Chemical Bonds and Valence Electrons

1. CHAPTER 19 MOLECULES AND COMPOUNDS

2. 19.1-Chemical Bonds • MOST atoms are unstable unless they are combined with other atoms. • When atoms of different elements combine to make compounds, they form Chemical Bonds. • Chemical bonds form when atoms exchange or share electrons.

3. Valence Electrons & The Octet Rule • Electrons in atoms are found in Energy Levels surrounding the nucleus in the form of an electron cloud. • The higher the energy level, the more energy is required in order for an electron to occupy that part of the electron cloud. • The outermost energy level contains ValenceElectrons and is called the valence shell.

4. Valence Electrons & Octet cont… • The maximum number of valence electrons that an atom can have is EIGHT. • The exception to this rule is the first energy level, which only holds two electrons. • When an atom has eight valence electrons, it is said to have an Octet of electrons.

5. Valence Electrons & Octet cont… • Atoms form chemical bonds with other atoms by either sharing electrons, or transferring them in order to complete their octet and move to a lower energy state! • This is known as the Octet Rule! • Look at page 328 (Table 19.1) in your textbook!

6. EXCEPTIONS to the Octet Rule! • Some atoms are an exception to the octet rule! • Hydrogen, with only one electron, needs only one more to fill its valence shell. • Helium, with two electrons, has a full valence shell. This means that Helium is chemically stable and does not bond with other atoms.

7. What’s Needed for The Octet

8. Example for The Octet Rule • Carbon’s atomic number is six. • Two electrons belong in level one and four electrons belong in level two. • Therefore, Carbon has four valence electrons in its outer shell and four electrons are needed to have the octet rule!!!

9. Electrons & The Periodic Table • The periodic table arranges elements from left to right by the number of valence electrons. • The Alkali Metals (Group 1) have one valence electron. • Group 2 elements have two valence electrons. • The elements in groups 3-12 are called Transition Metals. These elements have electrons in the fourth & fifth energy levels.

10. Electrons & Periodic Table cont… • Group 17 have seven valence electrons and are called Halogens. • The Noble Gases (group 18) have no valence electrons and do not form chemical bonds with other atoms. • Look at page 329 in your textbook and identify each group and its name.

11. Why Chemical Bonds Form • Chemical Bonds are a form of potential energy. • Energy is released when atoms form chemical bonds. • So…It takes energy to separate bonded atoms, then the same energy must be released when the bond is formed!

12. Why Chemical Bonds Form cont… • Atoms are most stable whey they have either 2 or 8 electrons. • Other atoms form chemical bonds so that they can share electrons to reach the stable number of 8. • The Lewis Dot Diagram shows the element symbol surrounded by 1 to 8 dots representing the valence electrons.

13. Lewis Dot Diagrams • Neon= 8 valence electrons • Fluorine= 7 valence electrons • Oxygen= 6 valence electrons • Nitrogen= 5 valence electrons • Carbon= 4 valence electrons • Boron= 3 valence electrons • Beryllium= 2 valence electrons • Lithium & Hydrogen= 1 valence electron

14. Why Chemical Bonds Form cont… • Oxygen has six valence electrons, that means that oxygen needs two to get to eight. • One oxygen atom bonded with two hydrogen atoms (water) is one way to make eight. • One oxygen atom can also bond with one beryllium atom to make eight because it has two valence electrons.

15. Examples of The Lewis Diagrams

16. More Examples of Lewis Diagrams

17. Elements & Bonding

18. Some Elements Prefer to Lose Electrons • Some elements can achieve the stable eight electrons more easily by losing an electron than by gaining one. . . . . . . . . Na . . . . Cl Na+ + - Cl +

19. Ionic & Covalent Bonds • Electrons in an Ionic Bond are TRANSFERRED from one atom to another. • Atoms that either gain or lose an electron become Ions. They may have either a positive or negative charge. • The atom which takes the electron acquires an overall negative charge! • The positive & negative ions are attracted to one another and create a bond!

20. Ionic Bonding

21. Ionic & Covalent Bonds cont… • In a Covalent Bond the electrons are SHARED between atoms. • The electrons in a covalent bond act like ties between the two atoms. • Elements that have two to six valence electrons tend to form covalent bonds with each other since the tendency to take or receive electrons is more matched.

22. Covalent Bonding

23. Ionic & Covalent cont… • The alkali metals with one valence electron have a high tendency to give up an electron. • The halogen elements with seven valence electrons have a high tendency to take an electron.

24. How Do I Remember Each Bond?

25. The Formation of Bonds • Formation of Ionic Bond: • Example: When sodium loses one electron, it has 11 protons (+) and 10 electrons (-) and becomes an ion of +1. • Example: When chlorine gains one electron to complete its stable octet, it has 17 protons (+) and 18 electrons (-) and becomes an ion with a charge of -1.

26. Opposites Attract • When a sodium ion has a positive charge and the chlorine ion has a negative charge, the two atoms become attracted to each other and form an ionic bond.

27. Diatomic Molecules • Covalent bonds can form between two different types of atoms, or between two or more atoms of the same type. • Many elemental gases in our atmosphere exist in pairs of covalently bonded atoms. • Examples:

28. 19.2-Chemical Formulas • Atoms combine in certain ratios with other atoms. • These ratios determine the Chemical Formula for a compound. • For Example: Sodium Chloride is NaCl. This formula indicates that every formula unit of sodium chloride contains one atom of sodium and one atom of chloride, a 1:1 ratio.

29. Chemical Formulas cont… • When sodium loses one electron, it becomes an ion with a charge of +1. • When chlorine gains an electron, it becomes an ion with a charge of -1. • So… when these two ions combine to form an ionic bond, the net electrical charge is zero! This is because (+1) + (-1) = 0 • All compounds have an electrical charge of zero: they are neutral!!!

30. Chemical Formulas cont… • An Oxidation Number indicates how many electrons are lost, gained, or shared when bonding occurs. • When writing the oxidation number, the positive (or negative) symbol is written AFTER the number, not before it. • Look at page 335 in your textbook, and observe the oxidation numbers on the periodic table.

31. Oxidation Numbers for Common Elements

32. Predicting Oxidation Numbers From the Periodic Table • If you can determine how many valence electrons an element has, you can predict it oxidation number. • For Example: Beryllium (Be) on the periodic table is in group 2, and has 2 valence electrons. It is much easier to lose two electrons than to gain six electrons (to complete the octet rule.) Once it loses the two electrons, Beryllium becomes an ion with a charge of 2+.

33. Writing Chemical Formulas of Ionic Compounds • Ions that consist of a single atom are called Monoatomic Ions. • Writing the formula for compounds made of monoatomic ions is simple, just follow these rules: • Write the symbol for the monoatomic ion that has a positive charge first. • Write the symbol for the monoatomic ion that has a negative charge second.

34. Writing Formulas cont… 3. Add Subscripts below each element symbol so that the sum of the positive and negative oxidation numbers is equal to zero- a neutral compound. Subscripts tell you how many atoms of each element are in the compound. • Note: Some elements have more than one oxidation number. Roman numerals are used to distinguish the oxidation number.

35. Elements With Variable Oxidation Numbers

36. Example of Writing a Chemical Formula • Question: Write the formula for a compound that is made of iron (III) and oxygen. • Find the oxidation numbers of each element in the compound. • Iron (III) indicates that it has an oxidation number of 3+, so it is Fe3+. • Oxygen has an oxidation number of 2-, so it is O2-.

37. Writing a Chemical Formula cont… 2. Determine the ratios of each element and write the chemical formula. • It will take two iron (III) for every three oxygen ions to equal zero! This means that three iron will bind with two oxygen to get a neutral compound. It looks like this: 2(3+) + 3(2-) = 0 • So… The formula for a compound of iron (III) and oxygen is Fe2O3

38. Ionic Compounds Made of More Than Two Types of Atoms • Some ionic compounds contain Polyatomic Ions. These contain more than one type of atom. • The prefix poly means “many.” • An example would be ammonium (NH4+)

39. Polyatomic Ions

40. Rules for Writing Chemical Formulas of Ionic Compounds That Contain Polyatomic Ions • Write the chemical formula & oxidation number of the positive ion. If the positive ion is polyatomic, use table 19.2 to find the oxidation number of the polyatomic ion. • Write the chemical formula & oxidation number for the negative ion. Again, use table 19.2 if the ion is polyatomic.

41. Rules for Polyatomic Ions cont… 3. Add the oxidation numbers of the positive and negative ions. Do they add up to zero? If yes, write the formula for the positive ion first & the negative ion second. Do not include the oxidation numbers in the chemical formula. Be sure to write the subscripts! 4. If the oxidation numbers do not add up to zero, figure out how many of each ion you will need so that the oxidation numbers add up to zero.

42. Example of Writing a Chemical Formula for Aluminum Sulfate • Question: The chemical formula for Aluminum Sulfate is… • Find the formula & charge of the positive ion. • Aluminum is a monoatomic ion and its oxidation number is 3+, so it is Al3+. 2. Find the formula & charge of the negative ion. • Table 19.2 states that its chemical formula & oxidation number is SO4 2-.

43. Writing a Chemical Formula cont… 3. Determine how many of each ion are needed so the charges are equal to zero. • The oxidation numbers of (3+) and (2-) add up to (1+), not zero! So… You need 2 aluminum ions and 3 sulfate ions to make the charges add up to zero. • (Al3+) x (2) = (6+) (SO4 2-) x (3) = (6-) 4. Write the Chemical Formula for the compound. • Write the formula, enclosing sulfate in ( ), do not change the subscripts in the ion. • Answer is Al2(SO4)3

44. How Do You Name Ionic Compounds? • Compounds with only monoatomic ions: • Write the name of the first element in the compound. • Write the root name of the second element. For example, chlor- is the root name of chlorine. Simply subtract the –ine ending. • Adding the ending –ide to the root name. Chlor- becomes chloride.

45. Naming Ionic Compound cont… • Compounds that contain polyatomic ions: • Write the name of the positive ion first. Use the periodic table or an ion chart to find its name. • Write the name of the negative ion second. Use the periodic table or an ion chart to find its name.

46. Covalent Compounds • Covalent compounds that are made of more than two types of elements have their own special naming system. • Naming covalent compounds that consist of only two elements are called Binary Compounds. • The Greek prefixes are used in naming binary covalent compounds.

47. Covalent Compounds cont… • If the molecule contains only one atom of the first element, the prefix mono- is not used. It is used in the name of the second element. • For Example: CO is Carbon monoxide CO2 is Carbon dioxide

48. Prefixes Used in Naming Binary Covalent Compounds

49. Give It a Try!! • How do you say/write N2O4? • The answer is dinitrogen tetraoxide!

50. Empirical & Molecular Formula • The simplest whole-number ratios by which elements combine are written in a form called the Empirical Formula. • The actual number of atoms of each element in the compound is written in a form called the Molecular Formula.