Ions, Ionic Compounds, and Formulas

Our work today introduced students to the concept of ions: atoms with positive or negative charge.  Up to now, students have considered atoms to be neutral, because we have discussed atoms as having equal numbers of protons (positive charge) and electrons (negative charge).  While atoms can certainly remain neutral, many atoms exist in nature as ions.  Atoms gain or lose electrons in predictable ways to form ions, and ions partner up in predictable ways to form ionic compounds.

A neutral atom has equal numbers of protons and electrons.  When a neutral atom loses one or more electrons, the atom will have fewer electrons than protons, and thus will have a positive charge.  We call a positively charged atom a cation (an ion with a positive charge).  Cations are often metals.  When a neutral atom gains one or more electrons, the atom will have more electrons than protons, and thus will have a negative charge.  We call a negatively charged atom an anion (an ion with a negative charge).  Anions are often non-metals.

Just like the positive end of one magnet is attracted to the negative end of another magnet, cations and anions attract.  When a cation bonds with an anion, an ionic compound forms.  The bond between the ions is called an ionic bond.  Ionic compounds can be simple: one cation with a +1 charge bond will bond with one anion with a -1 charge.  Similarly, one cation with a +2 charge will bond with one anion with a -2 charge.  If a cation with a +2 charge bonds with an anion with a -1 charge, the +2 cation will actually bond with two -1 anions, creating an ionic compound with three ions: one cation and two anions.  This is because anions and cations bond together following the Rule of Zero Charge: the positive and negative charges sum to zero.  Ions commonly exist in charges of +1, +2, +3, -3, -2, -1.

The charge of an ion for a given element is predictable.  It’s actually built into the structure of the periodic table.  Focusing on the main group elements:

  • Group 1A elements readily give up one electron to form +1 cations.
  • Group 2A elements give up two electrons to form +2 cations.
  • Group 3A elements give up three electrons to form +3 cations.
  • Group 4A elements don’t often give up or take electrons and thus remain neutrally charged (no charge).
  • Group 5A elements take three electrons to form -3 anions.
  • Group 6A elements take two electrons to form -2 anions.
  • Group 7A elements greedily take one electron to form -1 anions.
  • Group 8A elements don’t give up or take electrons and remain neutrally charged (which is why they are called Noble Gases – they don’t interact with other elements).

We then practiced writing formulas of ionic compounds using our improved understanding of the periodic table.

Finally, we extended our understanding of ions to include cases where cations and/or anions consist of multiple atoms bonded together.  We call such cations and anions polyatomic ions.  The ammonium ion (NH3+) is a common polyatomic cation.  Hydroxide (OH-) is a common polyatomic anion.  Polyatomic ions commonly have charges ranging from +1 to -3 depending on the atoms that come together to form the polyatomic ion.  Just like cations and anions attract, polyatomic cations attract anions and polyatomic anions.  Similarly, polyatomic anions attract cations and polyatomic cations.

To practice writing formulas of polyatomic compounds, students received a handout containing common ions and their charges to use as a resource for completing the Polyatomic Ions POGIL activity on Moday.

Notes from class:

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For extra help, the video below will review ions:

For additional support writing ionic formulas, students are encouraged to watch the video below: