Cations and Anions: Formation and Naming
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Cations and Anions
When a neutral atom loses one or more electrons, the total number of electrons decreases while the number of protons in the nucleus remains the same. The result is that the atom becomes a cation—an ion with a net positive charge.
The opposite process can also occur. When a neutral atom gains one or more electrons, the number of electrons increases while the number of protons in the nucleus remains the same. The result is that the atom becomes an anion—an ion with a net negative charge.
| H+ | H | H- | |
|---|---|---|---|
| Classification | cation | neutral atom | anion |
| No. of protons | 1 | 1 | 1 |
| No. of electrons | 0 | 1 | 2 |
| Net charge | 1+ | 0 | 1− |
Naming Monatomic Anions
Most often, when we name monatomic anions, we add the suffix -ide to the end of the element's name.
Example: What is the chemical formula of calcium bromide?
A) Ca2Br
B) BrCa
C) CaBr2 -----THIS IS CORRECT
D) CaBr3
Why:
Calcium forms Ca2+, and bromide refers to a Br- anion.
In order for the charges on the ions to cancel, we need one Ca2+ cation to bond with every two Br- anions. This is because the total charge contributed by calcium will be:
1 x (2+) = 2+
The total charge contributed by bromide will be:
2 x (1-) = 2-
The 2+ and 2- exactly cancel.
Therefore, CaBr2 is the correct answer.
Example: What is the name of the compound SrF2?
A) Fluoride strontium
B) Strontium fluoride
C) Strontium fluorine
D) Sulfur fluorine
Why:
F is the anion, so it will have the suffix -ide. F is the chemical symbol for fluorine; here, fluorine has gained an electron to form the anion fluoride, F-.
Also, when naming ionic compounds, the name of the element that contributed the cation comes first, followed by the name of the anion—which should end in -ide.
Sr is the chemical symbol for strontium, a group 2 element. Based on strontium's position on the periodic table, we would predict it to form Sr2+ cations.