Biology 171 2.2: Elements found in living systems

Octet Rule

Rule to describe patterns of chemical bonding in main group elements that require a total of eight electrons to complete their outer electron shell.

• requires one filled s orbital and 3 filled P orbitals (with the exception of helium)

Valence Electrons

An electron in the outermost energy level of an atom

• The chemical behavior of an element reflects how many of these 8 positions are filled

• If it’s almost filled, it will gain electrons, if it only has 1 or 2 it will lose them

Noble gases

elements with 8 electrons in their outer energy level (non reactive)

Halogens

elements with 7 electrons in their outer energy level (very reactive)

Alkali metals

only has 1 electron in outer level (very reactive)

Which 12 elements are found in significant amounts in living systems?

(C, H, O, N, P, S, Na, K, Mg, Fe, Cl, Ca)