1. Chapter 4 Formation of Compounds
Section 4.2
How Elements Form Compounds

2. Objectives: Model two types of compound formation- ionic and covalent, Demonstrate how and why atoms achieve chemical stability by bonding, Compare the effect of covalent and ionic bonding on the physical properties of compounds

3. Compound formation
When elements react, atoms of the elements must collide
Reactions between atoms only involve their electron clouds- v.e. of colliding atoms interact

4. Noble Gases Group 18 elements Unreactive elements, extreme stability
Noble gases are used in light displays, neon lights 
Lack of reactivity of noble gases indicates that atoms of these elements must be stable
Each noble gas has 8 valence electrons, except helium has 2

5. NOBLE GAS CONFIGURATION
Achieving the same configuration of v.e.’s (a stable octet) as one of the noble gases

6. OCTET RULE
Atoms can become stable by having eight electrons in their outer energy level (or 2 for H and He)
If atoms collide with enough energy, their outer electrons may rearrange to achieve a stable octet of v.e.’s → the atoms will form a compound

7. Valence electron rearrangement
2 possibilities:
Transfer (ionic) - metal and nonmetal
Sharing (covalent) - 2 nonmetals

8. Electron Transfer (Ionic Compound)
Can use Lewis dot structures to represent atoms and the rearrangement of v.e.’s.
Colliding atoms transfer electrons only when one atom has stronger attraction for valence electrons than the other atom
Electrons will move from one atom (metal) to the other (nonmetal)

9. Electron Transfer (Ionic Compound)
Example: Transfer gives both Chlorine and Sodium stable octet
Na • + • Cl : → [Na] + + [ ; Cl : ] • • • •
Try these: Ca and Cl, Li and S, K and P
Atoms are no longer neutral- become ions

10. Electron Transfer (Ionic Compound)
ION: Atom or group of combined atoms that has a charge because of the loss or gain of electrons.
Ions always form when v.e.’s rearrange by electron transfer between atoms

11. Electron Transfer (Ionic Compound)
IONIC COMPOUND: A compound that is composed of ions
Once the ions have formed they are strongly attracted to each other.
IONIC BOND: Strong attractive force between ions of opposite charge

12. Electron Transfer (Ionic Compound)
Ions in salt do not arrange themselves into isolated sodium ion/chloride ion pairs. They arrange themselves into a well-organized definite cube structure
CRYSTAL: Regular repeating arrangement of atoms, ions or molecules

13. Electron Transfer (Ionic Compound)
Ionic attractions are strong attractive forces so it is hard to break ionic compounds and they have high melting point temperatures

14. Properties of Ionic Compounds
Generally, ionic compounds: 1) Crystalline solids at room temperature (Why?) 2) Hard, rough and brittle (Why?) 3) High temperatures in order to melt (Why? What does this look like?) 4) They tend to conduct electricity when they dissolve in water or are melted (Why? What does this look like?) ELECTROLYTE: Compound that conducts electricity when melted or dissolved in water.

15. 1. Would NaCl dissolved in water conduct electricity?
Predict:
1. Would NaCl dissolved in water conduct electricity?
Would this be an electrolyte? Why or why not?
2. Would NaCl dissolved in ethanol conduct electricity?
3. Would distilled water conduct electricity?
Why or Why not?

16. Chemical Formulas Formulas of compounds tell
What elements make up the compound
How many atoms of each element are present in the compound
Ex: Water – H2O has 2 hydrogen atoms and 1 oxygen atom
Sodium Chloride- NaCl has 1 sodium and 1 chlorine atom

17. Electron Sharing/ Covalent Compounds (molecules)
When atoms collide with enough energy to react, but neither attracts electrons strongly enough to take electrons from the other atom, the atoms combine by sharing v.e.’s

18. Electron Sharing/ Covalent Compounds (molecules)
Ex: Sharing gives both H and O stability
Hydrogen has 2 valence electrons and oxygen has an octet when they share electrons.
• • • •
H+ • O : → H : O : (O has 7 e-s)
• •
• • • •
H : O : + H → H :O :
• •

19. Electron Sharing/ Covalent Compounds (molecules)
COVALENT BOND: Attraction of two atoms for a shared pair of electrons In a covalent bond, atoms (2 nonmetals) share electrons and neither atom has an ionic charge COVALENT COMPOUND: Compound whose atoms are held together by covalent bonds

20. Electron Sharing/ Covalent Compounds (molecules)
MOLECULE: Uncharged group of two or more atoms held together by covalent bonds
Covalent bonds= molecular compounds

21. Sharing More than 2 e-‘s
More than 2 electrons can be shared in some cases of electron sharing
Example: Carbon Dioxide
: O • + • C • + • O : →
• • •
: O : :C : :O :
•• • •

22. Properties of Covalent bonds
Attractive force between molecules are usually weak INTERPARTICLE FORCES: Forces between particles that make up a substance

23. Properties of Covalent bonds
Many covalent compounds 1) Are liquids or gases at RT or are solids that melt at low temperatures 2) Do not conduct electricity 3) do not usually dissolve in water 4)In general, are usually less soluble in water than ionic compounds.

24. Lewis Dot Structures (Structural Formulas)
Steps to determine the number of bonds:
1) Have -Total number of V.E.
2) Need - Total number of V.E. (Happy)
3) Subtract Need – Have
4) Divide by 2
5) Draw with that number of bonds

25. Polyatomic Structures