What term is use to describe the positive and negative charge of an atom after the transfer of electron?

If an atom, or atoms, has a balanced number of electrons (negative charge) and protons (positive charge) they are neutral overall. However, if they are not balanced, they will be charged. These charged species are called ions.

Cations are ions that are positively charged. Anions are ions that are negatively charged. Ions are charged atoms or molecules. If a balanced atom loses one or more electrons, it will become a positively charged cation. If a balanced atom gains one or more electrons, it will become a negatively charged anion. 

What is a cation?

A cation has more protons than electrons, consequently giving it a net positive charge. For a cation to form, one or more electrons must be lost, typically pulled away by atoms with a stronger affinity for them. The number of electrons lost, and so the charge of the ion, is indicated after the chemical symbol, e.g. silver (Ag) loses one electron to become Ag+, whilst zinc (Zn) loses two electrons to become Zn2+.

What is an anion?

An anion has more electrons than protons, consequently giving it a net negative charge.

For an anion to form, one or more electrons must be gained, typically pulled away from other atoms with a weaker affinity for them. The number of electrons gained, and so the charge of the ion, is indicated after the chemical symbol, e.g. chlorine (Cl) gains one electron to become Cl-, whilst oxygen (O) gains two electrons to become O2-.

Cation vs anion chart

The main differences between cations and anions are summarized in the table below.

Metallic atoms hold some of their electrons relatively loosely. Consequently, they tend to lose electrons and form cations. Conversely, most nonmetallic atoms attract electrons more strongly than metallic atoms, and so gain electrons to form anions. Therefore, when atoms from a metallic and a nonmetallic element combine, the nonmetallic atoms tend to draw one or more electrons away from the metallic atoms to form ions. These oppositely charged ions then attract one other to form ionic bonds and produce ionic compounds with no overall net charge. Examples include calcium chloride (CaCl2), potassium iodide (KI) and magnesium oxide (MgO).

Cation vs anion periodic table

It can be possible to predict whether an atom will form a cation or an anion based on its position on the periodic table. Halogens always form anions, alkali metals and alkaline earth metals always form cations. Most other metals form cations (e.g. iron, silver, nickel), whilst most other nonmetals typically form anions (e.g. oxygen, carbon, sulfur). However, some elements are capable of forming both cations and anions given the right conditions. One example is hydrogen, which may gain (H-) or lose (H+) an electron, forming hydride compounds such as ZnH2 (where it is an anion) and hydron compounds such as H2O (where it is a cation).

Elements in group 18 of the periodic table – the “noble gases”, tend not to form ions due to the arrangement of their electrons which makes them generally unreactive.

Cation vs anion size

Cations and anions come in many sizes across the periodic table, as seen in this video.

Utilizing ionic properties

Ionic properties can be exploited by chemists for a range of purposes. Ion-exchange chromatography for example relies on the affinity of the molecules being separated for the stationary phase based on their charge properties to enable separation.

Ionic properties are central to the function of batteries too. Batteries have two electrodes made of conductive material, the cathode which is the positive end where the electrical current leaves/electrons enter, and the anode where the electrical current enters/ electrons leave. In between the electrodes is an electrolyte liquid or gel that contains charged particles – ions. As this ionic substance reacts with the electrodes it generates electrical current. In single use, dry cell batteries, zinc is commonly used as the anode whilst manganese dioxide is a popular choice for the electrolyte cathode. The zinc anode also acts as the battery’s container in zinc-carbon batteries so as it oxidizes during use, the contents can start to leak over time.

A zinc-carbon dry cell battery (left) and alkaline battery (right).

In rechargeable batteries, such as many lithium-ion batteries, this chemical process is reversible and the internal structure different which allows the batteries to be recharged.

Due to the ionic properties of salt water, scientists are now striving to exploit the ionic electricity-generating potential of salinity gradients where salt water and fresh water mix as a green source of energy generation for the future.

Learning Objectives

• Define the two types of ions.

Most atoms do not have eight electrons in their valence electron shell. Some atoms have only a few electrons in their outer shell, while some atoms lack only one or two electrons to have an octet. In cases where an atom has three or fewer valence electrons, the atom may lose those valence electrons quite easily until what remains is a lower shell that contains an octet. Atoms that lose electrons acquire a positive charge as a result because they are left with fewer negatively charged electrons to balance the positive charges of the protons in the nucleus. Positively charged ions are called cations. Most metals become cations when they make ionic compounds.

A neutral sodium atom is likely to achieve an octet in its outermost shell by losing its one valence electron.

\[\ce{Na \rightarrow Na^{+} + e^{-}} \nonumber \]

The cation produced in this way, Na+, is called the sodium ion to distinguish it from the element. The outermost shell of the sodium ion is the second electron shell, which has eight electrons in it. The octet rule has been satisfied. Figure \(\PageIndex{1}\) is a graphical depiction of this process.

Figure \(\PageIndex{1}\): The Formation of a Sodium Ion. On the left, a sodium atom has 11 electrons. On the right, the sodium ion only has 10 electrons and a 1+ charge. Neutral sodium atom on left has 11 protons and 11 electrons. Sodium ion on right has 11 protons and 10 electrons, with a +1 overall charge.

Some atoms have nearly eight electrons in their valence shell and can gain additional valence electrons until they have an octet. When these atoms gain electrons, they acquire a negative charge because they now possess more electrons than protons. Negatively charged ions are called anions. Most nonmetals become anions when they make ionic compounds.

A neutral chlorine atom has seven electrons in its outermost shell. Only one more electron is needed to achieve an octet in chlorine’s valence shell. (In table salt, this electron comes from the sodium atom.)

\[\ce{e^{-} +Cl -> Cl^{-}} \nonumber \]

In this case, the ion has the same outermost shell as the original atom, but now that shell has eight electrons in it. Once again, the octet rule has been satisfied. The resulting anion, Cl−, is called the chloride ion; note the slight change in the suffix (-ide instead of -ine) to create the name of this anion. Figure \(\PageIndex{2}\) is a graphical depiction of this process.

Figure \(\PageIndex{2}\): The Formation of a Chlorine Ion. On the left, the chlorine atom has 17 electrons. On the right, the chloride ion has 18 electrons and has a 1− charge. Neutral chlorine atom on left has 17 protons and 17 electrons. Sodium ion on right has 17 protons and 18 electrons, with a -1 overall charge.

The names for positive and negative ions are pronounced CAT-eye-ons and ANN-eye-ons, respectively.

In many cases, elements that belong to the same group (vertical column) on the periodic table form ions with the same charge because they have the same number of valence electrons. Thus, the periodic table becomes a tool for remembering the charges on many ions. For example, all ions made from alkali metals, the first column on the periodic table, have a 1+ charge. Ions made from alkaline earth metals, the second group on the periodic table, have a 2+ charge. On the other side of the periodic table, the next-to-last column, the halogens, form ions having a 1− charge. Figure \(\PageIndex{3}\) shows how the charge on many ions can be predicted by the location of an element on the periodic table. Note the convention of first writing the number and then the sign on a ion with multiple charges. The barium cation is written Ba2+, not Ba+2.

Figure \(\PageIndex{3}\): Predicting Ionic Charges. The charge that an atom acquires when it becomes an ion is related to the structure of the periodic table. Within a group (family) of elements, atoms form ions of a certain charge.

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