1. Bohr Diagrams:
Bohr diagrams show how many electrons appear in each electron shell around an atom.
Each shell holds a maximum number of electrons (2, 8, 8, 18, 18)
Electrons in the outermost shell are called valence electrons
If the valence shell is full = the atom is stable
If the valence shell is not full = the atom is not stable
Note: Think of the shells as being 3D like spheres, not 2D like circles!

2. Bohr Diagrams What element is this?
It has 18 protons and it has = 18 electrons.
It has 8 electrons in the outer (valence) shell
18 p
22 n
Argon!

3. Bohr Diagrams for the first 20 elements
Note: The noble gas elements have full electron shells, and are very stable.

4. Bonding Types:
When two atoms get close together, their valence electrons interact.

5. Electrons are transferred from the cations to the anion
Ionic Bonds:
Metals give electrons to non-metals (transfer of valence electrons)
cations (+ ions) and anions (- ions) form
For example, lithium and oxygen form an ionic bond in the compound Li2O
Lithium
Oxygen +
Electrons are transferred from the cations to the anion
Li+ O2- Li+
Lithium oxide,
Li2O

6. Bohr Diagram for LiF - Lithium fluoride
On board

7. Covalent Bonds: Formed between two or more non-metals
Valence electrons are shared between atoms
A group of covalently bonded atoms are called a molecule
Hydrogen
Fluorine +
Electrons are shared
Hydrogen fluoride

8. Bohr Diagram for CH4 – Methane (or carbon tetrahydride)
On Board

9. Lewis Diagrams: Like simplified Bohr diagrams
Only valence electrons are shown
Dots representing electrons are placed around the element symbols (on 4 sides, imagine a box around the symbol)
Electron dots are placed singularly, until the fifth electron is reached, then they are paired.
Example: Nitrogen atom

10. Lewis Diagrams:
Note: the Lewis diagrams are the same (except for the symbols) for elements in the same family because they have the same number of valence electrons

11. Lewis Diagrams for Ions:
For positive ions: one electron dot is removed from the valence shell for each positive charge of the ion.
For negative ions: one electron dot is added to each valence shell for each negative charge of the ion.
Square brackets and the charge are placed around each ion
Example: Nitrogen ion

12. Lewis Diagrams For Covalent Bonds:
valence electrons are drawn to show sharing of electrons.
Remember: All atoms “like” to have a full valence shell
The shared pairs (“bonding pairs”) of electrons are usually drawn as a straight line
“lone pairs” are the electrons not shared

13. Lewis Diagrams For Ionic Bonds:Be Cl
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Each beryllium has two electrons to transfer away, and each chlorine wants one more electron
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• • ••
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Since Be2+ wants to donate 2 electrons and each Cl– wants to accept only one, two Cl– ions are necessary
The ionic compound Beryllium chloride is formed 2+ –
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