An atom is the smallest particle of an element that retains the chemical properties of the element. Atoms are electrically neutral, with a positively charged nucleus that binds one or more electrons in motion around it.
An ion-An atom or molecule that has acquired a charge by either gaining or losing electrons. An atom or molecule with missing electrons has a net positive charge and is called a cation*; one with extra electrons has a net negative charge and is called an anion*.
An atom is what you would see on the periodic table e.g. hydrogen and nitrogen are atoms. Atoms have no overall charge and the charge of the electrons is equal to the charge of the protons. (but they are opposite charges- so protons have a +1 charge and electrons have a -1 charge).
An ion is when one or more electrons are either added or removed from an atom to form a charged particle. The atoms are then strongly attracted to each other.
So take, for example, sodium and chlorine atoms. When the atoms react the sodium ion gives up its outer electron and becomes a Na+ ion. This means it has a charge of +1 (due to it losing an electron, the forces are no longer balanced as there is one more proton than electrons). The chlorine gains the electron that sodium lost and becomes a Cl- ion. This meant it has a charge of -1 (due to it gaining an electron there is one more electron than protons, causing to forces to become unbalanced). Both ions then have full outer shells and are attracted to each other due to being oppositely charged.
To sum it up, an atom is what you see on the periodic table and they always have no charge and an ion is when an atom loses or gains at least one electron causing it to have a positive or negative charge.
It has to be intensive, which means it does not depend on the amount of the substance. Boiling point, melting point, and density are three intensive physical properties.
