Ionic Bonds and Properties of Ionic Compounds

 Recall that atoms lose or gain electrons to form ions.  Metals tend to lose electrons and form positively charged ions called cations.  Nonmetals tend to gain electrons and form negatively charged particles called anions.  They do this to acquire a stable octet (full outer energy level – usually 8 valence electrons).  For the representative elements (groups 1,2 &1-17) we can tell the charge of the ion by the number of valence electrons. Remember: 1+, 2+, 3+, skip, 3-, 2-, 1-, zip! Ions

 Because opposites attract, cations and anions are attracted to each other and form neutral compounds often referred to as salts.  Since the positive and negative charges have to zero out, the formula for an ionic compound will give the ratio of each element.  Individual particles of an ionic compound are called formula units, but they do not exist in isolation. They form a repeating crystalline structure called a crystal lattice. Ionic Compounds

 Think about it for a minute – what ionic compound are you very familiar with? Table salt – sodium chloride. You worked in lab with several other salts. What state of matter were they all at room temperature?  If you said solids you are correct!! Now think about why that might be.  It is because of the strong attractive forces among the ions and formula units Properties of Ionic Compounds Let’s fill in the chart!!

 Have you ever tried to melt table salt? If so it probably didn’t work very well. Why?  Melting and boiling points of ionic compounds are relatively high. Why?  Those strong bonds that make them solids at room temperature also require lots of energy to break thus resulting in high melting and boiling points. Melting & Boiling Points

 Generally when we talk about solubility, we are referring to solubility in water.  Ionic compounds are usually soluble in water. Why?  Water molecules are polar (we’ll talk more about that later) and will pull the ions apart. Solubility

 What does that mean???  Basically it means if you hit it with a hammer, what happens???  Yep, it crumbles – that’s because ionic compounds tend to be brittle. Why?  In the crystal lattice, ions have to be lined up with opposite charges and if that gets shifted, then like charges repel - it breaks!! Response to Stress or Deformation

 As solids, ionic compounds are not good conductors of electricity.  However, when dissolved in water or in a molten state they are good conductors. Why?  In order to move electrons (that’s what an electrical current is) you need charged particles – in this case ions- for them to move through. Within the crystal lattice, there are no loose charged particles for transferring the electrons!! Conductivity