A long time ago, a Greek scientist named Aristotle believed that there were only four elements: earth, water, air, and fire. We now know that there are over 100 chemical elements. Why do you think Aristotle's elements (earth, water, fire and air) are NOT considered to be chemical elements as we know them today? 

Hi (warning change the wording or you may get charged for plagiarism!)

The idea of the atom is dated back way back to Aristotle's day as you know. But you may not have known that there were many ideas about what the atom would be. some believed that there was only 1 fundamental particle, others however believed that there was no atom at all and that you could just keep cutting the cake forever and never stop. Aristotle's four elements (fire earth air and water) where believed to be fundamental units that everything was made of. Different things such as would and glass would be a combination of earth fire and air perhaps. You may see now how different this idea is from the atom nowadays. there are magnitudes of different types of atoms. the 118 elements you know of are only a fraction of the thousands of isotopes (an isotope is an atom with slightly more or less nuetrons than the normal one with as many nuatrons as protons).

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.