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What are Anions and Cations?

Anions and Cations

We will explain from a chemical point of view what an anion is and what a cation is and the most common types of anion.

What is an anion?

Anions are negatively charged ions.

Anión y Catión en el agua

Anions and Cations in water

What is a cation?

Cations are positively charged ions.

How are a cation and anion formed?

They are formed when a metal loses electrons, and nonmetals gain those electrons. The electrostatic attraction between positives and negatives attracts the particles to each other and creates an ionic compound, for example sodium chloride.

On the other hand, a metal reacts with a nonmetal to form an ionic bond. You can often determine the charge for an ion that you normally have had by the position of the element on the periodic table:

  • Alkaline metals (the elements IA) lose an electron to form a cation with a 1+ charge.
  • Alkaline metals on earth (the IIA elements) lose two electrons to form a 2+ cation.
  • Aluminum, a member of the IIIA family, loses three electrons to form a 3+ cation.
  • Halogens (elements VIIA) have seven valence electrons. All halogens gain one electron to fill their valence energy level. And they all form an anion with a single negative charge.
  • VIA elements gain two electrons to form anions with a 2- charge.
  • VA elements gain three electrons to form anions with a 3- charge.

The first table shows the family, element, and ion names for some of the more common mono-atomic cations (one atom). The second table shows the same information for some of the more common mono-atomic anions.

Some examples of the most common nanoatomic cation



Ion name

Lithium cation
Sodium Cation
Potassium cation
Beryllium cation
Magnesium cation
Calcium cation
Strontium cation
Barium cation
Silver cation
Zinc cation
Aluminium Cation


Some examples of the most common nanoatomic anion



Ion name

Nitride anion
Phosphide anion
Anion oxide
Sulfur anion
Fluoride anion
Chloride anion
Anion bromide
Iodide anion

It should be noted that it is more difficult to determine the number of electrons than the members of the transition metals (the B families). In fact, many of these elements lose a variable number of electrons so that they form two or more cations with different charges.

Also that the electric charge that an atom reaches is sometimes called: oxidation state. Many of the transition metal ions have different oxidation states. The following table shows some common transition metals that have more than one oxidation state.

Some base metals with more than one state of oxidation



Ion name

VIB Chrome Chrome (II) or chromium
Chrome (III) or chrome
VIIB Manganese Manganese (II)
Manganese (III)
VIIIB Iron Fierro (II) or ferrous
Iron (III) or ferric
Cobalt Cobalt (II)
Cobalt (III) or cobaltic
IB Copper Copper (I) or cuprous
 Copper (II) or cupric
IIB Mercury Mercury (I)
Mercury (II) or Mercuric
IVA Tin Pewter (II) or Tin
Tin (IV)
Lead Lead (II) or Leaded
Lead (IV) or lead

Note that these cations can have more than one name. The current way of naming ions is to use the name of the metal, such as chromium, followed in parentheses by the ionic charge written as a Roman numeral, such as (II).

How to remove an anion or cation from water?

The two types of ion exchange units are water softeners and anion exchange devices. Water softeners remove cations (positively charged ions like calcium and magnesium) and replace them with sodium. Anion exchange devices remove anions (negatively charged ions like arsenic and nitrate) and replace them with chloride.

Mixed media ion exchange units remove both cations and anions. A typical mixture would be 60 percent cation exchange material and 40 percent anion exchange material. Units generally need to be regenerated in a central processing plant. Two-bed deionizers, using separate cation and anion vessels, can be backwashed to remove trapped particles.

Water softening (cation exchange)

Water softeners are the most widely used household water treatment devices. They remove scale-forming minerals on water heaters and soap film on sinks. Softening the water is sometimes called water conditioning.

Do you need to soften water or remove cations?

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