Atoms of the same element that have different masses are called isotopes. Different masses result when there are different numbers of neutrons. All atoms of a particular element have the same number of protons.
To identify the different isotopes of an element, the element symbol is written followed by the number of neutrons. For example, two common isotopes of uranium are written as U-236 and U-238.
An atom typically has the same number of neutrons and electrons as the number of protons. When an atom has a different number of electrons, the atom's electric charge becomes either positive, if fewer electrons are present, or negative, if additional electrons are present. Atoms with a positive or negative charge are called ions. When an atom has a different number of neutrons, because neutrons have no electric charge, the atom's electric charge is not affected. Isotopes do have a different atomic weight, though.
On the periodic table, the atomic weight, or atomic mass, is given for each element. The number is not a whole number, since it reflects the natural abundance of isotopes of an element. For example, hydrogen's atomic mass is 4.003. In chemistry calculations, the atomic mass is used with the assumption that a sample of the element includes the natural range of isotopes for the element.