4.2 Representing Molecular Compounds

Agenda Practice writing and naming ionic compounds Take up homework Lesson 4.2 Representing Molecular Compounds Read pages Vocabulary Learning Check page 156 #1-2 Practice Problem page 157 #1 Handout

Learning Goals Students will define the following terms: molecular compound, molecule, binary molecular compound. Students will name binary molecular compounds. Students will write formulas for binary molecular compounds.

Forming Molecular Compounds Most of the compounds you encounter each day do not contain ions. Most everyday compounds are molecular or made of only non-metals. Molecular compounds are also known as covalent compounds. E.g. sugar, plastic and water Molecular compounds are composed of individual molecules. Each molecule is composed of a set number of atoms of each element.

Covalent Bond Molecular compounds are formed when 2 non-metal atoms share their outer valence electrons to fill each others outer orbits. This sharing of electrons results in a covalent bond that holds the atoms together as a molecule. Atoms become more stable when their outer orbit is full. Electrons are not transferred so the atoms remained uncharged.

The term molecule is also used to describe 2 or more atoms of the same element that are joined by a covalent bond. They are called diatomic molecules. Table of Diatomic molecules Name of ElementChemical SymbolFormula of molecule HydrogenH OxygenO FluorineF BromineBr IodineI NitrogenN ChlorineCl

Naming Molecular Compounds When you are asked to write the name of a compound: 1.First check the formula to see if it includes a metal. 2.If the first element is a metal, the substance is an ionic compound and should be named using the steps for an ionic compound as previously discussed.

Naming Binary Molecular Compounds If the compound consists of only non-metals, then it is a molecular compound. Naming molecular compounds involves using prefixes to indicate the number of atoms. The prefix is attached to the name of the element to which it refers. The prefix “mono” is only used for the second element in a compound.

Prefixes PrefixNumber of atomsExample mon(o)-1Carbon monoxide CO di-2 tri-3 tetra-4 penta-5

Common Names Some molecular compounds have common names like : H 2 O = water NH 3 = ammonia CH 4 = methane H 2 O 2 = hydrogen peroxide O 3 = ozone

Steps: 1.Write the names of both elements in the same order as in formula. phosphorus chlorine 2. Replace the ending of the second element with ``ide`` chlorine becomes chloride

3. Add prefixes a.Count the number of atoms of the first element in the chemical formula. b.Write the appropriate prefix. There is only one phosphorus but we never use mono for the first element, therefore it remains as: phosphorus Remember the prefix ``mono`` is never used for the first element. C. Count the number of atoms of the second element. d. Write the appropriate prefix. There are three chlorines altogether. You can tell by looking at the number in the subscript. Therefore you add the prefix ``tri`` The name for the second element becomes: trichloride 4. Put together the name: Phosphorus trichloride

Step 1: Write the names of both elements. nitrogen oxygen

Step 2: Replace the ending of the second element with ``ide`` oxide

Step 3: Add prefixes a.Count the number of atoms of the first element in the chemical formula. There are 2 atoms of nitrogen (look at the subscript) b. Write the appropriate prefix. dinitrogen

c. Count the number of atoms of the second element. There are 4 atoms of oxygen d. Write the appropriate prefix. tetroxide If the prefix ends with an a or o, this letter is dropped before adding oxide You would not write tetraoxide.

Step 4: Put the name together. dinitrogen tetroxide

Writing Formulas for Binary Molecular Compounds Steps: 1.Write the chemical symbol for the first element. 2.Determine the number of atoms of the first element based on the prefix. If there is no prefix for the first element this means that there is only one atom. 3.Write the chemical symbol of the second element. Keep in mind that the ``ide`` ending is not part of the element`s name. 4.Determine the number of atoms of the second element based on the prefix. This number will appear in the final chemical formula. 5.Write the formula using the appropriate subscripts.

Write the Chemical Formula for Phosphorus Trichloride Step 1: Write the chemical symbol of the first element. Symbol - P

Step 2: Determine the number of atoms of the first element. There is 1 phosphorus atom

Step 3: Write the chemical symbol of the second element. Symbol - Cl

Step 4: Determine the number of atoms of the second element based on the prefix. Trichloride- prefix is tri – means 3 There are 3 atoms of chlorine

Another Example: Write the formula for Disulfur Dinitride Step 1: Write the chemical symbol for the first element. Symbol - S

Step 2: Determine the number of atoms of the first element based on the prefix. Disulfur-- di means 2 There are 2 atoms of sulfur

Step 3: Write the chemical symbol of the second element. Symbol - N

Step 4: Determine the number of atoms of the second element based on the prefix. Dinitride- di means 2 There are 2 atoms of nitrogen

Success Criteria I am able to define the following terms: molecular compound, molecule, binary molecular compound. I am able to name binary molecular compounds. I am able to write formulas for binary molecular compounds.