CHEMICAL BONDING AND VALENCY OF ELEMENTS

This is a presentation about chemical bonding and valency of elements appropriate for chemistry lessons.

1. CARLO JAMES Q. SABLAN BONDING SHS Science Teacher CHEMICAL

2. LEARNING OBJECTIVES: Apply th e octet rule in the formation of molecular covalent compounds. 01 03 Draw the Lewis structure of ions. 04 Write the formula of molecular compounds formed by the nonmetallic elements. Draw Lewis structure of molecular covalent compounds. 02

3. Key concept Atom seldom exists as independent particles in nature. The oxygen you breathe, the water you drink, and nearly all other substances consist of combinations of atoms that are held together by chemical bonds. A chemical bond is a mutual electrical attraction between nuclei and valence electrons of different atoms that binds the atom together.

4. CHEMISTRY CONCEPT Chemical bonds are defined as the forces that hold elements in molecules and ions. It was Gilbert N. Lewis who described that the tendency of atoms to combine with other atoms to form compounds is based on their need to achieve a stable electron configuration through losing, gaining, or sharing electrons. Valence electrons are the electrons in the outermost energy level of an atom. These electrons determine what kind of chemical bonds, if any, the atom can form. Electron configurations may be used to determine the number of valence electrons of an element. Dots are often used to represent the valence electrons in atoms and molecules. These structures are referred to as Lewis structures , electron-dot structures , or Lewis electron- dot structures .

5. THE OCTET RULE: The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds. When discussing the octet rule, we do not consider d or f electrons. Only the s and p electrons are involved in the octet rule, making it useful for the main group elements. Atoms will react to get in the most stable state possible. A complete octet is very stable because all orbitals will be full. Atoms with greater stability have less energy, so a reaction that increases the stability of the atoms will release energy in the form of heat and light. Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons.

6. VALENCY TABLE OF CHEMICAL ELEMENTS

7. LEWIS STRUCTURES OF SOME ELEMENTS The dots are placed on the four sides of the symbol and each side can accommodate up to two electrons. In general, we spread out the dots as much as possible.

8. LEWIS STRUCTURES OF SOME ELEMENTS

9. PRACTICE: Cl Ne F P Ar 1 2 3 1 2 3 4 5

10. The formation of chemical bonds involves changes in the electron configuration of the atoms involved. Since the atoms of noble gases generally do not bond, they are stable as solitary atom ( inert or unreactive ). The three ways by which an atom can alter its electron configuration to obtain the octet (or duet) of electrons of a noble gas are:

11. A metal may lose one to three electrons to form a cation with electron configuration of the previous noble gas (the one with the next lowest atomic number). A nonmetal may gain one to three electrons to form an anion with the electron configuration of the next noble gas (the one with the next highest atomic number). Atoms (usually two non metals) may share electrons with other atoms to obtain access to the number of electrons in the next noble gas.

12. Types of Ionic Bonding It is the direct transfer of electron(s) from a metallic substance to a non-metallic substance to form a product called formula unit or simply compound. The atoms are linked with the connection called “ionic bond” and sometimes referred as “electrovalent bond.” Chemical bonds Example: Bond of NaCl Sodium is a metal with 1 valence electron, therefore it will lose electron. Chlorine is a nonmetal with 7 valence electrons, therefore it will gain electron from sodium atom. Note: Dot structures of metal ions have no dots, only nonmetal ions; must include charge

13. Types of Covalent Bonding It is the mutual sharing of electrons from 2 nonmetallic atoms to form an end product called molecule. It is usually presented with shared pair of electrons with a single line or dash from each opposite sides of atom’s symbol. The atoms are bonded through chemical bonds called “covalent bonds.” Chemical bonds Example: Bond of HCl Hydrogen needs 1 more electron to complete the duet rule. Chlorine needs 1 more electron to complete the octet rule. A dash line represents the sharing of electrons. Lone pairs (dots) represent unshared electrons.

14. EXAMPLE: The common convention is to show each shared electron pair as a line and any unshared electron pairs (also called lone pairs or nonbonding pairs ) as dots. Therefore, one might predict that 7A elements, such as F, would form one covalent bond to achieve an octet; 6A elements, such as O, would form two covalent bonds; and so on.

15. PRACTICE QUESTIONS 1. Draw the Lewis electron dot diagram for each element. bromine gallium 2. Draw the Lewis electron dot diagram for each ion. Mg +2 S −2

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