Lesson 4 – Forming Compounds


  • to explain, using the periodic table, how and why elements combine to form compounds


  • 1 class


  1. What is the difference between an atom and an ion?
  2. Which subatomic particle is the main cause of chemical reactions?
  3. How many valence electrons does an atom of sulphur have?

Background Information:

Remember, that atoms start out neutral, but by losing or gaining electrons they become charged ions. Just like the south and north pole of a magnet attract each other, positively and negatively charged ions attract each other. This is the basis of how compounds form. There are two types of compounds that can form: ionic and molecular.

Ionic Compounds

  • Have a positive ion (cation) and a negative ion (anion)
  • Valence electrons are ripped off the cation and joined to the anion
  • This gives each ion a full outer valence of electrons
  • Are very stable and strong due to the opposite charges

Bonding of Sodium Chloride

  • Usually form rigid crystal structures, called a crystal lattice
  • When melted, or dissolved in water, the ions split apart
  • Ions are what cause electricity to be conducted


Molecular Compounds

  • Form when two anions share electrons in a type of bond called a covalent bond
  • Electrons are shared in order to complete the valence level
  • The elements can share 1 electron each (single bond) 2 electrons each (double bond) or even 3 electrons each (triple bond)
  • Many non-metals do this naturally and are called diatomic (di = 2) or polyatomic (poly = many)

Oxygen covalent bonds

  • Examples are:  H2, O2, N2, F2, Cl2, Br2, I2, P4 and S8
  • (They make a “7” on the periodic table, or to be truly Canadian, they make a hockey stick, with Hydrogen being the puck!)
  • Molecular compounds can share 1 electron each (single bond), 2 electrons each (double bond) or even 3 electrons each (triple bond)!
  • Molecular compounds do not bond as strongly, so have lower melting and boiling points.
  • They do not usually conduct electricity, since there are no charged particles

An animated comparison of ionic and molecular bonding:



Use modelling clay or some other type of dough (or the ion building kits from lesson 3) to model ionic and molecular compounds. Use a big ball of clay for a nucleus, and smaller balls to represent electrons. For molecular compounds, students may use rubber bands, string, or pipe cleaners to represent the covalent bonds. Students should draw dot diagrams of the models they have created.

Ionic compounds to create:

KF, MgO, AlP, Na2S, Ca3N2

Molecular compounds to create:

CO2, SO2, Cl2, CH3, N2

Self Check:

1. Why do you think ionic compounds have much higher melting and boiling points than molecular compounds? Explain your reasons using what you know about the formation of compounds.

2. Use an electron dot diagram to predict how Hydrogen and Nitrogen would bond together.

3. You have samples of 2 unknown elements. You have determined that the ionic charge for element X is 2+, and for element Y it is 0.

a) Which is a metal? How do you know?

b) You introduced oxygen into the mixture, predict how you think element X and Y will react. Explain your reasoning.

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2 Responses to “Lesson 4 – Forming Compounds”

  1. This is awesome stuff, its superior to be inside the know.

  2. Many thanks, for any terrific article

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