The properties of an element can be predicted to some extent by its position in the Periodic Table. In the table, each element is represented by a symbol. The number of protons in an atom is called the atomic number , and by convention the atomic number is listed at the lower left of the symbol as a subscript. Isotopes are forms of an element with different atomic masses that occur because the number of neutrons in the nucleus can vary without essentially changing chemical properties of the element. In other words, isotopes behave exactly alike in terms of how they interact electrically with other elements, even though their masses differ, because neutrons are electrically neutral.
Most elements have two or more isotopes.
Most hydrogen atoms are made of one proton and one electron. However, in nature, about 1 out of every 6,500 hydrogen atoms has a neutron as well as a proton in its nucleus.