Topic 6 Molecular Compounds

There are over 112 elements so far! When elements join together, they form compounds. Elements are held together by chemical bonds. These bonds are formed when elements gain, lose, or share electrons. A molecular compound is formed when atoms share electrons.

Properties of Molecular Compounds Formed from only non-metallic elements (REMEMBER: what side of the periodic table are non-metals located?) Do NOT form ions in solution Usually do NOT conduct electricity Solid, liquid or gas at room temperature

What’s in a Chemical Formula? Symbol for the element oxygen Symbol for chemical element hydrogen H2O(l) This means the state of the compound: (l)- liquid (g)- gas (s)- solid (aq)- aqueous The small number here means 2 atoms of hydrogen (subscripted number) No small number means 1 atom of oxygen

A chemical formula uses symbols and numerals to represent the composition of a pure substance. Each symbol in a chemical formula represents an atom of an element. If there is more than one atom of an element, a small number written below the line ( a subscript) follows the elements symbol The subscript shows the number of atoms. The Law of Definite Composition tells us that every pure substance has a fixed and definite composition. Ex: (H2O) ALWAYS contains two atoms of hydrogen for each atom of oxygen.

# of atoms of each element per molecule Total # of atoms per molecule Formula Elements # of atoms of each element per molecule Total # of atoms per molecule # of molecules Total # of atoms N2 CH4 5Ca(OH)2

MOLECULAR COMPOUNDS A molecule is the smallest independent unit of a pure substance and is generally a cluster of atoms joined together. Diatomic molecules are molecules made of two atoms of the same element. Examples: oxygen O2 nitrogen N2 hydrogen H2

The Rule of “-gen” These are all of the DIATOMIC elements in the periodic table Includes: All of group 17 (halogens) oxygen nitrogen hydrogen

The BONDING between ATOMS is strong, but the attraction between MOLECULES is weak. When you melt or vaporize a molecular compound, you must supply enough energy to overcome the attraction between the molecules.

This is ELECTROLYSIS! WATER MOLECULES Oxygen atom Hydrogen atom Hydrogen molecules Oxygen molecules

IUPAC makes recommendations on how compounds should be named. Not only do elements have universal symbols that are recognized world-wide, but compounds are also named according to global guidelines. The international Union of Pure and Applied Chemistry (IUPAC) is an organization of scientists responsible for setting standards in chemistry. IUPAC makes recommendations on how compounds should be named. There are rules to naming compounds. Ex: A compound made up of two elements is called a binary compound. The names of molecular binary compounds follow these rules.

Language Name of element Symbol English hydrogen H French hydrogene German wasserstoff Italian idrogeno Portuguese hidrogenio Spanish hidrogeno

Naming molecular compounds! Hmm… what is this? CO2 (Use your periodic table) Write the entire name of the 1st element carbon Change the ending on the name of the 2nd element to “-ide” oxygen  oxide Use a prefix to indicate the number of each type of atom in the formula carbon dioxide

PREFIXES REMEMBER: You only use MONO- for the second element! 1- mono 2- di 3- tri 4- tetra 5- penta 6- hexa 7- hepta 8- octa 9- nona 10- deca REMEMBER: You only use MONO- for the second element!

Let’s do a couple examples: SO CCl4 S3O6 P4O10

Name these: H2O2 HCl7 CO SO2 H2O

If you are changing from the written name to the symbol: Write the symbols for the elements in the same order as they appear in the name Use subscripts to indicate the numbers of each type of atom

Let’s do these: carbon tetrachloride Silicon dioxide Sulfur dioxide

Some molecular compounds are known by common names as well as by chemical names FORMULA DESCRIPTION CO(g) Carbon tetrachloride Silicon dioxide SO2(g)

To do: Naming Molecular Compounds Molecular Model Kits