1. Chapter 20: Molecules and Compounds

2. Chemical Bonds
A chemical compound occurs when 2 different elements bond together so that both become stable.

3. Why do atoms form chemical bonds?
Chemical bonds form to lower the energy of the system.
Components of the system become more stable by forming bonds.
Everything wants to be more stable - its easier to lie down than to stand up.
Bonding is nature's way of allowing the elements to lie down.

4. Atom Stability
* In order for atoms to be stable, they need to have their outer energy level completely filled. * In most cases, atoms need to have 8 valence electrons in their outer energy level (8 is great). * Atoms with 8 valence electrons are said to have octets.

5. The Noble Gases (group 18) naturally have octets and are said to be chemically stable. All other atoms must either lose or gain electrons to become stable.
There are exceptions to the octet rule: atoms with only one energy level need only two electrons to be stable, ex. hydrogen and helium.

6. Electron Losers & Gainers
Atoms want to become stable by doing the least amount of work possible.
Consider the following analogy:
You are sitting at a table all by yourself . 7 of your friends are sitting at the table next to you. Will it be less work for you to join your friends or for all 7 of your friends to join you?
Of course, it takes less work for you to join your friends.

7. Who gains, who loses??
Is it easier for group one atoms to lose their 1 valence electron or to gain 7 more?
Group 1 = Lose 1 electron
Is it easier for group two atoms to lose their 2 valence electrons or to gain 6 more?
Group 2 = Lose 2 electrons
Is it easier for group seventeen atoms to lose their 7s electron or to gain 1 more?
Group 17 = Gain 1 electron

8. In general, metals are electron losers, and nonmetals are electron gainers.

9. Metals * All metals form cations Metals are electron losers
* All atoms that lose electrons and form positive ions are called cations.
* All metals form cations
Metals are electron losers
(so they become positive.)

10. Example: Lithium
An atom of lithium has 3 protons and 3 electrons
An atom of lithium is electrically neutral because the (+) = (-), however, it is unstable.
This energy level can hold 8 electrons, it needs 7 more to be stable e
Valence electron + e + + e

11. Lithium will lose its 1 valence electron to become stable
Lithium will lose its 1 valence electron to become stable. Once Lithium loses its 1 valence electron, the number of (+) = (-)
Now lithium has 1 more (+) than (-). Lithium has become an ion with a charge of 1+. Written as Li1+ e + + + e

12. Nonmetals * All non-metals form anions.
* All atoms that gain electrons and form negative ions are called anions.
* All non-metals form anions.
* Non-metals are electron gainers

13. An atom of fluorine has 9 protons and 9 electrons.
Example: Fluorine
An atom of fluorine has 9 protons and 9 electrons.
An atom of fluorine is electrically neutral; however, it is unstable. e e e e
This energy level can hold 8 electrons, it needs one more to become stable + + + e + + + + e + + e e e

14. Now, fluorine has a complete outer shell. + + + + + e + + e + + e e e
Fluorine will gain 1 electron to become stable. Once fluorine gains an electron, the number of (+) = (-)
Now, fluorine has 1 more (-) than positive. Fluorine has become an ion with a charge of 1- Written as F1- e e e e
Now, fluorine has a complete outer shell. + + + e + + e + + e + + e e e

15. Bonding
Atoms become stable by transferring or sharing electrons with other atoms.
Ionic Bonds are formed when atoms transfer electrons (occur between metals and non-metals)
ex. NaCl (salt)
Covalent Bonds are formed when atoms share electrons (occur between non-metals)
ex. F2

16. Hydrogen Bonds into Helium
Hydrogen is a very reactive element. When it bonds with itself, it becomes helium, which is non-reactive.
Hydrogen reaction was very evident in the zeppelin (blimp) Hindenburg disaster.

17. The Hindenburg Disaster
Broadcast Recording:

18. Ionic Bonds Occur between a metal and a non-metal.
Ionic bonds form when electrons are gained or lost.

19. Covalent Bonding and the Octet Rule:
Covalent Bonds
Equal sharing of electrons between two atoms in a molecule
                                  
Covalent Bonding and the Octet Rule:
Atoms will share electrons so that each atom will fill its valence shell

20. Hydrate vs Anhydrous Compounds
A hydrate compound is one that has water chemically attached to its ions and written in the formula (H20). If water is attached, the formula is followed by a dot (*):
CaSO4 * 2H20
A anhydrous (without water) compound means that water has been removed, so it reverts back to the original compound/

21. Molecules
When a neutral particle forms as a result of covalent bonding, it is called a molecule. Sometimes one atom is much larger than the other in bonding (ex: Oxygen is larger than Hydrogen). So electrons being shared in a molecule are held more closely to the atoms with the larger nucleus.

22. Polar Molecules
When the larger element in the molecule has a stronger negative hold, then the smaller elements on the ends have a slightly positive charge. A polar molecule has a slightly positive end and a slightly negative end, causing them to attract similar molecules. This is what gives water its unique properties.

23. Polar Molecule

24. Nonpolar Molecules
When electrons are equally shared in bonds so that the molecule does not have oppositely charged ends. Molecules may form identical atoms or molecules that are symmetric (CCl4).

25. Why should two atoms share electrons?
A covalent bond is a chemical bond in which two or more electrons are shared by two atoms.
Why should two atoms share electrons?
7e-
7e-
8e-
8e- F F + F
Lewis structure of F2
lone pairs F
single covalent bond
single covalent bond F
9.4

26. Lewis structure of water
single covalent bonds
2e-
8e-
2e- H + O + H O H or
Double bond – two atoms share two pairs of electrons
8e-
8e-
8e- O C or O C
double bonds
Triple bond – two atoms share three pairs of electrons
8e- N
8e- or N
triple bond
9.4

27. Ionic and Covalent Song

28. Polyatomic Ion
Poly means many, so polyatomic is many atoms. A polyatomic ion is a positively or negatively charged, covalently bonded group of atoms. For example, baking soda is hydrogen carbonate HCO3.

29. Oxidation numbers
Oxidation numbers indicate the number of electrons an atom will lose or gain to become stable.
Elements in the same family have the same oxidation number.
The transition metals have variable oxidation numbers.
All metals have a (+) oxidation numbers (meaning they lose electrons to stable)
All nonmetals have a (-) oxidation numbers (meaning they gain electrons to be stable)

30. What’s Next?
Now you are ready to use this information to write chemical formulas.

31. Chemical Formulas
A chemical formula is a shorthand method that uses symbols to tell which elements are in a compound, and their ratios. Example:
3CaF2 means that there are
3 Molecules of Calcium Fluoride
There are a total of 9 atoms in the 3 molecules

32. Rules for Writing Formulas:
The sum of the oxidation numbers for each element must equal zero. All compounds must have a neutral charge.
The ion with the positive oxidation number is written first. The ion with the negative oxidation number is written second.
Do not include the charges of the oxidation numbers in the formula.

33. Writing formulas for Binary Compounds
Definition: compounds made of two different elements.
Steps:
Write the symbol of the element with the positive oxidation number as a superscript.
Write the symbol of the element with the negative oxidation number as a superscript.
Criss-cross the oxidation numbers so they become subscripts. Leave the (+) and (-) signs behind.

34. The correct formula is: Na2O
Examples:
Sodium and Oxygen
Determine Ox. #s: Na 1+ O 2-
Criss-Cross: Na O 2-
Na2O1
The one is not needed.
The correct formula is: Na2O

35. The correct formula is: MgCl2
2) Magnesium and Chlorine
Determine the Ox #s: Mg Cl 1-
Criss Cross: Mg Cl 1-
Mg1Cl2
The one is not needed.
The correct formula is: MgCl2

36. Criss-Cross with Subscripts
Remember, you never change the subscript. If you use the criss-cross method and there is an existing subscript, DO NOT CHANGE IT. It just means that you will need to put that element or compound in parentheses and put the new subscript outside of the parentheses.

37. Examples
Al+3 + CO32- →Al2 (CO3)3 NH4 + CO32- → (NH4)2CO3 Notice that, if there is already a subscript, you put the new subscript outside of a parentheses.

38. Breaking Down Formulas
When you have a compound that you need to break down, then just “undo” the criss-cross method. Subscripts stay in place if they are next to an element. If the subscript is outside of parentheses, then they represent the oxidation number of the opposite element or compound.

39. Criss- Cross: POLYATOMIC IONS!
Before you do the criss-cross method, always check the oxidation number of polyatomic ions (page 619.) These have to be written first, before doing the criss-cross method. Cation Anion Ex: K3PO4 K PO4 then K PO43- FIND THE OXIDATION NUMBER FOR POTASSIUM (K) ON THE PERIODIC TABLE K 1+ PO43-

40. Breaking Down Formulas
Examples:
Cation (+) Anion (-)
NaOH Na OH
KNO K NO3
Ca(OH) Ca OH
Because the 2 is outside the parentheses, it has to be the oxidation number of the first element or compound.

41. Writing Names of Binary Ionic Compounds
You can write the name of a binary formula by
Write the name of the positive ion
If the positive ion can form more than one oxidation number, then you write the charge using roman numerals in parentheses.
Example copper can be +1, +2 or +3, so the formula would say Cu(I) Cu (II) or Cu (III). This is only the case if the positive ion can have more than 1 oxidation number. Otherwise, you just write the positive ion first.

42. Writing Names of Binary Ionic Compounds
3) Write the root name of the negative ion, The root is the first part of the element’s name. For example: Chlorine = chlor- Oxygen = ox- 4) Add the ending ide to the root. For example: BaF2 is Barium Fluoride. The subscript does not influence the name of ionic compounds.

43. Writing Names of Polyatomic Ionic Compounds
You would always write the name of the positive ion first.
Look on page 619 for the endings of polyatomic ions. You would write this ending after the name of the positive ion.

45. Conclusion Valence electrons determine the reactivity of an atom.
All atoms seek full outer shells (typically, an outer shell of eight electrons).
Atoms form ionic or covalent bonds to achieve stability.
Oxidation numbers assist in the development in the chemical formulas.