Valence Electrons

1
Valence Electrons

• The electrons responsible for the chemical
  properties of atoms are those in the outer energy
  level.
• Valence electrons - The electrons in the outer
  energy level or shell
• the highest occupied energy level
• Inner electrons -those in the energy levels
  below.

2
Keeping Track of Electrons

• Atoms in the same column...
• Have the same outer electron configuration.
• Have the same valence electrons.
• Easily found same as the main group number on
  the periodic table.
• Group 2 Be, Mg, Ca, Sr, Ba.
• 2 valence electrons

3
Electron Dot diagrams

• A way of keeping track of valence electrons.
• How to write them?
• Write the symbol.
• Put one dot for each valence electron
• Dont pair up until they have to.

X
4
The Electron Dot diagram for Nitrogen

• Nitrogen has 5 valence electrons.
• First we write the symbol.

N

• Then add 1 electron at a time to each side.

• Until they are forced to pair up.

5
Electron Configurations for Cations

• Metals lose electrons to attain noble gas
  configuration (full outer shell).
• They make positive ions (cations)
• If we look at the electron configuration, it
  makes sense to lose electrons
• Na 2,8,1 1 valence electron
• Na 2,8 noble gas configuration

6
Electron Dots For Cations

• Metals will have few valence electrons (usually 3
  or less)

Ca
7
Electron Dots For Cations

• Metals will have few valence electrons
• These will come off

Ca
8
Electron Dots For Cations

• Metals will have few valence electrons
• These will come off
• Forming positive ions

Ca2
noble gas configuration
9
Electron Configurations for Anions

• Nonmetals gain electrons to attain noble gas
  configuration.
• They make negative ions (anions)
• Halide ions- ions from chlorine or other halogens
  that gain electrons
• S (2,8,6) - 6 valence electrons
• S2- (2,8,8) - a noble gas configuration.

10
Electron Dots For Anions

• Nonmetals will have many valence electrons
  (usually 5 or more)
• They will gain electrons to fill outer shell.

P
P3-
11
Stable Electron Configurations

• All atoms react to achieve noble gas
  configuration.
• Noble gases have 8 outer shell (valence)
  electrons.
• This is also called the octet rule.

Ar
12
Ionic Bonding

• Between metals and non-metals
• Transfer of electrons
• Full outer shells
• Oppositely charged ions created
• Ions held to one another by electrostatic
  attraction
• Giant structure formed

13
Ionic Bonds

• Characteristics of ionic compounds
• Usually soluble in water
• Electrical conductors when molten and when in
  aqueous solution.
• High m.p.
• Brittle, hard crystals

14
Ionic Bonding

• Anions and cations are held together by opposite
  charges.
• Ionic compounds are called salts.
• Simplest ratio is called the formula unit.
• The bond is formed through the transfer of
  electrons.
• Electrons are transferred to achieve noble gas
  configuration.

15
Ionic Bonding
Na
Cl
16
Ionic Bonding
Na
Cl-
17
Ionic Bonding

• All the electrons must be accounted for

Ca
P
18
Ionic Bonding
Ca
P
19
Ionic Bonding
Ca2
P
20
Ionic Bonding
Ca2
P
Ca
21
Ionic Bonding
Ca2
P 3-
Ca
22
Ionic Bonding
Ca2
P 3-
Ca
P
23
Ionic Bonding
Ca2
P 3-
Ca2
P
24
Ionic Bonding
Ca
Ca2
P 3-
Ca2
P
25
Ionic Bonding
Ca
Ca2
P 3-
Ca2
P
26
Ionic Bonding
Ca2
Ca2
P 3-
Ca2
P 3-
27
Ionic Bonding
Ca3P2
Formula Unit
Calcium Phosphide
28
Classwork problems (a)

• Show the bonding between the following
• Magnesium and Oxygen
• Potassium and Chlorine
• Calcium and Fluorine
• Lithium and Nitrogen
• Aluminium and Fluorine

29
Properties of Ionic Compounds

• Crystalline structure, usually solids
• A regular repeating arrangement of ions in the
  solid
• Ions are strongly bonded together.
• Structure is rigid.
• High melting points
• Electrical conductors when melted
• Electrical conductors in solution

30
Crystalline structure
31
Conducting electricity

• Conducting electricity is allowing charges to
  move.
• In a solid, the ions have charges but are locked
  in place.
• Ionic solids are insulators.
• When melted, the charged ions can move around.
  Melted ionic compounds conduct electricity.
• To melt NaCl, it must get to about 800 ºC.
• Dissolved in water (aqueous) the ions are now
  free to move and they conduct electricity.

32
Ionic solids are brittle
33
Ionic solids are brittle

• Displacing the top layer (diagram) puts ions of
  the same charge near one another creating strong
  repulsion that breaks the crystal apart.

34
Solubility

• Ionic solids are often (but not always) soluble
  in water.
• Solubility usually (but not always) increases
  with increasing temperature.