Why aren't Chlorine-35 and Chlorine-37 two different elements?
Chlorine-35 and chlorine-37 are not different elements because an atom of chlorine-35 and an atom of chlorine-37 each contain the same number of protons. The number of protons an atom has, also known as the atom's atomic number, determines which element it is. All atoms which contain 17 protons are called chlorine atoms. Adding or removing a proton from an atom's nucleus changes that atom's atomic number and creates a different element.
Chlorine-35 and chlorine-37 are both isotopes of the element chlorine. The number after the name 'chlorine' is called the mass number. The mass number is a tally of the number of protons and the number of neutrons in an atom's nucleus. Since all atoms of chlorine contain 17 protons, chlorine-35 and chlorine-37 differ in the number of neutrons each one has. An atom of chlorine-35 contains 18 neutrons (17 protons + 18 neutrons = 35 particles in the nucleus) while an atom of chlorine-37 contains 20 neutrons (17 protons + 20 neutrons = 37 particles in the nucleus).
Adding or removing a neutron from an atom's nucleus creates isotopes of a particular element. Why does changing the number of protons in an atom change which element that atom is but changing the number of neutrons doesn't? Protons carry a positive charge. Each proton in an atom's nucleus must be balanced with a negatively charged electron in one of the 'shells' outside the nucleus. The number of electrons in an atom's outer shell determines the atom's chemical properties. Adding or removing protons changes the number and arrangement of electrons in the outer shell which changes how that atom reacts with other atoms. Neutrons don't change things greatly because they do not carry an electrical charge. Neutrons can be added or removed from an atom and the electrons around the atom really don't care that much.
Steve Gagnon, Science Education Specialist (Other answers by Steve Gagnon)