COMPOUNDS

• Compounds are pure substances that contain 2 or more elements combined in a definite proportion by mass.
• Compounds have unique properties compared to their component elements. For example, although both Na and Cl are extremely reactive and poisonous substances, compound from combination of them, NaCl , is a relatively harmless flavor enhancer.
  

  

• Compounds are formed from elements by combining in a definite, fixed composition.
• The first chemist to formally state this relationship was Joseph Proust, who developed the law of constant composition, which states:

All samples of a given compound have the same proportions of their constituent elements.

• The ratio of hydrogen to oxygen is variable in a mixture, while it is fixed in a compound such as water.

LAW OF CONSTANT COMPOSITION

• In a pure compound, the elements are always present in the same definite proportion by mass.

Sample 1

Sample 2

• Based on this law, the mass of an element can be determined from its mass percent in a compound.

Examples:

1. Barium iodide, , contains barium by mass. How many grams of barium does an 8.50 g sample of barium iodide contain?

2. Mercuric sulfide, HgS, contains mercury by mass. What mass of mercuric sulfide can be prepared from 60.0 g of mercury?
3. When 12.66 g of calcium are heated in air, 17.73 g of calcium oxide is formed. What is the percent of oxygen in this compound?

CHEMICAL FORMULAS

• A chemical formula is an abbreviation for a compound.
• It shows the symbols and the ratio of the atoms of the elements present in the compound.
  

  

Examples:

1. Which formula represents the greatest number of atoms?
   a)
   b)
   c)
   d)

TYPES OF FORMULAS

• Chemical formulas can be categorized into 3 types:

1. Molecular formula
2. Empirical formula
3. Structural formula

• A molecular formula is the actual number of atoms of each element in a compound. For example, the molecular formula for hydrogen peroxide is , because a molecule of hydrogen peroxide actually contains 2 hydrogen and 2 oxygen atoms.
• An empirical formula is the simplest whole-number ratio of atoms of each element in a compound. For example, hydrogen peroxide would have an empirical formula of HO , since this is the smallest ratio of its atoms.
• A structural formula uses lines to represent chemical bonds and shows how atoms in a molecule are connected to each other. For example, hydrogen peroxide would have the structural formula shown below:

Examples:

1. Give the empirical formula that corresponds to each molecular formula shown below:
   a)
   b)
   c)
   d)

MOLECULAR VIEW OF ELEMENTS & COMPOUNDS

• Recall that all pure substances can be classified as element or compound. We can further subcategorize elements and compounds according to the basic units that compose them.
• Elements may be either atomic or molecular, while compounds can be either molecular or ionic.
• The smallest particles of matter can be therefore atoms, molecules or ions.
• Atomic elements are those that exist as single atoms. Most elements fall
  

  

  
  into this category.
• Molecular elements are those that exist naturally as diatomic molecules. Among these are hydrogen, nitrogen, oxygen as well as the four halogens: and .
• Molecular compounds are compounds of 2 non-metals. Two examples are water and dry ice .
• Ionic compounds are composed of one or more cations paired with one or more anions. In most cases, the cations are metals and the anions are non-metals. The basic unit of ionic compounds is not a molecule, but a formula unit. Examples are sodium chloride, NaCl , which is composed of and ions.

Examples:

1. Classify each substance as atomic element, molecular element, molecular compound or ionic compound:
   a) NO
   

   

   
   b) Chlorine
   c) Au
   d)
   e)

IONIC CHARGES & FORMULAS

• The formula of an ionic compound indicates the number and kinds of ions that make up the ionic compound.
• The sum of the ionic charges in the formula is always zero, which indicates that the total number of positive charges is equal to the total number of negative charges.
• For example, the +1 charge on the sodium ion is cancelled by the -1 charge on the chloride ion, to form a net zero charge.
  

  

• When charges between the two ions do not balance, subscripts are used to balance the charges.
• For example, since each magnesium loses 2 electrons, and each chloride gains one electron, 2 chlorides are needed to balance the charge of the magnesium ion. Therefore, magnesium chloride is written as .
  

  

WRITING IONIC FORMULAS

• When writing ionic formula, knowing the charge of the ions are important since the net charge on the compound must be zero.
• Some elements produce only one ion (Type I) while others produce two or more ions (Type II).
• Differentiating between type I and II ions is important, since the naming system is different for each. Shown below are the common ions of each type:

• Note that most main-group elements are type I (except Sn and Pb), and most transition elements are type II (except Ag, Cd and Zn).

WRITING IONIC FORMULAS (TYPE I & II)

Binary Ionic Compounds:

• Binary compounds contain only two elements.
• In these compounds, charges of the cations must equal the charges of the anions since the net charge is zero.
• Subscripts are used to balance the charges between cations and anions.

Examples:

1. Write formulas for ionic compounds formed from the following elements:
   a) calcium & chlorine:
   b) sodium & sulfur:
   c) aluminum & nitrogen
   d) copper (I) & phosphorus
   e) iron (III) & sulfur

WRITING IONIC FORMULAS (POLYATOMIC IONS)

• Some ionic compounds contain polyatomic ions, an ion composed of several atoms bound together.
• Some common polyatomic ions are:

• When writing formulas for compounds containing polyatomic ions, treat the polyatomic ion as one group.

Examples:

1. Write formulas for the ionic compounds formed from the following ions:
   sodium & carbonate:
   ammonium & sulfide:
   magnesium & bicarbonate:

NAMING IONIC COMPOUNDS

• When naming ionic compounds, we must first identify it as one. Any formula that contains a metal and a nonmetal can be categorized as an ionic compound.
• Ionic compounds must then be categorized by the type of cation they possess: Type I or Type II.
  

  

Binary Ionic Compounds (Type I):

• Type I cations are those that form only one ion. Most main-group cations are of this type (exceptions are Pb and Sn ).
• These compounds are named by naming the cation (same as the atom), followed by the name of the anion with the ending -ide.
  

  

Examples:

1. Name each of the following Type I ionic compounds:

NAMING IONIC COMPOUNDS

Binary Ionic Compounds (Type II):

• Type II cations are those that form more than one ion. Many transition metals are of this type (exceptions are and Zn ). A partial list of these cations is shown below:

• When naming compounds formed from these ions, include the ionic charge as Roman numeral, in parentheses, after the metal's name.
  

  

Examples:

1. Name each of the following compounds that include type II cation:

NAMING IONIC COMPOUNDS

Examples:

2. Name each of the following compounds that include type II cations:
   :
   :

Ionic Compounds containing Polyatomic ions:

• Polyatomic ionic compounds are named by naming the cation first, followed by the polyatomic ion.

Examples:

3. Name the following polyatomic compounds:
   :
   
   :

NAMING & WRITING MOLECULAR FORMULAS

Binary Molecular Compounds:

• These compounds are named similar to ionic compounds, with the second element named based on its root and suffix "-ide".
• Greek prefixes are used to indicate the number of atoms in these compounds:

• The first atom uses a prefix only when more than one atom is present.
• The second atom always uses a prefix.

Examples:

1. Name the following molecular compounds:
   :
   :
2. Write formulas for the following molecular compounds:
   carbon tetrachloride:
   dichlorine monoxide:

NOMENCLATURE FLOWCHART FOR BINARY COMPOUNDS

NAMING ACIDS

• Acids are molecular compounds that form ions when dissolved in water.
• Acids can be categorized into two groups: binary and oxyacids.
  

  

Naming Binary Acids:

• Formulas are written similar to binary ionic compounds, assigning a +1 charge to hydrogen.
• When naming the acids, use hydro- prefix, followed by the name of the non-metal with an -ic ending, followed with the word acid.
  

  

Naming Oxyacids:

• Oxyacids are acids that contain oxyanions which are listed in the table of polyatomic ions. Some of the important acids in this group and the oxyanions they form are listed below.

FORMULA MASS

• Recall that the atomic mass of elements was defined as average atomic mass of the isotopes that compose that element and measured in atomic mass unit (amu).
  

  

• Similarly, the formula mass of molecules or formula units can be defined as the sum of the atomic masses of all the atoms in its formula.
  

  

Examples:

1. Lithium sulfide,
2. Aluminum nitrate,