon, and argon are shown) have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially-filled valence shells and gain or lose electrons to achieve a stable electron configuration.

Detailed explanation-5: -Atoms that have less than eight electrons in their outer shell are unstable. Helium, however, is stable even though it has less than eight electrons, since its outer shell is full with two electrons.