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Counting Electrons for an Atom or Ion
Dr. MJ Patterson

Counting Electrons for an Atom

To count the number of electrons found in an atom, look at the periodic table.  The atomic number tells you the number of electrons in a neutral atom (along with the number of protons).

Example 1:
How many electrons are in the following neutral atoms?
a.    H
b.    Fe
c. Ar
d. Pb

Solution 1:
a.    H = 1 electron
b.    Fe = 26 electrons
c.    Ar = 18 electrons
d.    Pb = 82 electrons

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Counting Electrons in an Ion

The only difference between a neutral atom and a monatomic ion is that the ion will have either gained or lost electrons, depending on its charge.  Since electrons have a negative charge, gaining electrons makes the ion negative.  Losing electrons makes the ion positive.

If you need to determine how many electrons are in the sodium ion Na⁺, start with the neutral atom.  Neutral sodium has 11 electrons.  To achieve a single positive charge, the neutral atom must lose one electron.  The net result is that the sodium ion Na⁺ has 10 electrons.

If you need to determine how many electrons are in the chloride ion Cl^-, start with the neutral atom.  Neutral chlorine has 17 electrons.  To achieve a single negative charge, the neutral atom must gain one electron.  The net result is that the chloride ion Cl^- has 18 electrons.

To summarize:

1. Start with the neutral atom''s electron count.
2. For a cation (positive ion), subtract the charge from the neutral atom''s electron count.
3. For an anion (negative ion), add the size of the charge to the neutral atom''s electron count.

Example 2:
How many electrons are in the following ions?
a.    S^2-
b.    Al³⁺
c. Fe³⁺
d. Fe²⁺
e.    N^3-

Solution 2:
a.    S^2-  = 16 + 2 = 18 electrons
b.    Al³⁺  = 13 - 3 = 10 electrons
c.    Fe³⁺  = 26 - 3 = 23 electrons
d.    Fe²⁺  = 26 - 2 = 24 electrons
e.    N^3-  = 7 + 3 = 10 electrons

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