Overview

It is interesting to look back at the lessons you have learned about matter when you were in Grades 7 and 8. Do you still remember them? Yes! In Grade 7. you were able to describe the properties of metals and non-metals and to recognize elements and compounds. In Grade 8, you observed different common changes such as evaporation, condensation, boiling, and melting that helped you learn about the particles that matter is made of. You have also learned that the elements are systematically arranged and grouped in the Periodic Table of Elements. Your knowledge about matter is continuously growing. In fact, in the previous unit you were introduced to how electrons in different atoms are distributed. You have learned a lot so far!

Studying this module will certainly increase your understanding about matter.

Get your periodic table. What do you notice about the electronic configuration of the noble gases? You're right! Except for helium, all of them have eight electrons at the outermost energy level. The sharing, or the complete transfer of electrons causes an atom to have the same electronic configuration as that of the nearest noble or inert gas. The sharing or the complete transfer indicates that the atom has attained stability. Either the sharing or the complete transfer of electrons leads to the formation of compounds.

Going through this module will make you understand what is happening in the atoms during the formation of compounds. Look at the periodic table. Did you notice the vertical arrangement of the elements? This is called family or group. Notice the number in each group? Do you know what information it gives you? It tells you the number of valence electrons. Do you still remember the meaning of valence electrons? Valence electrons give you the number of electrons at the outermost energy level of the atom. This is the information you need to know in order for you to determine whether atoms transfer, accept, or share electrons to become stable. Why do we need to talk about the transfer or the sharing of electrons? You will discover the answer to this question as you study this module.

The valence electrons are the electrons directly involved in forming bonds to form compounds. It is important that you know the number of valence electrons so that can illustrate how bonds are formed. It is good that you have found out that metals have low electronegativity and non-metals have high electronegativity because this property plays an important role in forming compounds.

Electronegativity is a measure of the tendency of an atom to attract electrons, the higher its value, the higher its tendency to attract electrons. How about ionization energy? Did you know that ionization energy is the energy needed to pull or remove one or more electron/s from a neutral atom? The lower the ionization energy the easier it is to remove its valence electrons.

Do you know why atoms form compounds? Have you heard about the Octet Rule? Atoms always strive to attain the most stable arrangement of electrons. Atoms are stable if their electrons have the same kind of arrangement as that of noble gases, where the s and p orbitals are filled with electrons except for helium, where only the s orbitals are filled up. All the noble gases except for helium have 8 valence electrons. The Octet Rule tells you that elements gain or lose or share electrons to achieve the electronic configuration of the nearest noble gas. Thus after chemical bonding, elements become isoelectronic with the nearest noble gas in the periodic table.

Metals have low electronegativity and ionization energy, thus they tend to transfer or loose electrons. Non-metals have high electronegativity and ionization energy. They have a greater tendency to attract electrons towards themselves. Thus non-metals tend to gain electrons.

Try aluminum and chlorine. Will they form an ionic bond?

Al: EN = 1.5

:CI: EN = 3.0

Based on the difference in the EN values of Al and Cl, the difference is only 1.5. These two elements cannot form an ionic bond. In this case, there is not enough energy to facilitate the complete transfer of electrons. Instead, another bond is formed, the covalent bond, in which sharing of electrons takes place. This sharing the Al and Cl atoms attain stability.

There are two types of covalent bond: the polar covalent bond and the nonpolar covalent bond. If the electronegativity difference is equal to 0.4 or less, it results to a nonpolar covalent bond. If the electronegativity difference is less than 1.9 and more than 0.4, polar covalent bond is formed. Try to do the next activity to find out if covalent bonding takes place. Take note of the Octet Rule in considering the number of bonds that will be formed between atoms. These steps will help you figure out if sharing of electrons will take Place.

a. Get the total available valence electrons in a compound.

For CO₂

carbon atom has 4 valence electrons oxygen atom has 6 valence electrons

Total Available Valence Electrons (TAVE) = (1 C atom x 4) + (2 0 atoms x 6)

= 4+ 12

= 16

b. Compute for the Octet Rule requirement that each atom should have 8 valence electrons to become stable.

Number of Electrons based on Octet Rule = (1 C atom x 8) + (2 O atoms x 8) =8+16

= 24

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.

Number of bonds =

(24-16)

2

= 4

Thus, there will be 4 bonds surrounding a carbon atom as shown in the Lewis Structure:

:O::C::O: or :0=C=0:

In covalent bonding, a pair of shared electrons is equal to one (1) bond. Notice that after the sharing of electrons, each of the atoms in the compound attains a stable configuration and a covalent compound is formed. Such compound could exist as independent units called molecules. As a whole, the molecule does not carry a charge.

Recall that an ionic bond is formed when a metal bonds with a non-metal

while a covalent bond exists between or among non-metals. However, there are cases when a polar covalent bond involves a metal and a non-metal, like in the case of aluminium chloride (AIC).

Summary:

Let us have a synthesis of the concepts you have learned in this module:

The valence electrons are the outermost electrons which are directly involved In chemical bonding.

Lewis symbol is composed of the symbol of the element and dots which represent the number of valence electrons of an atom that can easily be determined through the family/group number in the Periodic Table of Elements

Atoms form bonds with one another to become stable and attain the electronic configuration of the noble gas nearest it.

An ionic bond involves complete transfer of electrons: thus, ions are formed. It involves metals with low electronegativity and non-metals with high electronegativity.

lonic compounds conduct electricity when in solution but not in solid phase.

Ionic compounds are generally soluble in water and in polar solvents.

A covalent bond involves the sharing of electrons that results in the formation of covalent compound whose representative particle is a molecule. As a whole, a molecule does not carry a charge.

Covalent bonds may be polar or nonpolar compounds

Two identical non-metallic atoms always form nonpolar covalent such as N, O, H2, F2 and other diatomic molecules.

Non-identical atoms with electronegativity difference higher than 0.4 and lower than 1.9 produce polar covalent bond.

Covalent compounds are non-conductors of electricity in the solid phase and in solution. They have a lower melting temperature than compounds formed by ionic bonds,

Metallic bonding exists in metals through the attraction between the freely moving valence electrons and the positively charged metal atom. The valence electrons of these metal atoms are usually called "sea of electrons."

Thermal and electrical conductivity in metals are due to the free flow of electrons in the solid phase. Aside from these properties, metals are lustrous, malleable, and ductile. These properties are related to the kind of bonding metals have.