Ionic and Molecular Compounds

Forming Ions  Why are noble gasses the least reactive?  The last level is filled up!  Why are the alkali metals so reactive?  They are close to being filled up! (only one electron in their last shell that they can give off)  We really only need to look at the last level of electrons to see how reactive the element will be. This is called the “valence level”.

 Elements become more stable when their valence level is filled up.  When elements gain or lose electrons they become charged. We call this an ion.  When an atom gains an electron, it becomes negatively charged since electrons are negative.  When an atom loses an electron, it becomes positively charged, since it is losing a negatively charged particle.  Elements can never lose or gain protons or neutrons.

 For example: A sodium ion that has 11 protons and 10 electrons has what charge?  +1  We represent it by Na+  A Magnesium ion has what charge?  Look on periodic table… what group is it in?  Will it lose or gain electrons?  Lose? It becomes positive…  how many does it lose?  2  Charge?  Mg 2+

You try it!  Write the ion of the following:  Lithium:  Potassium:  Flourine:  Sulfur:  Chlorine:  Boron:  Nitrogen: Li+ K+ Fl- S2- Cl- B 3+ N 3-

Ionic Bonds  Compounds are made of more than one different element and are chemically linked together.  We call them chemical bonds  One type of bond is called an ionic bond.  You put one or more positive ions with negative ion(s) in order to make it neutral.  You have a metal (+) with a non-metal (-) to form an ionic bond.

Physical Properties of Ionic Compounds  Most are found in a solid arrangement called a crystal lattice  They are also readily dissolvable in water (high solubility)  They are good conductors of electricity when dissolved in water because of the ions are free to move around.

Covalent Bonds  A molecular compound forms when atoms share a pair of electrons to form a covalent bond.  The shared electrons are attracted to the nuclei of both atoms.  Unlike ionic bonds, the electrons are not transferred, they are shared.  It is kind of like a tug of war game, both atoms want the electrons, but they have a mutual pull on them.

 Forming molecular compounds is based on the same idea as ionic compounds: stability is associated with a full outer energy level.  Simple covalent compounds are made of two non-metals.  Compounds are made of 2 or more different elements. Molecules just have to be two or more elements (same element or different).  E.G. Oxygen gas is O 2 and is a molecule, but not a compound!

 The names of most molecular compounds tell you what element is in it, plus how many atoms of that element.  We use prefixes to tell us the # of elements:  E.G. Carbon dioxide = 1 C and 2 O’s. OR… C O 2

Prefixes mono1penta5 di2hexa6 tri3hepta7 tetra4octa8

Properties of Molecular Compounds  There is a weaker attraction between molecules than between ionic compounds.  Do not dissolve well in water  Plastics are made of polymers, which are large molecules that are very hard to break down.