Isotopes (from Greek words meaning "same position") refer to the different versions of a given element. They occupy the "same position" of an element in the Periodic Table, and are chemically identical. Each element is composed of a number of positively charged protons which are surrounded by negatively charged electrons. Hydrogen, being the simplest element, is composed of a single electron moving around a single proton. However, elements may possess various number of neutrons in the nucleus of the atom; the various number of neutrons determine the various number of isotopes (some elements have many isotopes, some have none.) Each isotope has a different atomic weight, since the nucleus may have different quantities of neutrons in the nucleus. In 1816, an English physician named William Prout (1785-1850) suggested that all atoms were constructed of multiple units of the hydrogen atom. However, his theory was disregarded; as atomic weights began to be determined, many elements had fractional weights instead of whole numbers as the theory required. Around the turn of last century chemists and physicists began experimenting with radioactivity, and began to realize that elements can have multiple forms, and therefore multiple weights, varying only by the number of neutrons, and it was this variation that caused the fractional weights in the atomic weight. Once the neutrons were considered, Prout's theory was vindicated, as each element and each of its isotopes all have integer weights.
"The New Intelligent Man's Guide to Science," I. Asimov, Basic Books, Inc., 1965, pg. 258.
Atoms are made of three different types of particles. Protons and neutrons are in the nucleus; electrons are found in outer shells, circling the nucleus. An atom of any element has a set number of protons; this helps determine, among other things, the element's position in the periodic table. But the number of neutrons can vary, and this determines different isotopes of the element. Some isotopes of some elements are radioactive--they break down into other isotopes of the element, and emit radiation.
remember that all atoms are made up of protons, neutrons, and electrons.
we know that proton and electrons ahve to be the same for the atom to be balanced
but when the nutrons change and the protons and electrons stay the same, that is called an isotope
chem. X chem Y
protons: 9 9
in this case, chem y is an isotope of chem X since only neutrons differ.