Chemical Formulas

• a molecular formula indicates the total number of atoms  in one molecule

• an empirical formula is the simplest whole number ratio of atoms in the compound.

• Consider hydrogen peroxide (H2O2 ) as an example.

• the molecular formula is H2O2. • the empirical formula is HO. (lowest ratio is 1:1)

• In some cases the molecular formula and empirical formula are the same. – For example, both the molecular formula and the

empirical formula for water is H2O. (the lowest whole number ratio is 2:1)

• It is important to recognize however that the empirical formula only describes the ratio of one atom to another, and not the actual number of atoms of each type in the compound.

• The formula for glucose (a simple sugar) is C6H12O6.

5.6 How Elements Form Compounds

There are 2 types compounds :

1: Molecular compounds2: Ionic compounds

1)     Molecular compounds are formed when 2 or more

nonmetals combine together. When dissolved in water the

molecules do not separate into charged particles and do not

carry an electrical charge (non electrolyte)

2)      ionic compounds – formed when a metal loses 1 or more valence electrons to a non-metal, forming a positive (cation) and a negative ion (anion)which then are attracted to each other and are held together by an attraction called an ionic bond.

Ionic compounds dissolve in water and separate into positive ion and negative ions. These charged particles carry electric current through the water (electrolyte) 

See Dissolving Videos

Molecular Compounds

Molecular Compounds (COVALENT)

 Most of the compounds you encounter each day do not contain ions. Most everyday compounds are molecular or made of only nonmetals.

E.g. sugar, plastic and water 

Molecular compounds are formed when 2 non-metal atoms share their outer 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 Some elements exist as molecules rather than atoms for this reason (Diatomic Elements) 

I2 , Br2 , Cl2 , F2 , O2 , N2 , H2

We will deal with five categories of molecular compounds:

1.mono atomic elements

2.diatomic elements

3.polyatomic elements

4. trivial (common) compounds

5.binary molecular compounds

• 1.  Mono-atomic elements• If we look at the name of the heading, mono -

means one, so these are the non-metals that that exist in nature as individual atoms.  Although these are not compounds we have included them here because we will reference them many times.

• Noble Gases (group 18)– He Helium– Ne Neon– Ar Argon– Kr Krypton– Xe Xenon– Rn Radon

• 2.  Diatomic molecular elements

• As the heading suggest these are elements composed of two (di) nonmetal  atoms.  We seen these in the last lesson. (just remember the gen's)

oxygenO2

hydrogen H2

nitrogen N2

the halogens

(group 17)

fluorine F2

chlorine Cl2

bromine Br2

iodine I2

• 3.  Polyatomic molecular elements

• These are non-metal elements composed of many (poly) atoms. – O3 ozone– S8 sulfur– P4 phosphorus (red)– P10 Phosphorus (white)

• 4.  Trivial Names: Common names.• Despite the IUPAC rules for naming many

molecular compounds still go by their trivial (common) names.

• Here is a list of the molecular compounds with trivial names that you need to know.

• O3 Ozone

• H2O Water

• H2O2 Hydrogen peroxide• NH3 Ammonia• CH4 Methane• CH3OH Methanol• C2H5OH Ethanol

• C12H22O11 Sucrose

• 5.  Binary Molecular Compounds

• These are molecular compounds that contain two different non-metal atoms.– binary - 2 – molecular - non-metal atoms only – compound - two different atoms

• Carbon dioxide - CO2 is a common example. It consists of carbon atoms and oxygen atoms.

• The IUPAC rules for naming and formula writing involve use of a prefix system that indicates the number of atoms present.

IUPAC Prefixes from 1 - 10

1 mono-

2 di-

3 tri-

4 tetra-

5 penta-

6 hexa-

7 hepta-

8 octa-

9 nona-

10deca-

• What do you do when you are given the formula and you are asked to write the IUPAC name?

• NAMING BINARY MOLECULAR COMPOUNDS:

1. Write the name of the first element of the formula in full.

2. Shorten the name of the second element and add the ide ending.

3. Use prefixes to indicate the number of atoms of each element in the molecular formula.

4. The prefix mono on the first name is optional.

• Sample Exercise: • Write a IUPAC name for CCl4.

– Answer

1. The first element is C. o Its full name is carbon.

2. The second element is chlorine. o Its name is shortened to chlor, and the prefix -ide is added to give

chloride.

3. The prefix mono (1) is added to carbon, and the prefix tetra (4) is added to chloride to give the name: monocarbon tetrachloride.

4. The prefix mono is omitted from the first element name to give carbon tetrachloride.

• Write the molecular formula for each of the following compounds:

1.  nitrogen triiodide                

2.  triphosphorus pentabromide      

3.  carbon disulfide                          4.  sulfur trioxide                              

Writing Formulas for Molecular Compounds

 The number of electrons an atom wants to share to become stable is a clue to the number of covalent bonds that atom will form. The combining capacity of a non-metal is a measure of the number of covalent bonds.

These combining capacities are listed below : 

4 3 2 1H

C N O FSi P S Cl

As Se BrI

 What is the formula for the compound made of carbon and oxygen?

4 2C O = C2O4= CO2

simplify

Naming Molecular Compounds Some molecular compounds have common names like : 

H2O = waterNH3 = ammoniaCH4 = methaneH2O2 = Hydrogen PeroxideO3 = Ozone

 

Or they can be named using prefixes which represent the number of each atom in the compound 

H2O = diHydrogen monOxideNH3 = Nitrogen triHydrideCH4 = Carbon tetraHydride

Notice the second part of the name still ends in “ide” like many of the ionic compounds. You must know the following prefixes:

Mon(o) = 1 (optional on first name not the second)

Di =2 Hex(a) = 6

Tri =3 Hept(a) = 7

Tetr(a) = 4 Oct(a) = 8

Pent(a) = 5 Non(a) = 9Dec(a) = 10

e.g. CO2 is carbon dioxideP2 H4 is Diphosphorous TetrahydrideCO is carbon monoxideSiO2 is silicon dioxide

P3Br8 =

= Tetrasulfur nonoxide

N2O2 =

= HeptaSelenium DecaIodie

Homework:• Molecular compound

worksheet

Assignment

• A8

• Homework : 1-6 pg 204