Bohr’s Model of Electron Arrangement
Atomic Structure
 Atomic number = the number of protons
 Atoms are neutral, so the number of protons = the number of
electrons
 Atomic mass ≈ the number of protons + the number of
neutrons
 Protons and neutrons have the same mass, which is 1800 times
bigger than the mass of electrons.
Examples:
Element
Symbol
Atomic
Mass
Atomic
Number
# of
Protons
# of
Neutrons
Oxygen
O
16
8
8
8
Lithium
Li
7
3
3
4
Bohr Model
• 1913 – Bohr explains movement of electrons around the
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•
•
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nucleus of the atom.
Bohr’s model:
Electrons orbit around the nucleus
Each orbit is a certain distance from the nucleus
Each orbit has a definite energy level
Each orbit has a definite number of electrons
• 1st orbit
• 2nd orbit
• 3rd orbit
2
8
8
Examples:
Name
Symbol
Electrons
Bohr Diagram
Nitrogen
7
Phosphorus
15
•Valence Shell – the outer shell of electrons
•Valence electrons – the electrons in the outer shell of an atom
•Valence – the number of electrons in the outer shell of an atom
Do Bohr Diagrams of
first 20 elements/atoms
Bohr Models and the Periodic Table
 Most elements in the same family have the same number of valence
electrons (an exception is Helium)
 Elements in the same period have valence electrons in the same shell.
 The period number indicated the number of shells that have electrons.
 The number of electrons in a shell corresponds to the number of elements
in that period.
Valence shells and Stability
 Atoms become stable when they have a full outer shell of electrons
 Noble gases are un-reactive because they have a full outer shell of electrons.
 Atoms in other families will gain or lose electrons to have a full outer shell.

Having an uneven number of protons and electrons gives an atom a charge.

Charged atoms are called ions.
o Atoms  protons = electrons
o Ions  protons ≠ electrons

Metals lose electrons to become positive ions.

Non-metals gain electrons to become negative ions.
Atom
3 electrons and 3 protons
Ion
2 electrons and 3 protons

The charge of an ion is equal to the sum of the charges of the protons and
electrons.

Ions are written as the chemical symbol with a superscript of their charge
Bohr Models of Ions
Ion
Atomic
Number
# of Protons
# of Electrons
Sulphur Ion
16
16
18
12
12
10
Ion Charge:
2-
Magnesium
Ion
Ion Charge
2+
Bohr Model
Do Bohr Diagrams of
first 20 ions