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Atoms with a positive or negative charge.

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The incredible green lights in this cold northern sky consist of charged particles known as ions. Their swirling pattern is caused by the pull of Earth’s magnetic north pole. Called the northern lights, this phenomenon of nature shows that ions respond to a magnetic field. Do you know what ions are? Read on to find out.

Atoms Are Neutral

The northern lights aren’t caused by atoms, because atoms are not charged particles. An atom always has the same number of electrons as protons. Electrons have an electric charge of -1 and protons have an electric charge of +1. Therefore, the charges of an atom’s electrons and protons “cancel out.” This explains why atoms are neutral in electric charge.

Q: What would happen to an atom’s charge if it were to gain extra electrons?

A: If an atom were to gain extra electrons, it would have more electrons than protons. This would give it a negative charge, so it would no longer be neutral.

Atoms to Ions

Atoms cannot only gain extra electrons. They can also lose electrons. In either case, they become ions. Ions are atoms that have a positive or negative charge because they have unequal numbers of protons and electrons. If atoms lose electrons, they become positive ions, or cations. If atoms gain electrons, they become negative ions, or anions. Consider the example of fluorine (see Figure below). A fluorine atom has nine protons and nine electrons, so it is electrically neutral. If a fluorine atom gains an electron, it becomes a fluoride ion with an electric charge of -1.

Some Common Ions
Cations Anions
Name of Ion Chemical Symbol Name of Ion Chemical Symbol
Calcium ion Ca2+ Chloride Cl-
Hydrogen ion H+ Fluoride F-
Iron(II) ion Fe2+ Bromide Br-
Iron(III) ion Fe3+ Oxide O2-

Q: How does the iron(III) ion differ from the iron(II) ion?

A: The iron(III) ion has a charge of +3, so it has one less electron than the iron(II) ion, which has a charge of +2.

Q: What is the charge of an oxide ion? How does its number of electrons compare to its number of protons?

A: An oxide ion has a charge of -2. It has two more electrons than protons.

Q: Why do you think atoms lose electrons to, or gain electrons from, other atoms?

A: Atoms form ions by losing or gaining electrons because it makes them more stable and this state takes less energy to maintain. The most stable state for an atom is to have its outermost energy level filled with the maximum possible number of electrons. In the case of metals such as lithium, with just one electron in the outermost energy level, a more stable state can be achieved by losing that one outer electron. In the case of nonmetals such as fluorine, which has seven electrons in the outermost energy level, a more stable state can be achieved by gaining one electron and filling up the outer energy level. You can learn more about why ions form by watching the video at this URL:

http://www.youtube.com/watch?v=CV53wfl-oV8 (9:35)



  • Atoms have equal numbers of positive protons and negative electrons, so they are neutral in electric charge.
  • Atoms can gain or lose electrons and become ions, which are atoms that have a positive or negative charge because they have unequal numbers of protons and electrons.
  • The process in which an atom becomes an ion is called ionization. It may occur when atoms are exposed to high levels of radiation or when atoms transfer electrons to or from other atoms.
  • Ions are reactive, attracted or repulsed by other charged particles, and deflected by a magnetic field.


  • ion: Positively or negatively charged form of an atom that has lost or gained electron(s).


At the following URL, scroll down to the middle of the page and download “Ion Worksheet.” Then fill in the missing information in the worksheet.



  1. Why are atoms neutral in electric charge?
  2. Define ion.
  3. Compare and contrast cations and anions, and give an example of each.
  4. Describe how ions form.
  5. List properties of ions.
  6. The model in the Figure below represents an atom of lithium (Li). If the lithium atom becomes an ion, which type of ion will it be, a cation or an anion? What will be the electric charge of this ion? What will the ion be named? What symbol will be used to represent it?

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