7.3: Cations

 

Figure \(\PageIndex{1}\) (Credit: User:Gold Guru/Wikipedia; Source: http://commons.wikimedia.org/wiki/File:Nugsrandt.jpg(opens in new window); License: Public Domain)

Have you ever gone digging for gold?

When the prospectors during the California Gold Rush (1848-1855) searched for gold nuggets in the earth, they were able to find the nuggets because gold is an unreactive material that exists in its elemental state in many places. Not everyone was fortunate enough to find significantly large gold nuggets, but a number of miners did become very wealthy (of course, a large number of others went back home broke).

Many of the elements we know about do not exist in their native form. They are so reactive that they are found only in compounds. These non-elemental forms are known as ions. Their properties are very different from those of the elements they come from. The term comes from a Greek word meaning "move" and was first coined by Michael Faraday, who studied the movement of materials in an electrical field.

Cations

Figure \(\PageIndex{2}\): Sodium loses an electron to become a cation. (Credit: CK-12 Foundation; Source: CK-12 Foundation; License: CK-12 Curriculum Materials License(opens in new window))

Some elements lose one or more electrons in forming ions. These ions are known as "cations" because they are positively charged and migrate toward the negative electrode (cathode) in an electrical field. Looking at the periodic table below, we know that the group 1 elements are all characterized by having one \(s\) electron in the outer orbit; group 2 elements have two \(s\) electrons in the outer orbit. These electrons are loosely attached to the atom and can easily be removed, leaving more protons in the atom than there are electrons, so that the resulting ion has a positive charge. Cations can also be formed from electron loss to many of the transition elements.

Figure \(\PageIndex{3}\): Periodic Table of Elements, notated with group numbers. (Credit: Christopher Auyeung; Source: CK-12; License: CC BY-NC 3.0(opens in new window))

The cations are designated by the symbol for the parent element and a plus charge as a superscript after the element symbol—the potassium cation would be indicated as \(\ce{K^+}\). Note that the charge is placed after the symbol and not before it. The potassium ion is monovalent, meaning that it has lost one electron and has a \(+1\) charge. The symbol for the magnesium cation would be \(\ce{Mg^{2+}}\) or \(\ce{Mg^{++}}\) to indicate that it has lost two electrons and has a \(+2\) charge, so the magnesium cation would be referred to as a divalent cation.

The cations are simply named as the parent element. The sodium cation is still called "sodium". Often, the charge would be attached for clarity, so the sodium cation might be referred to as "sodium one plus".

Applications of Cations

Cations play important roles in our daily lives. Sodium, potassium, and magnesium ions are essential for such processes as blood pressure regulation and muscle contraction. Calcium ions are an important part of bone structure. Sodium ions can be used in water softeners to remove other harmful elements. We put sodium chloride (table salt) on our food and use it as a preservative.

Summary

• Cations are formed by the loss of one or two electrons from an element.
• Groups 1 and 2 elements form cations.
• Cations are named according to the parent element.
• Cation charges are indicated with a superscript following the chemical symbol.

Review

1. What is a cation?
2. Write the symbol for the barium cation.
3. Write the symbol for the cesium cation.
4. List three ways cations are useful.