In chemistry, the main difference between an atom and an ion is that an atom is a neutral particle, while an ion has a positive or negative electrical charge.
Atoms
An atom is basic building block of matter made of protons, neutrons, and electrons. The identity of an element is determined by the number of positively charged protons in the atom’s nucleus.
Usually, when chemists talk about atoms, they mean neutral particles that contain the same number of protons and electrons. However, a broader definition of an atom includes all possible variations in the number of neutrons (isotopes) and electrons (ions). Under this broader definition, an atom can be an ion, but not all ions are atoms. The difference between an atom and an ion has to do with net electrical charge.
Examples of atoms include hydrogen (H), helium (He), iron (Fe), and any other element on the periodic table.
Ions
An ion is an atom or collection of atoms with a net positive or negative charge.
When electrons are added or removed, the neutral atom becomes an ion. If electrons are removed, the net charge of the ion is positive and it is known as a cation. When electrons are added, the net charge of the ion is negative and it is known as an anion.
Ions can also be collections of atoms. Polyatomic ions form when atoms combine, but the chemical bonds between the atoms don’t balance the electrical charge.
Here are some examples of ions:
- Fluoride ion: F– – This anion has 9 protons and 10 electrons.
- Sodium ion: Na+ – This cation has 11 protons and 10 electrons.
- Copper ions: Cu+ and Cu2+ – These cations have 29 protons and 28 and 27 electrons, respectively.
- Ammonium ion: NH4+ – This ion contains one nitrogen and four hydrogen atoms with a net charge of +1.
- Sulfate ion: SO42- This ion contains one sulfur atom and four oxygen atoms with a net charge of -2.
- Nitrite ion: NO2– This ion contains one nitrogen atom and two oxygens for a net charge of -1.
For more polyatomic ions, check out this handy table.