Ask your student to recall Activity 2, the Lewis symbols or electron dot formulas do not include the inner electrons of the atom. Tell them that it only shows the valence electrons as dots. For example, fluorine has seven valence electrons. Thus, to form the fluorine molecule, the two fluorine atoms will share electrons. Each fluorine atom has eight electrons (an octet) in its valence shell, just like the electronic configuration of the nearest noble gas element, neon (Ne). It is important to emphasize to the learners that after chemical bonding atoms became isoelectronic with noble gases. Discuss also that there is a pair of bonding electrons between the two F atoms and three pairs (six electrons) of nonbonding electrons belonging to each atom as shown below:
.. ..
:F:F:
. . . .
The bonding electrons are counted as belonging to both atoms. The nonbonding electrons are those that are not shared with another atom.
You can detect the number of bonding and nonbonding electrons through a computation based on octet rule. Introduce to your students a mind-set of determining the total available valence electrons and detecting electrons needed to attain stability. It will help them identify the number of shared electrons (bonds) and unshared electrons through the following computations:
Bonding by Sharing of Electrons
Activity
4
DRAFT
April 29, 2014
19 a. Get the total available valence electrons in a compound (TAVE).
For H2S
hydrogen atom has 1 valence electron sulfur atom has 6 valence electrons
Total Available Valence Electrons = (2 H atoms x 1) + (1 S atoms x 6) = 2 + 6
= 8
b. Compute for the Octet Rule requirement that each atom should have 8 valence electrons to become stable except for hydrogen, it only needs 2electrons to become stable.
Number of Electrons based on Octet Rule = (2 H atom x 2) + (1 S atom x 8) = 4 + 8
= 12
c. Subtract a from b, then divide the difference by 2 because a pair of shared electron is equal to 1 bond. The quotient will give you the number of bonds around the central atom.
(12 – 8) Number of bonds =
2 = 2
Thus, there will be two pairs of shared electrons and two pairs of unshared electrons. . .
H S:
H
DRAFT
April 29, 2014
20 Completion of Table 2 will facilitate the acquisition of the above-mentioned skills.
Table 2. Types of Covalent Bonds Compound Chemical
Formula
Lewis Structure Type of Bond (polar covalent/nonpolar
hydrogen gas H2 H:H nonpolar covalent
phosphine PH3
DRAFT
April 29, 2014
21
chlorine gas Cl2
.. ..
:Cl:Cl:
. . . . nonpolar covalent
Note:
Emphasize to the students that there are molecules and ions which have more than eight (8) valence electrons around the central atom. These are elements which are non-metals from Period 3 or higher, which have d orbitals that are available for the two extra electrons to occupy. Thus, sulfur (S) in sulfur dioxide is surrounded by 10 electrons.
Answers to Questions:
Q1. How do covalent bonds form between atoms?
Covalent bonds form between atoms due to the sharing of electrons to attain stability.
Q2. What kind of element usually forms covalent bond? Is it possible for metals and non-metals to form nonpolar covalent bonds? Why? How about polar covalent bonds?
Why?
Generally, non-metals form covalent bonds. However, there are cases that metals and non-metals also form polar covalent bond. It is impossible for a metal and a non-metal to form a nonpolar covalent bond.
Q3. Why is it that diatomic molecules always form nonpolar covalent bonds?
Diatomic molecules always form nonpolar covalent bonds because of the equal electronegativity values resulting to equal sharing of electrons.
Q4. Differentiate polar covalent bond from nonpolar covalent bond.
Polar covalent bond involves unequal sharing of electrons while nonpolar covalent bond involves equal sharing of electron.
DRAFT
April 29, 2014
22 Metals have low ionization energy so they easily lose their outermost electrons. A large number of metal atoms can share their valence electrons through a special type of bond called metallic bonding. This type of bonding is different from the covalent and the ionic bond. In metallic bonding, the electrons are not moving around one nucleus. The positive atomic nuclei of the metal are surrounded by electrons moving freely throughout the piece of metal. These moving electrons in metals are called a “sea of electrons.” So, what holds the metal together are the strong forces of attraction between the positive nuclei and the freely moving electrons.
You may ask your students to draw how a metallic bond looks like. This is in order for you to find out their mental models about metallic bonding after you have explained what takes place in metallic bonding.
This is a simplified model of metallic bonding. It cannot account for the differences in properties of individual metals. The bond theory of metals will be able to explain the individual differences among metals. This bond theory will be explained in chemistry lessons at the university level.
Answers to Questions:
Q1. What do you think will make bonding among metals possible?
Metals tend to lose electrons to become stable. This property makes metallic bonding possible the positive atomic nuclei are surrounded by moving electrons. Since the latter are negatively charged, they are attracted to the positively charged nuclei.
Table 3. Metallic Properties
Metallic Property Explanation
Luster Metals are lustrous because when light strikes the surface of the metal, the free valence electrons reflect the light giving the metal a shiny appearance.
Malleability Metals can be flattened or can be formed into sheets when being hammered because of the ability of the metal atoms to slide over one another without breaking the metallic bond.
Ductility Metals can be drawn into fine wire because of the free moving electrons which enable the metal atoms to slide over each other.
Good Conductor of electricity
Metals are good conductors of electricity because the electrons are free to move within the metal.
Good thermal conductor
Metals are good conductors of heat because the positive metal nuclei are close together and can easily transfer the heat. The motions of the moving electrons also transfer heat.