1. 4.2 How Elements Form Compounds

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

3. How Elements Form Compounds
In the 19th Century chemists tried to picture how atoms combine.
Early models included atoms with “hooks” that allowed them to attached to one another.

4. When Atoms Collide Atoms collide when they react
These collisions determine what kind of compounds are formed.
When atoms collide it is the electron clouds that interact with one another.
To be more precise it is the valence electrons that interact.
Remember: valence electrons are responsible for the chemical properties of elements

5. Valence Electrons and Bonds
To understand how valence electrons affect the bonds of compounds let look at elements with unusual properties….the noble gases

6. Noble Chemical Stability
Noble Gases are located in Group 18
They are inert or unreactive
This lack of reactivity makes them useful as incandescent light bulbs (argon or krypton) and neon lights (orange-neon, blue-argon, yellow-helium)
Occur naturally in the environment

7. Noble Gases
Not a single compound of these elements has ever been found naturally in the environment.
In 1960 chemists were able to react fluorine with krypton and xenon under high temperature and pressure.
Since then a few additional compounds of xenon and krypton have been synthesized, but none with helium, neon or argon.

8. The Octet Rule
Lack of reactivity of the noble gases must mean that they are stable.
Elements in the same group have similar valence arrangements.
Noble gases have 8 valence electrons, except for helium that has 2.
Because the electron arrangement determines chemical properties  unreactive nature of noble gases

9. Octet Rule Atoms combine because they become more stable by doing so.
Modern model of bonding is based on the fact that the stability of noble gases is a result of their valence electron arrangements.
Octet Rule- atoms can become stable by having eight electrons in their outer energy level (or two electrons in the case of some of the smaller atoms).

10. Noble Gas Configuration
Atoms obtain stability by achieving the same configuration of valence electron as found in noble gases; most stable configuration.

11. Valence Electron Review
How many valence electrons do you find in Group 1, 2, 13, 15, 16 and17?

12. Review
What is the octet rule?

13. Ways to Achieve a Stable Outer Energy Level
4.2 Continued
Ways to Achieve a Stable Outer Energy Level

14. Forming Compounds
When atoms collide with enough force their outer electrons may rearrange to achieve a stable octet of valence electrons.
When it occurs they achieve a noble gas configuration and the atoms form compounds

15. How do the electrons rearrange?
Electrons are transferred from one atom to another
Electrons are shared between atoms

16. Electrons Can Be Transferred
Example Sodium (Na) and Chlorine (Cl)
Na has one valence electron
Cl has seven valence electrons
Na transfers its 1 to Cl 7  Na now has 8 electrons in its outer shell and Cl now has 8 in its outer shell
Na now has 1 less electron  has +1 charge
Cl now has 1 more electron has a -1 charge

17. Ions
Atom or group of atoms that has a charge b/c they have either lost or gained electrons
Ions form when valence electrons rearrange by transfer
Compounds composed on ions are ionic compounds

18. Table 4.2 p. 134

19. Ions Attract Each Other
Opposites attract
Positive sodium is attracted to the negative chloride ion.
Positive ions are called cations
Names stays the same
Negative ions are called anions
Add –ide to the end
Sulfur  Sulfide ion

20. Ionic bond Strong attractive force between ions of opposite charge
Hold ions together in an ionic compound
Crystal is a regular, repeating arrangement of atoms, ions or molecules

21. The Results of Ionic Attraction
b/c of the strong attractive forces and the degree of organization  solid at room temp
Raising temp causes particles to move faster  particles overcome attractive forces and the crystal organization breaks down

22. Representing Compounds with Formulas
Sodium Chloride
Formula: NaCl
Write cation first followed by the anion
Always in the smallest whole number ratio

23. Review What is a negatively charged ion called?
What is a positively charged ion called?
What is the noble gas configuration?

24. Electrons Can Be Shared
What happens when hydrogen and oxygen collide?
Oxygen has 6 valence electrons
Hydrogen has 1 valence electron
Can hydrogen lose its one valence electrons?

25. Colliding atoms
Transfer electrons only when one atom has a stronger attraction for valence electrons
In the case of sodium and chlorine, chlorine attracts sodium’s valence electrons strongly and sodium is holding its electrons weakly
In the case of hydrogen and oxygen neither atom attracts strongly enough to take electrons from the other.

26. Hydrogen and Oxygen They will share their electrons
When atoms collide with enough energy to react, but neither atom attracts electrons strongly enough to take electrons from the other, the atoms combine by sharing valence electrons.

27. Let’s Look at Water

28. Formation of Compounds
Like the formation of sodium chloride all the components present before the reaction are still present after the reactions
The valence electrons no longer reside in the same position.
They have rearranged

29. Electrons Sharing Produces Molecules
Covalent bond- the attraction of two atoms for a shared pair of electrons
Electrons are shared
Neither atom has an ionic charge
Covalent compound- compound whose atoms are held together by a covalent bond, also called molecular compounds
Water is an example
Molecules-uncharged group of two or more atoms held together by covalent bonds (water molecules)

30. More Than Two Electrons Can Be Shared
Let’s look at Carbon Dioxide
Carbon has 4 valence electrons
Oxygen has 6 valence electrons

31. Covalent and Ionic bonds
Two nonmetallic elements usually form molecular or covalent compounds by sharing electrons
Nonmetallic and a metallic element usually form ionic compounds

32. Review What is a covalent bond?
What is the definition for a molecular compound?
Give me an example of a molecular compound?

33. Review What is a covalent bond?
What is the definition of a molecular compounds?
What are two examples of a molecular compound?

34. How do ionic and covalent compounds compare?

35. Bonding
When elements combine they either form ions or molecules…there are no other possibilities.
When particles change dramatically, when they change from sodium to sodium ions or from hydrogen and oxygen to water molecules.
These changes explain why compound have different properties from the elements they are composed of.

36. Explaining the Properties of Ionic Compounds
Physical properties are a result of the well-organized, tightly bound ions.
Ions forms strong, three-dimensional crystals

37. Properties of Ionic Compounds
Generally ionic compounds are crystalline solids at room temperature.
Generally hard, rough and brittle
High melting points and boiling points
This is because of the strong attraction between ions
Usually dissolve in water

38. Properties of Ionic Compounds
Solutions will conduct electricity
Liquid state (melted state) will conduct electricity
They are electrolytes- any compound that conducts electricity when melted or dissolved in water
In order to conduct electricity the ions must be free to move. Must take on or give up electrons
Solid state does not conduct electricity. Why?

39. Explaining the Properties of Covalent Compounds
Composed of molecules
Held together by covalent bonds-make the molecule a stable unit
Molecules have no ionic charge
Attractive forces between molecules are usually weak
Interparticle forces- forces between particles that make up a substance

40. Properties of Covalent Compounds
Many are liquids or gases at room temperature
Low melting points and boiling points
Do not conduct electricity
Some do not dissolve in water
Vegetable oil and gasoline
In general they are less soluble in water than ionic compounds

41. Ionic Vs. Covalent
It is the strength between interparticle forces in covalent compounds compared to the strong attractive forces of ions in ionic compounds is what explains many of the differences in their physical properties
Because covalent compounds do not have ions you would expect that they don’t conduct electricity
Ionic compounds are more soluble in water because ions are attracted by the water molecules, while molecular compounds are not

42. Ionic Vs. Covalent Compounds
High melting point
High boiling point
Solid at room temperature
Soluble in water
Excellent conductor of electricity
Many crystalline
Low melting point
Low boiling point
Liquid or gas at room temperature
Less soluble in water
Poor conductor of electricity