Compounds and Formulas Matter, elements, and such.

Stability and Compounds  Sodium is a soft, silvery metal.  Chlorine is yellow-green gas.  Why is sodium chloride (NaCl) a brittle crystalline solid?

Compounds  Some of the matter around you is in elemental form. It is not combined with any thing (Oxygen you’re breathing)  Most of the elements, however, are in the form of compounds.

Compounds  Compounds have their own properties, and they are completely different than the individual elements.  Iron + Oxygen makes rust.  Copper + Sulfur makes verdigris.  Silver + sulfur makes tarnish.

Formulas  The chemical symbol for sodium is Na. The symbol for Chlorine is Cl.  Compounds have a chemical formula. NaCl is the formula for sodium chloride.  H 2 O is the formula for water.

Subscripts H2OH2O  Notice that the 2 is written in a subscript.  The subscript tells how many atoms of that specific element are present in a compound.  There are 2 hydrogen for every one oxygen.  Fe 2 O 3 ?

Stability  Some elements are more stable when ionized, than when in their natural state.  How stable an element is, depends on the arrangement of electrons in the valence shell of that element.

Noble Gases  Noble gases are the most stable elements.  The reason is the configuration of their electrons: They all have complete valence shells.

Chemical Stability  An atom is only stable when its valence shell is full of electrons.  But, its hard to capture and hold electrons alone.

Other elements  Hydrogen has one valence electron.  But its outer shell needs 2 to be complete.  Hydrogen cannot capture another electron by itself, so it must find another way.  Hydrogen will share an electron with another hydrogen.  They will both have complete shells, and will both be stable.

Helium.  Helium is a noble gas, and has 2 valence electrons.  Helium is stable, and will rarely form compounds.

Chemical bond  When atoms gain, lose or share electrons with other atoms, they will stay stuck together.  There is energy stored in the link between them.  It is called a chemical bond.