In very early human history, complex sanitation systems were not needed because settlements were usually very small, like the village shown below. however, as cities became larger and population densities increased, one major advancement was the development of which to channel wastewaters away from human populations more effectively than in the past.

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements

the electron configuration that you gave me has 16 electrons ergo it has 16 protons ergo it is the element sulfur.

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8