What is an ion and how is it different from an atom?

#"An ion is a species with an excess or deficiency of electrons."#

A given atom, say #"chlorine"# or #"sodium"# has a given number of protons, fundamental positively charged NUCLEAR particles, which is given by #Z#, the atomic number. For #Cl#, #Z=17#; for #Na#, #Z=11#. Chlorine atoms tend to gain electrons to form negatively charged ions with 18 electrons (the difference between protons and electrons determines the charge, how?); sodium tends to lose an electron to form #Na^+#. Because the nuclear charge remains the same in each case, the element is still sodium or chlorine, but now we have #Na^+#, and #Cl^-# ions.

But because there is now a mismatch between NUCLEAR charge and ELECTRONIC charge, an ion results.............. Most elements form ions of different sorts, which may be predicted on a Periodic basis.

An atom is neutral when it has equal numbers of protons and electrons.

An ion is an atom that is no longer neutral because it has either lost or gained one or more electrons. When an atom loses one or more electrons, it becomes a positively charged ion, called a cation . When an atom gains one or more electrons, it becomes negatively charged, called an anion .