Read the pages indicated by A-4 and study behavioral objectives 12 and 14. Molecules are combinations of atoms. The atoms may be of the same element (H2) or from different elements (CO2). There are two types of bonding which form the molecules present in living systems, ionic and covalent. Both of these types use the electrons in the outer shell. The term used for these electrons is valence shell. During ionic bonding there is a transfer of outer shell electrons from one atom to a second thus forming a charged particle (atom which contains and uneven number of electrons and protons) called an ion. Ions are atoms that have lost or gained electrons and they may either have a positive charge or a negative charge. Ions are formed during a chemical reaction where two or more reactants react to form products. During a chemical reaction, both atoms which are capable of giving up electrons and atoms capable of accepting electrons, must be present. How are ions formed?
Rules for forming ions: (write in your notes)
Atoms which contain three or less electrons in its valence shell will tend to lose all of its electrons. Since the atom has lost electrons the resulting charge will be positive since it contains more protons than electrons. (Example sodium has one valence shell electrons. The ion sodium may be written as Na+, calcium has two valence shell electrons. The ion calcium may be written as Ca++ or Ca +2)
(valence shell refers to the outer shell)
Atoms which contain five to seven electrons in its valence shell will gain enough electrons to fill it. Since the atom has gained electrons the resulting charge will be negative since it contains more electrons than protons. (Example flourine has seven valence shell electrons. The chlorine ion may be written as F-, sulfur has six valence shell electrons. The ion sulfur may be written as S = or S-2)
(valence shell refers to the outer shell)
Recall that an atom with only one shell can only hold two electrons in its outer shell and that all other atoms can only hold eight electrons in their outer shell. (in the scope of this course hydrogen will be the only atom requiring two electrons to fill its outer shell, all other atoms will require eight electrons)
Atoms which contain four electrons will neither gain nor lose electrons and tend to demonstrate covalent bonding. Covalent bonding will be investigated in the next lesson.T
The resulting charge number will be equilivent to the number of electrons gained or lost. (Ex. loses two electrons = -2, loses three electrons = -3, gains two electrons = +2)
Chlorine (Cl) atoms contain seven valence electrons and sodium (Na) atoms contain one valence electron.
1. Chlorine ions formed during the chemical reaction between these two atoms will have a charge of:
Press here to check your answer.
2. Sodium ions formed during the chemical reaction between these two
atoms will have a charge of:
Press here to check answer.
Since both ions have an opposite charge they will attract one another
electomagnetically and form a molecule of NaCl (sodium chloride or table
salt).
Write the equation above in your notes.
Note: The positive ion is always written first followed by the negative ion.
Sulfur (S) atoms contain six valence electrons and potassium (K) atoms contain one valence electron.
3. Sulfur ions formed during the chemical reaction between these two atoms will have a charge of:
Press here to check your answer.
4. Potassium ions formed during the chemical reaction between these
two atoms will have a charge of:
Press here to check answer.
Since sulfur ions have a negative two charge and potassium has a positive
one charge, one oxygen ion will attract two potassium ions form a molecule
of K2 S (potassium sulfide).
Note: The positive ion is always written first followed by the negative ion.
Note: The over all positive and negative charges of a molecule must balance out. (+1 balances a -1, two +1 must be balanced with two -1 or one -2).
Write the equation above in your notes.
Oxygen (O) atoms contain six valence electrons and iron (Fe) atoms contain three valence electrons.
5. Oxygen ions formed during the chemical reaction between these two atoms will have a charge of:
Press here to check answer.
6. Iron ions formed during the chemical reaction between these two
atoms will have a charge of:
Press here to check answer.
Since oxygen ions have a negative two charge and iron has a positive
three charge, three oxygen ions will attract two iron ions form a molecule
of Fe3 O2 (iron oxide or rust).
Note the over all positive and negative charges of a molecule must balance out. (One iron has +3 and one oxygen as -2; common number = 6; therefore arrange the molecules with 6+ and 6- which would be two +3 and three -2).
Write the equation above in your notes.
Study behavioral objective 5. An atom of phosphorus (atomic number 15) has an atomic mass of 32 amu.
7. Determine the following information concerning phosphorus.
- Number of electrons, protons and neutrons in this atom.
- Number of shells and the number of valence electrons.
- Charge found on a phosphorus ion.
Study behavioral objective 13. Atoms are indicated by the symbol without
a charge, ions are indicated with a (+) or (-) charges and molecules usually
consist of two or more atoms.
8. Which of the following represent an ion?
- a) H
- b) H+
- c) H2
- d) Hy
9. Which of the following represent an atom?
a) H
b) H+
c) H2
d) Hy
Press here to check your answer.
10. Which of the following represent a molecule?
- a) H
- b) H+
- c) H2
- d) Hy
Salts, Acids, Bases:
Study behavioral
objective 15 and 16. And read the pages indicated by A-5
in the text.
Molecules which contain ionic bonding can be classified as a salt, acid, or base. When ionic compounds are placed in water they will break up (disassociate) into their individual ionic parts.
Example: NaCl ---------> Na+ + Cl-
Reactant Products
This equation represents a molecule of NaCl on the right (reactants) which when placed in water will form the ions of Na+ and Cl- (products) in a one to one ratio.
Example: Fe2
O3 ---------> 2 Fe+3
+ 3 O-2
Reactant
Products
Example: Na2 O
---------> 2 Na+ +
O-2
Reactant Products
11. Write a reaction showing the disassociation of potassium oxide when place in water indicating the reactants and products. potassium oxide = K2O
Press here to check your answer.
Read behavioral
objective 11. Electrolytes conduct electricity and nonelectrolytes
do not contain electricity. Therefore compounds with ionic bonding are
electrolytes and compounds with covalent bonding are nonelectrolytes. The
solutions above are considered as electrolytes. Rubber and organic solvents
such as benzene are nonelectrolytes.
pH ranges:
Study behavioral objectives 17 and 18 and examine the pH scale in the text. (Reading A-5).
An acid is a substance that gives off hydrogen ions. When HCl is added to water H+ and Cl- ions are formed. Pure water contains HOH which disassociates into H+ and OH- ions in equal numbers. -OH ions are called hydroxide ions. Therefore when an acid (HCl, HNO3 ,) is added to water the number of hydrogen ions (H+) will be greater than the number of hydroxide ions (OH-); therefore the solution will be an acid. A base is a substance which combines with H+ and thereby reduces their number. Sodium hydroxide (NaOH) will disassociate and the hydroxide (OH-) will combine with the H+ removing hydrogen from the solution. All hydroxides (-OH) are bases. Salts are ionic compounds which when placed in a solution will not effect the hydrogen ion concentration. NaCl, BaO, Fe2O3, AlCl3 are all salts because they do not add H+ or OH ions to the solution.
12. Identify the following as acid, base, or salt?
Ca(OH)2
Press here to check your answer.
13. Identify the following as acid, base, or salt?
MgS
Press here to check your answer.
14. Identify the following as acid, base, or salt?
H2 SO4
Press here to check your answer.
The pH of a solution is a measure of its concentration of hydrogen
ions.
Acid = 0 - 6.9
Neutral = 7
Basic = 7.1 - 14
The pH ranges near 7 are weak acid or base and the pH ranges at either end are strong acids or bases.
pH 2 = strong acid (stomach acid),
pH 6.8 = weak acid (saliva)
pH 12 = strong base (Draino cleaner),
pH 7.2 = weak base (milk)
Molecular Weight Determination:
The molecular weight of a molecules is the sum of all the individual atomic weights of the atoms present in an atom. The atomic weight is expressed in amu (atomic mass unit). Given the atomic weight of carbon is 12 amu and the atomic weight of oxygen is 16 amu.
15. Determine the molecular weight of carbon dioxide CO2.
Press here to check your answer.
Given the following atomic weights:
H = 1 amu, S = 32 amu, O = 16 amu.
16. Determine the molecular weight of sulfuric acid H2 SO4
Press here to check answer.
This is the end of the third lesson, ionic bonding and some
properties if ions. The second type of bonding which may hold atoms together
is called covalent bonding. Please go back to the Miniunit
Alpha Lesson Page and continue to lesson four concerning covalent
bonding.
For information on how to use this page, go to How
to Use This Site.
Created by the Center for Learning Technologies, Academic Technology Services.
Last modified October 22, 1997.