Cations| Definition, Formation, and Types

Ions

Protons and neutrons reside within the nucleus of an atom. Electrons encircle an atom and are often depicted in diagrams on rings.

The charge of an ion can be determined by finding the number of protons and electrons that they have. Protons are positively charged particles, while electrons are negatively charged particles. Neutrons are molecular particles that have no electric charge to them. The overall net charge is determined by finding the net difference between the number protons and electrons. Consider the molecular composition of the following molecule:

• 5 neutrons
• 6 protons
• 4 electrons

{eq}6 Protons - 4 Electrons = +2 Charge {/eq}

The above ion would have a +2 charge. This is because there are 4 electrons resulting in 4 negative charges and 6 protons resulting in 6 positive charges. The net difference is 2 positive charges, denoted as a +2 charge. Because neutrons have no charge, they are not considered in the calculation of ionic charges. Using this simple difference can help an individual determine if an ion is a cation or anion.

Cation and Anions: Characteristics and Formation

To better understand ions, it is important to understand how cations form. Protons and neutrons are found within the nucleus of an atom. Electrons are observed within the outer rings of an atom. Because the nucleus is difficult to access and thus hard to alter, cations are formed when electrons in the outer rings are lost resulting in a smaller number of negative charges. Similarly, anions are formed when additional electrons are picked up resulting in more negative charges. A helpful tool to remember cations and anions is to picture the "t" in the word "cation" as a plus sign. This tip can help one remember if a cation is positive or negative by associating a plus sign with positive.

Elements are identified by the number of protons within their nucleus. Elements generally do not become other elements, and thus protons are not lost or gained. The number of neutrons within a nucleus determines the form of an element. Carbon is a common element that differs in the number of neutrons it possesses, often having between 12 and 16 neutrons. Because neutrons do not effect charges, the number of neutrons does not change the ionic charge value.

To become a cation, a neutral atom or molecule (which is just multiple atoms bound together) must lose an electron. Generally speaking, molecules hold onto electrons unless they are exposed to enough energy that forces them to lose an electron. The amount of energy required to remove an electron from its molecule is called the ionization energy.

Alternatively, anions form by gaining electrons. How easily a molecule will become an anion is determined by its electron affinity, or likelihood to pick up extra electrons. Molecules with full electron rings have lower affinities than ones with emptier rings. Molecules with more rings that are further from the nucleus will have higher affinities than those with fewer, closer rings.

Cations and anions can also be identified based on what they're attracted to. Cations will travel towards a cathode, which is a negatively charged electrode that will attract positive charges. Anions will travel towards anodes, which are positively charged electrodes that attract negative charges. This is easily remembered when recalling that, in chemistry, opposites always attract.

Charged States and Types of Cations

Cations and anions are not just positive and negatively charged, but have different levels of charge to them. For instance, a cation can have a +1, +2, +3, or +4 charge. Generally speaking, these charges can be observed by understanding an elements place on the periodic table. The periodic table is a chart that organizes the chemical elements based on like properties. Knowing the placement of elements on the periodic table can help answer questions such as: "why do metals form cations?"

Understanding the placement of elements on the periodic table can help explain trends that result in cations or anions.

Metals are unique on the periodic table. They are likely to become cations, but can be neutral as well. More common trends are identified based on the column or group that an element resides within.

The first column from the left of the periodic table forms cations with a +1 charge. The second column forms +2 charges. Sodium commonly forms a +2 ion and is a classic cation example. Columns 3 thru 12 represent the transitional metals, which have varying charges which are not predictable based on their position on the periodic table. Column 13 typically forms cations with a +3 charge, and column 14 can produce cations with a +4 charge. Columns 14 and up typically produce anions with negative charges. The exception to this is column 18, the Noble gases. Noble gases do not form bonds or ions unless under extreme circumstances.

Cations can be further categorized based on their type:

• Carbocations
• Radical cations
• Organic cations

Carbocations

Carbocations are any ions that are attached to carbon atom with a positive charge. Carbocations can receive their entire charge from the carbon atom or may receive part of it from the carbon atom and part of it from another positively charged ion within their molecular makeup.

Radical Cations

Radical cations are cations that have a radical, or unpaired electron. These occur when a single electron is removed from a neutral molecule resulting in a positively charged ion. Radical cations can form inside of a mass spectrometer, which is a tool chemists use to identify a mass-to-charge ratio of an ion.

Organic Cations

An organic substance in chemistry is one that contains carbon. Often, organic substances are composed of carbon bonded to nitrogen, hydrogen, and oxygen. An organic cation is an organic compound with a positive charge.

Role of Cations in Molecular Bonding

Cations are important in ionic bonding. Ionic bonding occurs when an atom or molecule donates an electron to another atom or molecule. By losing an electron, the donating atom becomes positively charged. By gaining an electron, the receiving atom becomes negatively charged. The resulting ions are then attracted to each other because of their opposite charges.

Sodium and chlorine bond ionically. Sodium donates an electron to chlorine and the resulting charged ions are then attracted to each other.

A cation does not need to be a single atom. A large molecule can be considered a cation if all of the bonded molecules together still result in a larger number of protons than electrons, thus creating an overall positive charge. Thus, there are both monoatomic cations (cations with an individual atom) and polyatomic cations (cations with multiple atoms).

Elements that Become Cations

Atoms are more stable when they have full electron rings. Thus, an element is more likely to become an anion or cation based off the ease of having a full ring. The first ring of an atom can hold 2 electrons, and every subsequent ring can hold 8 electrons. If an atom has 3 electrons, then it has a full ring plus a ring with a single electron. It would be easier to lose a single electron to have a single full ring than it would be to gain 7 more electrons. Thus, the atom is more likely to become a cation than an anion and electron configuration can help explain how cations are formed.

Looking at the organization of basic elements makes it easier to determine what type of elements form cations. Families on the periodic table are likely to become cations based off the number of electrons in their rings:

• Alkali metals often become +1 cations
• Alkaline earth metals often become +2 cations
• Metalloids may become +3 cations, +4 cations, remain neutral, of become -3 anions
• Nonmetals and transitional metals vary greatly on potential ionic charges
• Halogens almost always become -1 anions
• Noble gases typically do not bond or form ions

Lesson Summary

An ion is an atom or molecule with an electrical charge. This charge results from an imbalance in the number of electrons (negative particles) and protons (positive particles). Neutral particles, or neutrons, do not effect ionic charge. A positively charged ion is called a cation, while a negatively charged ion is called an anion.

The number of protons within an ion do not change, so charge is determined based on the gain or loss of electrons. The amount of energy required to remove an electron is called the ionization energy, while the likelihood for a molecule to pick up an extra electron is its electron affinity. Cations are attracter to negatively charged electrodes called cathodes, while anions are attracted to positively charged electrodes called anodes.

Cations can be monoatomic cations with only one atom or polyatomic cations with multiple atoms as long as the total net charge remains positive. The likelihood for an element to become an anion or cation is often observable based on the number of electrons in its outer ring, which can be determined in the organization of the elements in the periodic table.

There are multiple common classifications for cations:

• Carbocations are ions attached to a positively charged carbon atom
• Radical cations are cations that are positive because of the loss of a single electron from a pair
• Organic cations are cations that are organic, which means they are composed primarily of carbon