Formation of Ions: Cations and Anions
- Have you ever wondered how table salt ($NaCl$) 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 and anions.
Cations: Positive Ions
Definition
Cation
A cation is formed when an atom loses one or more electrons, resulting in a net positive charge.
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:
$$ \text{Na} \rightarrow \text{Na}^+ + e^- $$
Anions: Negative Ions
Definition
Anion
An anion is formed when an atom gains one or more electrons, resulting in a net negative charge.
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:
$$ \text{Cl} + e^- \rightarrow \text{Cl}^- $$
Tip
Remember that metals form cations by losing electrons, while non-metals form anions by gaining electrons.
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., $\text{Na}^+$, $\text{K}^+$).
- Group 2 (Alkaline Earth Metals): Lose 2 electrons to form $+2$ ions (e.g., $\text{Mg}^{2+}$, $\text{Ca}^{2+}$).
- Group 13: Lose 3 electrons to form $+3$ ions (e.g., $\text{Al}^{3+}$).
- Group 15: Gain 3 electrons to form $-3$ ions (e.g., $\text{N}^{3-}$, $\text{P}^{3-}$).
- Group 16: Gain 2 electrons to form $-2$ ions (e.g., $\text{O}^{2-}$, $\text{S}^{2-}$).
- Group 17 (Halogens): Gain 1 electron to form $-1$ ions (e.g., $\text{Cl}^-$, $\text{Br}^-$).
Example
Predicting the charge of a calcium ion
- Calcium (Ca) is in Group 2.
- Its electron configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$.
- Calcium loses its two outermost electrons to achieve the noble gas configuration of argon ($1s^2 2s^2 2p^6 3s^2 3p^6$).
- Therefore, calcium forms a $+2$ ion: $\text{Ca}^{2+}$.
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.
Example
- Iron (Fe) can form $\text{Fe}^{2+}$ (losing 2 electrons) or $\text{Fe}^{3+}$ (losing 3 electrons).
- Copper (Cu) can form $\text{Cu}^+$ (losing 1 electron) or $\text{Cu}^{2+}$ (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, $\text{Fe}^{2+}$ is called iron(II), and $\text{Fe}^{3+}$ is called iron(III).
Example
Predict the possible charges of manganese (Mn)
- Manganese has the electron configuration $[Ar] 3d^5 4s^2$.
- It can lose 2 electrons from the $4s$ orbital to form $\text{Mn}^{2+}$.
- It can also lose additional electrons from the $3d$ orbital to form ions like $\text{Mn}^{3+}$ or $\text{Mn}^{4+}$.
Common Mistake
- Do not assume that all transition metals form the same ions.
- For example, zinc ($\text{Zn}$) only forms $\text{Zn}^{2+}$ because it has a full $d$ orbital ($3d^{10}$).
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.
Self review
- What type of ion does sulfur form, and what is its charge?
- Why can iron form both $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$ ions?
- Predict the charge of an ion formed by aluminum.
