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Earlier we learned that the atom was comprised of a very small positively charged core of protons and neutrons surrounded by a much large "cloud" of orbiting electrons.


While chemistry doesn't usually involve changes in the number of protons or neutrons in an atom, changes in the number of electrons in an atom is central to the science of chemistry.

If electrons are removed or added to a neutral atom, a charged particle called an ion is formed. There are two types of ions:

  • Cation - a positively charged ion
  • Anion - a negatively charged ion

For example, a neutral sodium atom has a nuclear charge of +11 and contains 11 electrons. If we strip off one electron we form a sodium cation:


This process can also be represented in short-hand notation.


A neutral chlorine atom has a nuclear charge of +17 and contains 17 electrons. If we add one electron we form a chlorine anion:


Another example. Zn likes to lose 2 electrons to make a divalent cation:


You can use the periodic table to predict how many electrons an element will lose or gain when it becomes an ion. For example, here are the most stable ionic charges on monoatomic ions:


Generally, Aluminum likes to form the cation Al3+ and Zinc likes to form the cation Zn2+.

Here is a "rough rule" you can use to figure out how many electrons an element will gain or lose:

Elements tend to gain or lose electrons to achieve the same number of electrons as the nearest noble gas.

For groups in the middle of the periodic table, it is not as simple. After you learn about quantum mechanics, however, you will have a better idea of how to predict the stable ion charges for these groups.

A final note: you will often hear chemists use the term proton interchangeably with hydrogen cation, H+. Can you explain why?

Homework from Chemisty, The Central Science, 10th Ed.

2.15, 2.19, 2.21, 2.23, 2.25, 2.27, 2.33, 2.35, 2.37, 2.47, 2.49