MR. SURRETTE VAN NUYS HIGH SCHOOL
CHAPTER 4: FORMULAS AND COMPOUNDS
CLASSNOTES
COMPOUNDS
Compounds are pure substances that break down into fixed definite proportions.
MOLECULAR COMPOUNDS
When atoms within a compound bond tightly together and behave as a physical unit, they become molecular compounds or molecules.
CHEMICAL FORMULAS
The composition of a compound is shown in its chemical formula. For example, one carbon combines with two oxygen atoms to form carbon dioxide. The chemical formula for carbon dioxide is: CO2.
IONIC COMPOUNDS
Metal atoms tend to lose electrons and become cations. Nonmetal atoms tend to gain electrons and become anions. Because cations have a positive charge and anions have a negative charge, they attract each other.
IONIC COMPOUNDS
When a cation combines with an anion, the result is called an ionic compound, or salt.
IONIC CHARGES
An element’s ionic charge is determined by its group position on the periodic table.
CATIONS
The Group 1 elements (alkali metals) tend to lose a single electron and become + 1 ions. The Group 2 elements (alkaline metals) tend to lose two electrons and become + 2 ions.
CATIONS
Elements in Groups 3 – 12 (transition metals) and elements in Group 14 generate multiple cations with charges + 1 to + 7.
CATIONS
Elements in Group 13 tend to lose 3 electrons and become + 3 ions.
ANIONS
Elements in Group 15 tend to gain three electrons and become - 3 ions. Elements in Group 16 tend to gain two electrons and become - 2 ions.
ANIONS
Elements in Group 17 (the halogens) tend to gain one electron and become -1 ions.
NOBLE GASES
Elements in Group 18, the noble gases, are neutral and do not create ions.
TYPE I COMPOUNDS
Ionic compounds with well-defined charges are called type I. Examples of type I compounds are MgF2, KBr, and Zn3N2.
TYPE I COMPOUNDS
When determining the chemical name and formula of type I compounds, write the name of the cation first. Then look at the name of the anion. The suffix for the name of the anion changes to -ide.
Example 1. A chlorine and a sodium ion form a familiar compound. What is the name of this substance?
1A.
(1) Sodium is a metal cation, so its name comes first.
(2) Chlorine is the anion so its name changes from “chlorine” to “chloride.”
(3) sodium chloride
Example 2. Name the chemical formula KI.
2A.
(1) “K” = potassium and “I” = iodine
(2) potassium iodide
BINARY COMPOUNDS
Compounds composed of two elements are called binary compounds. The names of molecular binary compounds sometimes use greek prefixes to determine the amount of each atom.
BINARY COMPOUNDS
For example: mono = 1, di = 2,
tri = 3, tetra = 4, penta = 5, hexa = 6, and hepta = 7. (Reminder: Molecules share electrons, ionic compounds (salts) connect cations with anions.)
BINARY COMPOUNDS
Exception: The prefix “mono” is not traditionally used in front of the first atom in a chemical formula. For example, the formula CO is named “carbon monoxide,” not “mono-carbon monoxide.”
Example 3. Name the chemical formula SO3.
3A. sulfur trioxide
Example 4. Name the chemical formula P5F6.
4A. pentaphosphorus hexafluoride
CONSERVATION OF CHARGE
Conservation of charge is often used to determine the chemical formula of ionic compounds. It states that the overall charge on a compound must equal zero.
CONSERVATION OF CHARGE
This means that the sum of positive charges (cations) within a compound must equal the sum of the negative charges (anions).
Example 5. Y+3 cations (Y = “yttrium”) react with F-1 anions. What is the most likely chemical formula for the product?
5A.
(1) The net charge of the product must be zero.
(2) 1 x Y+3 = + 3
(3) 3 x F-1 = - 3
(4) YF3
Example 6. Al+3 cations react with O-2 anions. What is the most likely chemical formula for the product?
6A.
(1) 2 x Al+3 = + 6
(2) 3 x O-2 = - 6
(3) Al2O3
Example 7. Mg+2 reacts with O-2.
7a. What is the most likely chemical formula for the product?
A.
(1) 1 x Mg+2 = + 2
(2) 1 x O-2 = -2
(3) MgO
7b. What is the name of this product?
A. Magnesium oxide
TYPE II COMPOUNDS
Type II ionic compounds involve multiple charges and the transition metals. To avoid confusion, the charge of the cation must be included in the formula name.
[Chapter 4 Type II handout]
TYPE II COMPOUNDS
The names for binary (two-element) Type II ionic compounds have the following form:
Name of cation (charge of cation) + base name of anion
Example 8. Fe+3 reacts with oxygen.
8a. What is the most likely chemical formula for the product?
A.
(1) Oxygen is a Group 16 element, so its ionic charge is – 2.
(2) 2 x Fe+3 = + 6
(3) 3 x O-2 = - 6
(4) Fe2O3
8b. What is the name of this product?
A. iron(III) oxide
Example 9. Give the name for PbCl4.
9A.
lead(IV) chloride. Chlorine is a halogen (group 17 element), so it has a – 1 charge. Since there are four chlorine atoms in PbCl4, they must have a combined charge of – 4. PbCl4, like all compounds, has an overall charge of zero. Therefore, Pb must have a + 4 charge in order to balance the - 4 charge from the chlorine atoms. Pb is also a transition metal, so this is a type II compound.
POLYATOMIC IONS
Many ionic compounds contain ions that are composed of a group of atoms (for example, SO3-2). Compounds that contain polyatomic ions have three or more kinds of atoms. They also include the name of the polyatomic ion.
Example 10. Give the name for KCN.
10A. potassium cyanide
Example 11. Give the name for NaClO3.
11A. sodium chlorate
NAMING COMPOUNDS
When naming compounds, it is important to realize if they are ionic or molecular because there are different sets of rules.
BINARY IONIC COMPOUNDS
Ionic compounds follow the formula (metal cation) + (non-metal anion ~ide). For example, BeF2 is named beryllium fluoride (not beryllium difluoride).
MOLECULAR BINARY COMPOUNDS
Molecules (both covalent and polar) are named by (metal cation) + (greek prefix)(non-metal cation). For example, CCl4 is carbon tetra~chloride (not carbon chloride).
ELECTRONEGATIVITY TABLES
Electronegativity tables predict the types of bonds formed between atoms.
[Chapter 4 Electronegativity handout]
ELECTRONEGATIVITY
Electronegativity is an atom’s ability to attract electrons. Non-metals have the highest electronegativity values, and metals have the lowest.
Example 12. A fluorine and a lithium atom combine and form lithium fluoride. According to electronegativity values, what type of compound is lithium fluoride?
(A) covalent molecule
(B) polar molecule
(C) ionic compound
12A. (C) ionic compound. Fluorine has a 3.98 electronegativity and lithium has a 0.98 electronegativity. The electronegativity difference between these values is: 3.98 – 0.98 = 3.0. This means lithium fluoride is an ionic compound (salt).
Example 13. A carbon and an oxygen atom combine and form CO. According to electronegativity values, what type of compound is CO?
(A) covalent molecule
(B) polar molecule
(C) ionic compound
13A. (A) covalent molecule. Carbon has a 2.55 electronegativity and oxygen has a 3.44 electronegativity. The electronegativity difference between these values is: 3.44 – 2.55 = 0.89. This means CO is a covalent molecule.
Example 14. What is the name of CO?
14A.
Since CO is a molecule, we must use greek prefixes. Therefore, CO is carbon monoxide.
METAL OXIDES
Metal oxides (metal + oxygen) are exceptions to the electronegativity table. Metal oxides are considered ionic compounds.
Example 15. Provide the name for Fe2O3.
15A.
Even though the electronegativity difference between iron (1.83) and oxygen (3.44) is 1.61, it is a metal oxide. Therefore, the name for Fe2O3 is iron(III) oxide (not di-iron trioxide).
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