all of these can form

Each Atom Can Make a Defined Number of Covalent Bonds

Electrons move around thenucleusof an atom in clouds calledorbitals,which lie in a series of concentricshells,or energy levels; electrons in outer shells have more energy than those in inner shells. Each shell has a maximum number of electrons that it can hold. Electrons fill the innermost shells of an atom first; then the outer shells. The energy level of an atom is lowest when all of its orbitals are filled, and an atom’s reactivity depends on how many electrons it needs to complete its outermost orbital. In most cases, in order to fill the outermost orbital, the electrons within it form covalent bonds with other atoms. Acovalent bondthus holds two atoms close together because electrons in their outermost orbitals are shared by both atoms.

Most of the molecules in living systems contain only six different atoms: hydrogen, carbon, nitrogen, phosphorus, oxygen, and sulfur. The outermost orbital of each atom has a characteristic number of electrons:

all of these form covalent bond