Chemistry

1. Valence Electrons

2. What is a Valence Electron?
• The electrons that are in the
highest (outermost) energy level.
• That level is also called the
valence shell Of the atom.
• Electrons not in the outermost
shell are called core electrons.

5. Bohr Atomic Structures
• Electron Configuration of Na:
1s22s22p63s1
• The first energy level contains
2 electrons. (s orbital…1s2)
• The second level contains 8
electrons. (2s and 2p
orbitals…2s22p6)
• How many electrons do you
see in the outermost level?
3s1… 1electron!
This is the Valence number.
Sodium has 1 Valence electron.

6. Valence Electrons
•How many valence electrons are
in the following atoms?
•Ne
•O
•Na
•Cl

7. Valence Electrons
•How many valence electrons are
in the following atoms?
•1s22s22p6
•1s22s22p63s23p64s23d104p4
•[Kr]5s24d105p2
•[Ne]3s23p3

8. Valence electron trends
1)As you go down a group the number of
electrons in the valence shell stays the same
2) As you go across a period (left to right) the #
of e- in the valence shell of the main group
elements increases
one by one
- This ignores the transition metals

9. 9
Lewis dot Structures for the
atoms of groups 1-2 and 13-18 of
The periodic table
The Main group elements
Notice the e- trends in the Groups and Periods

12. The number of valence electrons in
an atom determines:
• The properties of the atom
• The way that atom will bond
chemically
• As a rule, the fewer electrons in
the valence shell, the more
reactive the element is.
• When an atom has eight electrons
in the valence shell, it is stable.

13. Noble Gas Stability
• Noble gases are usually unreactive
• This is because they have full valence shells
• An element with a full valence shell is a
happy element 
• For two atoms to join together atoms must
gain, lose or share electrons
• Elements with full valence shells do not
easily gain or lose electrons

14. Noble Gas Stability
• Atoms want to gain stability
• Atoms will try to gain or lose electrons
to have a full valence shell
• Metals try to lose electrons
• Non-Metals try to gain electrons

15. Oxidation Numbers
An oxidation number represents the number
of electrons an atom will typically gain or lose
to achieve the octet.
For example:
The oxidation number of Sodium is +1
The oxidation number of Oxygen is -2
Some Atoms can have more than one
oxidation number

16. Electron Dot Structure
•Electron-dot structure consists
of the element’s symbol
representing the nucleus,
surrounded by dots representing
the element’s valence electrons.

17. Lewis Theory
G.N. Lewis (1875-1946) recognized
valence (outmost) electrons
fundamental to bonding
electron transfer resulting in ionic
bonds
sharing electrons resulting in
covalent bonds
atoms tend to acquire a noble-gas
electronic configurations

18. Basic Chemical Bonding
Lewis Dot
Structure
Lewis wrote in a
memorandum
dated March 28,
1902

19. Lewis Structures
Lewis structures show ONLY valence electrons.
Examples:
Sodium (Na) is in group 1A,
so we know it has 1 valence
electron. Group 1A is
known as the alkali metals
Beryllium (Be) is in group
2A, so we know it has 2
valence electrons. Group
2A is known as the alkaline
earth metals

20. Lewis Structures
Lewis structures for oxygen
Oxygen is in group 6A, so it has 6 valence electrons.

21. How to Draw Lewis Structures
1) Look at the element you want to draw the structure for
and see what group it is in.
2) Draw dots around the symbol according to how many
valence electrons it has.
Example: Draw a Lewis Structure for calcium
1) We see calcium (Ca) is
in group 2A, so it has 2
valence electrons
2)
Ca

22. Formation of Cations
• A positively charged ion
Na  Na+ + e-

23. Mg  Mg2+ + 2e-

24. Formation of Anions
• Negatively charged particles
Cl + 1e-  Cl -

25. What is the Lewis electron dot diagram
for each ion?
• Ca2+
• O2−

26. Solution
 Having lost its two original valence electrons,
the Lewis electron dot diagram is just Ca2+.
Ca2+
 The O2− ion has gained two electrons in its
valence shell, so its Lewis electron dot
diagram is as follows:

27. Questions?

28. Thank you for listening! 

29. ASSESSMENT

30. Questions
1. What is a valence electron? Draw Na atom and
show the valence electrons.
2. How many valence electrons do the following
elements have:
Si, C, O, Ar, K
3. What does inert mean?
4. How many valence electrons do atoms like to
have?
5. What do “groups” on the periodic table have in
common?

31. Questions
1. Draw the Lewis electron dot
diagram for each element.
bromine
gallium
2. Draw the Lewis electron dot
diagram for each ion.
Mg2+
S2−

32. Questions
1. Draw the Lewis electron dot
diagram for each element.
bromine
gallium
2. Draw the Lewis electron dot
diagram for each ion.
Mg2+
S2−