Question

Write the formula of the common ion derived from each (a) \(\mathrm{Li},(\mathrm{b}) \mathrm{S},\) of the following: (c) \(\mathrm{I},(\mathrm{d}) \mathrm{N},\) (e) \(\mathrm{Al},\) (f) \(\mathrm{Cs}\), (g) \(\mathrm{Mg}\).

Short answer

(a) Li⁺, (b) S²⁻, (c) I⁻, (d) N³⁻, (e) Al³⁺, (f) Cs⁺, (g) Mg²⁺.

Step-by-step solution

Understanding Ion Formation

Elements typically form ions by losing or gaining electrons to achieve a stable electron configuration, often resembling the nearest noble gas configuration. Cations are formed by losing electrons, and anions are formed by gaining electrons.

Analyzing Each Element

We'll examine the position of each element in the periodic table to determine how many electrons it will likely lose or gain to become a stable ion. We'll then write the ionic formula for the derived ions. - (a) Lithium (Li) is an alkali metal in Group 1; it loses 1 electron forming a cation: Li⁺. - (b) Sulfur (S) is a non-metal in Group 16; it gains 2 electrons forming an anion: S²⁻. - (c) Iodine (I) is a halogen in Group 17; it gains 1 electron forming an anion: I⁻. - (d) Nitrogen (N) is a non-metal in Group 15; it gains 3 electrons forming an anion: N³⁻. - (e) Aluminum (Al) is a metal in Group 13; it loses 3 electrons forming a cation: Al³⁺. - (f) Cesium (Cs) is an alkali metal in Group 1; it loses 1 electron forming a cation: Cs⁺. - (g) Magnesium (Mg) is an alkaline earth metal in Group 2; it loses 2 electrons forming a cation: Mg²⁺.

Writing the Ion Formulas

For each element, we write out the formula of the ion by depicting the element's symbol with its corresponding charge derived from the electrons gained or lost. - For (a) Li: Li⁺ - For (b) S: S²⁻ - For (c) I: I⁻ - For (d) N: N³⁻ - For (e) Al: Al³⁺ - For (f) Cs: Cs⁺ - For (g) Mg: Mg²⁺.

Key concepts

Electron Configuration

When atoms form ions, they aim to achieve a stable electron configuration. This usually means they try to emulate the nearest noble gas, which is known for its stable octet configuration (except helium, which is stable at two electrons). The way they achieve this is by losing or gaining electrons. For instance:

• Elements like lithium (\( \mathrm{Li} \)) lose electrons to become positively charged cations.
• Elements like sulfur (\( \mathrm{S} \)) gain electrons to become negatively charged anions.

Whenever an atom loses an electron, it has more protons than electrons, resulting in a positive charge. Conversely, when an atom gains electrons, it has more electrons than protons, leading to a negative charge.
By achieving a full outer shell, an atom reaches a more energetically favorable and stable state.

Periodic Table

The periodic table is an essential tool in understanding how elements interact and form ions. Elements are arranged in the periodic table in a way that reflects their electron configuration.

• Elements on the left side of the table, such as alkali metals and alkaline earth metals, typically lose electrons to form cations because they have fewer electrons in their outer shell.
• Elements on the right side, such as halogens and other non-metals, usually gain electrons to form anions to complete their outer shell with eight electrons, aligning with the octet rule.

The group numbers often indicate the behavior of the element:

• Group 1 elements, like lithium (\( \mathrm{Li} \)), lose one electron to form \( \mathrm{Li}^+ \)
• Group 16 elements, like sulfur (\( \mathrm{S} \)), gain two electrons to form \( \mathrm{S}^{2-} \)

Understanding the position of an element in the periodic table helps predict the kind of ion an element will form.

Cations

Cations are positively charged ions formed by the loss of one or more electrons. Metals, especially those on the left side of the periodic table, tend to form cations. Some common examples include:

• Lithium (\( \mathrm{Li} \)), which forms \( \mathrm{Li}^+ \)
• Aluminum (\( \mathrm{Al} \)), which forms \( \mathrm{Al}^{3+} \)
• Magnesium (\( \mathrm{Mg} \)), which forms \( \mathrm{Mg}^{2+} \)

When an atom becomes a cation, it means it has lost enough electrons to achieve a stable electron configuration, thereby decreasing its atomic radius.
This is because there are fewer electron-electron repulsions, allowing the remaining electrons to be pulled closer to the nucleus. Cations are crucial in forming ionic compounds, as they bond with anions to create stable structures.

Anions

Anions are negatively charged ions formed when an element gains electrons. Non-metals, especially those on the right side of the periodic table, tend to form anions due to their higher electronegativity. Commonly, you'll encounter:

• Nitrogen (\( \mathrm{N} \)), which gains three electrons to form \( \mathrm{N}^{3-} \)
• Oxygen (\( \mathrm{O} \)), which forms \( \mathrm{O}^{2-} \)
• Iodine (\( \mathrm{I} \)), which forms \( \mathrm{I}^- \)

As an atom gains electrons, it increases its electron-electron repulsive forces, thus enlarging its atomic radius.
Anions are vital in forming ionic compounds by bonding with cations, resulting in electrically neutral compounds.

Alkali Metals

Alkali metals are found in Group 1 of the periodic table and are known for their reactivity and tendency to form cations. These metals include elements such as lithium (\( \mathrm{Li} \)) and cesium (\( \mathrm{Cs} \)), which lose one electron to form \( \mathrm{Li}^+ \) and \( \mathrm{Cs}^+ \), respectively.

• Alkali metals have one electron in their outermost shell.
• They are highly reactive, particularly with water, due to their desire to lose this single electron and attain a stable electron configuration like a noble gas.
• These elements typically form ionic compounds by bonding with non-metals that form anions.

Understanding alkali metals helps in predicting the behavior of many chemical reactions involving these elements.