Monatomic Ions Cations and
Anions
Ck12 Science
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Printed: January 25, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
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Chapter 1. Monatomic Ions Cations and Anions
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Monatomic Ions Cations
and Anions
Define cation.
Explain how cations are formed.
Describe the naming of cations.
Describe how cations are designated.
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 these nuggets because gold is an unreactive material that exists in its elemental state in many places. Not
everyone was fortunate enough to find large nuggets such as those shown above, but a number of these 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
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 and 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 that there are electrons, so the resulting ion has a
positive charge. Cations can also be formed from electron loss to many of the transition elements.
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FIGURE 1.1
Sodium loses an electron to become a
cation.
FIGURE 1.2
Periodic table of elements, notated with
group numbers.
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 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 Mg2+ or 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 ion is an important part of bone structure. Sodium ions can used in water softeners to remove other harmful elements. We put sodium chloride (table
salt) on our food and use it as a preservative.
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Chapter 1. Monatomic Ions Cations and Anions
Summary
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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.
Practice
Use the link below to answer the following questions:
http://dl.clackamas.edu/ch104-07/cations.htm
1.
2.
3.
4.
How many electrons are there in the outer shell of Group IA elements?
What is the charge of Group IA ions?
How many electrons are there in the outer shell of Group IIA elements?
What is the charge of Group IIA ions?
Review
1.
2.
3.
4.
5.
What is an ion?
What is the cathode?
Write the symbol for the barium cation.
Write the symbol for the cesium cation.
List three ways cations are useful.
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What does the amount of salt in seaweed tell us?
Before iodized salt was developed, some people experienced a number of developmental difficulties, including
problems with thyroid gland function and mental retardation. In the 1920s, we learned that these conditions could
usually be treated easily with the addition of iodide anion to the diet. One easy way to increase iodide intake was to
add the anion to table salt. This simple step greatly enhanced health and development. Large amounts of iodide ion
are also found in seaweed such as kelp (see picture above) and saltwater fish .
When a metal loses an electron, energy is needed to remove that electron. The other part of this process involves
the addition of the electron to another element. The electron adds to the outer shell of the new element. Just as
the loss of the electron from the metal produces a full shell, when the electron or electrons are added to the new
element, it also results in a full shell.
Anions
Anions are negative ions that are formed when a nonmetal atom gains one or more electrons . Anions are so named
because they are attracted to the anode (positive field) in an electric field. Atoms typically gain electrons so that
they will have the electron configuration of a noble gas. All the elements in Group 17 have seven valence electrons
due to the outer ns 2 np 5 configuration. Therefore, each of these elements would gain one electron and become an
anion with a 1− charge. Likewise, Group 16 elements form ions with a 2− charge, and the Group 15 nonmetals
form ions with a 3− charge.
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Chapter 1. Monatomic Ions Cations and Anions
Naming anions is slightly different than naming cations . The ending of the element’s name is dropped and replaced
with the –ide suffix. For example, F − is the fluoride ion, while O 2− is the oxide ion. As is the case with cations ,
the charge on the anion is indicated by a superscript following the symbol. Common anions are listed in the Table
below :
TABLE 1.1:
Anion Name
Symbol and Charge
fluoride
F−
chloride
Cl −
bromide
Br −
iodide
I−
oxide
O 2−
sulfide
S 2−
nitride
N 3−
Uses for Anions
Fluoride ion is widely used in water supplies to help prevent tooth decay. Chloride is an important component in ion
balance in blood . Iodide ion is needed by the thyroid gland to make the hormone thyroxine.
Summary
• Anions are formed by the addition of one or more electrons to the outer shell of an atom.
• Group 17 elements add one electron to the outer shell, group 16 elements add two electrons, and group 15
elements add three electrons.
• Anions are named by dropping the ending of the element’s name and adding - ide.
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Practice
Use the link below to answer the following questions:
http://preparatorychemistry.com/bishop_anion_names_formulas_help.htm
1. Why do elements form anions?
2. Why do group 17 elements form anions more readily than group 1 elements?
Review
1.
2.
3.
4.
5.
6.
What is an anion?
How are anions formed?
Why do anions form?
How are anions named?
List three examples of anions with names, charges, and chemical symbols.
List three ways anions are used.
• cation: Ions known as cations are because they are positively charged and migrate toward the negative
electrode (cathode) in an electrical field.
• monovalent: Has a +1 charge.
• divalent: Has a +2 charge.
• cathode: Negative electrode.
References
1.
2.
3.
4.
5.
6
. Gold Nuggets . Public Domain
Erik Dunmire. marin.edu . CC BY-NC-SA 3.0
Christopher Auyeung. CK-12 . CC BY-NC-SA 3.0
. Seaweed.
. Periodic Table.