Formation of Ions: Cations and Anions

Have you ever wondered how table salt ( $N a C l$ ) dissolves in water and allows electricity to flow?

The key lies in ions—charged particles formed when atoms gain or lose electrons.

What Are Ions?

Definition

Ion

An ion is an atom or group of atoms that has gained or lost electrons, resulting in a net electrical charge.

Ions are classified into two main types:

Cations: Positive Ions

Definition

Cation

A cation is formed when an atom loses one or more electrons, resulting in a net positive charge.

Formation: Metals, which are found on the left side of the periodic table, tend to lose electrons because they have relatively low ionization energies.

Example

Sodium (Na) loses one electron to form a sodium ion:
$Na \to {Na}^{+} + e^{-}$

Anions: Negative Ions

Definition

Anion

An anion is formed when an atom gains one or more electrons, resulting in a net negative charge.

Formation: Non-metals, which are found on the right side of the periodic table, tend to gain electrons because they have high electron affinities.

Example

Chlorine (Cl) gains one electron to form a chloride ion:
$Cl + e^{-} \to {Cl}^{-}$

Tip

Remember: Metals form cations by losing electrons, while non-metals form anions by gaining electrons.

From left to right: sodium atom, sodium cation, chlorine atom, chlorine anion.

Predicting the Charge of Ions

The charge of an ion can be predicted based on the electron configuration of the atom and its position in the periodic table. Atoms tend to lose or gain electrons to achieve a stable noble gas configuration (full outer shell).

Main Group Elements

Group 1 (Alkali Metals): Lose 1 electron to form $+ 1$ ions (e.g., ${Na}^{+}$ , $K^{+}$ ).
Group 2 (Alkaline Earth Metals): Lose 2 electrons to form $+ 2$ ions (e.g., ${Mg}^{2 +}$ , ${Ca}^{2 +}$ ).
Group 13: Lose 3 electrons to form $+ 3$ ions (e.g., ${Al}^{3 +}$ ).
Group 15: Gain 3 electrons to form $- 3$ ions (e.g., $N^{3 -}$ , $P^{3 -}$ ).
Group 16: Gain 2 electrons to form $- 2$ ions (e.g., $O^{2 -}$ , $S^{2 -}$ ).
Group 17 (Halogens): Gain 1 electron to form $- 1$ ions (e.g., ${Cl}^{-}$ , ${Br}^{-}$ ).

Example

Predicting the charge of a calcium ion

Calcium (Ca) is in Group 2.
Its electron configuration is $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2}$ .
Calcium loses its two outermost electrons to achieve the noble gas configuration of argon ( $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6}$ ).
Therefore, calcium forms a $+ 2$ ion: ${Ca}^{2 +}$ .

Common Mistake

A common mistake is assuming that all elements form ions with the same charge. For example, transition metals often form ions with varying charges, which we will discuss next.

Transition Metals and Variable Charges

Unlike main group elements, transition metals can form ions with different charges. This is because they can lose electrons from both their outermost $s$ orbital and their $d$ orbitals.

Examples of Transition Metal Ions

Iron (Fe): Can form ${Fe}^{2 +}$ (losing 2 electrons) or ${Fe}^{3 +}$ (losing 3 electrons).
Copper (Cu): Can form ${Cu}^{+}$ (losing 1 electron) or ${Cu}^{2 +}$ (losing 2 electrons).

Why Do Transition Metals Have Variable Charges?

Transition metals have partially filled $d$ orbitals, and the energy difference between the $s$ and $d$ orbitals is relatively small. This allows them to lose different numbers of electrons depending on the chemical environment.

Tip

When naming transition metal ions, the charge is indicated using Roman numerals. For example, ${Fe}^{2 +}$ is called iron(II), and ${Fe}^{3 +}$ is called iron(III).

Example

Predict the possible charges of manganese (Mn)

Manganese has the electron configuration $[A r] 3 d^{5} 4 s^{2}$ .
It can lose 2 electrons from the $4 s$ orbital to form ${Mn}^{2 +}$ .
It can also lose additional electrons from the $3 d$ orbital to form ions like ${Mn}^{3 +}$ or ${Mn}^{4 +}$ .

Common Mistake

Do not assume that all transition metals form the same ions. For example, zinc ( $Zn$ ) only forms ${Zn}^{2 +}$ because it has a full $d$ orbital ( $3 d^{10}$ ).

Periodic Table and Ion Charges

The position of an element in the periodic table provides valuable clues about the charge of its ions:

Groups: Elements in the same group form ions with the same charge because they have the same number of valence electrons.
Periods: As you move across a period, the tendency to lose electrons decreases, and the tendency to gain electrons increases.

Theory of Knowledge

How does the periodic table reflect the underlying quantum mechanical structure of atoms? Consider how the periodic trends in ion formation relate to the arrangement of electrons in orbitals.

Recap: Key Points

Cations are positive ions formed by losing electrons (typically metals).
Anions are negative ions formed by gaining electrons (typically non-metals).
The charge of an ion can be predicted based on its electron configuration and periodic table position.
Transition metals can form ions with variable charges due to their $d$ orbitals.
The periodic table is a powerful tool for understanding and predicting ion formation.

Self review

What type of ion does sulfur form, and what is its charge?
Why can iron form both ${Fe}^{2 +}$ and ${Fe}^{3 +}$ ions?
Predict the charge of an ion formed by aluminum.

Note

Understanding ion formation is a foundational concept for studying ionic bonding, which we will explore in the next section.

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S2.1.1 Formation of ions

Formation of Ions: Cations and Anions

What Are Ions?

Cations: Positive Ions

Anions: Negative Ions

Predicting the Charge of Ions

Main Group Elements

Predicting the charge of a calcium ion

Transition Metals and Variable Charges

Examples of Transition Metal Ions

Why Do Transition Metals Have Variable Charges?

Predict the possible charges of manganese (Mn)

Periodic Table and Ion Charges

Recap: Key Points

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Atoms and Ions