1. Noble Gases and Valence e- Ionization Energy and Bonding

2. A look back at the Periodic Table
Halogens, group 7A or 17: F, Cl, Br, I
Nonmetals
Element form is diatomic F Cl Br I2
At room temperature, F2 and Cl2 are gases, Br2 is liquid, I2 is solid
Reactivity decreases down group
F2 is the most reactive of all elements
I2 is the least reactive halogen

3. Trends in Element Families
Alkali metals, group 1A or 1: Li, Na, K, Rb, Cs
Metals
stored under oil to keep them from reacting with O2
Impart bright, characteristic colors to flames
Li = red Na = orange K, Rb, Cs = lavender
All react with water to produce H2
Reactivity increases down group
Cs is most reactive alkali metal
Li is least reactive alkali metal

4. Trends in Element Families
Noble gases, group 8A or 18: He, Ne, Ar, Kr, Xe
Nonmetals
Element form is monatomic
At room temperature, all are gases
Most significant property is that they are almost completely unreactive
Compounds of Xe, Kr, and recently Ar have been made with F and O, but they decompose very easily
No compounds of He have ever been prepared

5. How do Atoms Join?
The behavior of the noble gas, alkali metal, and halogen families is key to understanding bonding
Noble gases generally do not form compounds
Alkali metals form compounds by losing one electron to form a +1 ion (cation)
Halogens form compounds by gaining one electron to form a –1 ion (anion)

6. Ion sizes A cation is always smaller than its parent atom
An anion is always larger than its parent atom

7. Let’s Practice… Which ion is larger? Which ion is smaller? Se or Se-2
Al or Al+3

8. Ionization energy
Ionization energy (IE) is the energy needed to remove the outermost electron from an atom
Low IE electron is easier to remove
High IE electron is harder to remove

9. Ionization energy
IE decreases down a group (easier to remove e– as you go down a group)
IE generally increases across a period from left to right (harder to remove e– as you go across a period)

10. Ionization Energy

11. Let’s Practice… Which has a larger ionization energy?
Cs or F
Which has a smaller ionization energy?
Be or Mg

12. 2 3 He Li 9 10 11 F Ne Na 17 18 19 Cl Ar K Group 8A Noble gases
“happy to be me” 2 He 10 Ne 18 Ar 17 Cl 9 F 19 K 11 Na 3 Li
Group 1A
Alkali metals
“just one
less electron . . .”
Group 7A
Halogens
“just one
more electron . . .”

13. Valence electrons This behavior suggests that
The number of electrons in a noble gas is especially stable
That number of electrons forms a noble gas core of stable electrons unavailable for bonding
Only electrons outside the noble gas core participate in the formation of chemical bonds between atoms
Those electrons are called VALENCE ELECTRONS

14. Valence electrons 1A 2A 3A 4A 5A 6A 7A 8A FOR MAIN-GROUP ELEMENTS:H 2 He 1A 2A 3A 4A 5A 6A 7A 8A 3 Li 4 Be 5 B 6 C 7 N 8 O 9 F 10 Ne 11 Na 12 Mg 13 Al 14 Si 15 P 16 S 17 Cl 18 Ar 19 K 20 Ca 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr
FOR MAIN-GROUP ELEMENTS:
number of valence electrons = group number

15. Depicting Valence Electrons
Show the valence electrons for a main-group element using a LEWIS DOT STRUCTURE O • ••
Oxygen
Group 6A
6 valence electrons Li •
Lithium
Group 1A
1 valence electron C •
Carbon
Group 4A
4 valence electrons

16. Lewis Dot Structures Dot number is important, positions are not• •• N •• • N • •• N • •• N
Nitrogen
Group 5A
5 valence electrons

17. Lewis Dot Structures
All the atoms in the same family have the same dot structure •• • F • •• Cl • •• Br • •• I
Halogens
Group 7A
7 valence electrons

18. Let’s Practice…
Draw the Lewis Dot Structure for: Si Rb As Br

19. Chemical Bonding Atoms form chemical bonds by
The stability of the noble gases suggests that for main group elements,
Atoms form chemical bonds by
losing, gaining, or sharing valence electrons
in order to achieve
the same number of valence electrons as
the nearest noble gas

20. Octet Rule Noble gases have 8 valence shell electrons (except He)
Atoms lose, gain, or share valence electrons in order to achieve 8 valence shell electrons
This is the OCTET RULE
Atoms near He (H and Li) will try to achieve just 2 valence shell electrons

21. Let’s Practice…
Octet rule predicts that an atom of Flourine will try to _____(gain/lose) _____(1,2,3,4) electron(s) when forming a compound.
Octet rule predicts that an atom of Calcium will try to _____(gain/lose) _____ (1,2,3,4) electron(s)when forming a compound.

22. Three Types of Chemical Bonding
Chemical Bonds can be classified into 3 broad categories, based on how valence electrons are arranged:
Atoms
Bond
Character
Properties
Metal
+ nonmetal
ionic
e- transferred
Forms a salt= crystal of cations and anions
Brittle
Solid does not conduct electricity, but conducts electricity when melted or dissolved in water
Nonmetal
covalent
e- shared
Composed of neutral molecules
Does not conduct electricity in any state (solid, liquid, or dissovled in water)
+ metal
metallic
e- pooled
Lattice of cations embedded in “sea of electrons”
Malleable
Solid and liquid both conduct electricity

23. Ionic bonds Ionic bonds form between metals and nonmetals
Metal gives up electrons
Nonmetal accepts electrons
Ions form
Because of opposite charges, ions stick together to form a crystal
Resulting compound is a SALT
NaCl, salt

24. Ionic bonds: electron transfer
Metals transfer electrons to nonmetals to form ions:
Na has 1 valence e–, wants to lose it to attain a Ne core
Cl has 7 valence e–, wants to gain 1 to attain an octet, like Ar
Solution is to transfer 1 electron from Na to Cl
Electron transfer forms ions
Ions stick together because of opposite charges •• • Cl Na  1– 1+

25. Dot structures for ionic compounds
Draw dot structures for these ionic compounds
Show the atoms before and ions after e– transfer
Use an arrow to show e– transfer
Show charges on ions after e– transfer
Write ions near each other but not together (they are not sharing the electrons)
MgO Na2O CaCl2

26. Electronegativity Electronegativity is the ability to attract bond e–
The higher the EN, the “greedier” the atom

27. Electronegativity
Electronegativity increases across a period and decreases down a group

28. Covalent bonds: electron sharing
Nonmetals share electrons to form covalent bonds:
Each Cl has 7 valence e–, wants to gain 1 e– to get an octet like Ar
Neither atom is willing to give up an electron
Solution is to share their unpaired electrons
The shared pair of e– is a single covalent bond •• Cl • •• Cl • •• Cl 

29. Covalent bonds Covalent bonds form between nonmetals
Nonmetals share unpaired valence electrons
Each atom “owns” all of its bond electrons
Each atom achieves an octet (or 2, for hydrogen)
One shared pair of electrons is shown with a single line •• Cl •• Cl •• Cl or ––

30. Bonding in O2 Oxygen is a diatomic element
Each unpaired electron must get into a bond
The atoms cannot achieve a filled valence level with a single bond, so a double bond forms: they share two pairs of electrons •• O • •• •• •• •• •• •• O •• •• •• •• = •• • ••  O O O O or •

31. How to draw a dot structure
Count valence e–
Put atom with lowest EN in center
Arrange other atoms around it symmetrically
Form single bonds between atoms (1 line = 2 e–)
Put lone pairs around terminal atoms to give each an octet (2 for H), then finish central atom octet
If central atom does not get octet, move in lone pairs to make double or triple bonds

32. Lewis dot structures Draw a Lewis dot structure for each species
NH3 HBr CO2
OH1– NH HCN
H2CO NO31– PF3
SO2 C2H4 Cl2O

33. How equally do atoms share e– ?
Hydrogen and fluorine share one pair of e– in a single covalent bond
In the Lewis dot structure, they appear to share the electrons equally, but do they?

34. How equally do atoms share e– ?
When you put HF in an electric field, the molecules line up, as if the F end were negative and the H end positive.
The electrons are shared unequally.

35. How equally do atoms share e– ?
When bond e– are shared unequally, the bond is said to be a polar covalent bond.
A polar covalent bond has a dipole: one end is more negative than the other end

36. Electronegativity and bond polarity
Bond polarity depends on the difference in EN

37. Evaluating bond type H–Br H–Br – H = 2.1, Br = 2.8 ∆EN = 0.7
Bond is polar covalent with Br end more negative
H–Br –

38. Evaluating bond type C = O C=O – C = 2.5, O = 3.5 ∆EN = 1.0
Bond is polar covalent with O end more negative
Doesn’t matter whether bond is single or double
C=O –

39. K1+ Cl1– Evaluating bond type KCl K = 0.8, Cl = 3.0 ∆EN = 2.2
Bond is ionic; e– transferred from K to Cl
K Cl1–

40. Metallic bonding: a sea of electrons
Lattice of positive metal cations (nucleus + core e–)
Lattice surrounded by sea of mobile valence e–
Valence e– can move => metals conduct electricty
Cations can slide past each other without breaking bonds => malleable

41. Metal alloys Alloys are mixtures that contain at least one metal
Examples: brass, bronze, stainless steel
Alloys usually stronger and harder than the pure metal
Alloys are not compounds because they do not have a definite composition
Table salt is a compound of 1 Na + 1 Cl = NaCl
Water is a compound of 2 H + 1 O = H2O
Brass is a mixture of Cu and Zn in any proportions