Question

Give the names and charges of the cation and anion in each of the following compounds: \((\mathbf{a}) \mathrm{CaO},(\mathbf{b}) \mathrm{Na}_{2} \mathrm{SO}_{4},(\mathbf{c}) \mathrm{KClO}_{4},\) (d) \(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2}\), (e) \(\mathrm{Cr}(\mathrm{OH})_{3}\)

Short answer

a) \(\mathrm{CaO}\): Cation: Calcium ion \(\mathrm{(Ca^{2+})}\), Anion: Oxide ion \(\mathrm{(O^{2-})}\) b) \(\mathrm{Na_2SO_4}\): Cation: Sodium ion \(\mathrm{(Na^{+})}\), Anion: Sulfate ion \(\mathrm{(SO_4^{2-})}\) c) \(\mathrm{KClO_4}\): Cation: Potassium ion \(\mathrm{(K^{+})}\), Anion: Perchlorate ion \(\mathrm{(ClO_4^{-})}\) d) \(\mathrm{Fe(NO_3)_2}\): Cation: Iron(II) ion \(\mathrm{(Fe^{2+})}\), Anion: Nitrate ion \(\mathrm{(NO_3^{-})}\) e) \(\mathrm{Cr(OH)_3}\): Cation: Chromium(III) ion \(\mathrm{(Cr^{3+})}\), Anion: Hydroxide ion \(\mathrm{(OH^{-})}\)

Step-by-step solution

a) CaO (Calcium Oxide)

Calcium oxide has two elements: Calcium (Ca) and Oxygen (O). Calcium is in Group 2 of the periodic table, so it loses two electrons and forms a cation with a charge of +2. The chemical symbol with its charge for the cation is: \(\mathrm{Ca^{2+}}\). Oxygen is in Group 6 and gains two electrons to complete its octet, forming an anion with a charge of -2. The chemical symbol with its charge for the anion is: \(\mathrm{O^{2-}}\). Cation: Calcium ion \(\mathrm{(Ca^{2+})}\) Anion: Oxide ion \(\mathrm{(O^{2-})}\)

b) Na2SO4 (Sodium Sulfate)

Sodium sulfate has three elements: Sodium (Na), Sulfur (S), and Oxygen (O). Sodium is in Group 1 of the periodic table, so it loses one electron and forms a cation with a charge of +1. The chemical symbol with its charge for the cation is: \(\mathrm{Na^{+}}\). The anion in the compound is the sulfate ion (SO4). Sulfate is a polyatomic ion with a charge of -2. The chemical symbol with its charge for the anion is: \(\mathrm{SO_4^{2-}}\). Cation: Sodium ion \(\mathrm{(Na^{+})}\) Anion: Sulfate ion \(\mathrm{(SO_4^{2-})}\)

c) KClO4 (Potassium Perchlorate)

Potassium perchlorate has three elements: Potassium (K), Chlorine (Cl), and Oxygen (O). Potassium is in Group 1 of the periodic table, so it loses one electron and forms a cation with a charge of +1. The chemical symbol with its charge for the cation is: \(\mathrm{K^{+}}\). The anion in the compound is the perchlorate ion (ClO4). Perchlorate is a polyatomic ion with a charge of -1. The chemical symbol with its charge for the anion is: \(\mathrm{ClO_4^{-}}\). Cation: Potassium ion \(\mathrm{(K^{+})}\) Anion: Perchlorate ion \(\mathrm{(ClO_4^{-})}\)

d) Fe(NO3)2 (Iron(II) Nitrate)

Iron(II) nitrate has two elements: Iron (Fe) and the polyatomic ion Nitrate (NO3). The charge of Iron (Fe) in this compound is +2 as given in the name. The chemical symbol with its charge for the cation is: \(\mathrm{Fe^{2+}}\). The anion in the compound is the nitrate ion (NO3). Nitrate is a polyatomic ion with a charge of -1. The chemical symbol with its charge for the anion is: \(\mathrm{NO_3^{-}}\). Cation: Iron(II) ion \(\mathrm{(Fe^{2+})}\) Anion: Nitrate ion \(\mathrm{(NO_3^{-})}\)

e) Cr(OH)3 (Chromium(III) Hydroxide)

Chromium(III) hydroxide has two elements: Chromium (Cr) and the polyatomic ion Hydroxide (OH). The charge of Chromium (Cr) in this compound is +3 as given in the name. The chemical symbol with its charge for the cation is: \(\mathrm{Cr^{3+}}\). The anion in the compound is the hydroxide ion (OH). Hydroxide is a polyatomic ion with a charge of -1. The chemical symbol with its charge for the anion is: \(\mathrm{OH^{-}}\). Cation: Chromium(III) ion \(\mathrm{(Cr^{3+})}\) Anion: Hydroxide ion \(\mathrm{(OH^{-})}\)

Key concepts

Cations and Anions

In chemistry, ions are atoms or groups of atoms that have gained or lost electrons, giving them a net positive or negative charge. There are two main types of ions: cations and anions.
Cations are positively charged ions. They are typically formed when a metal loses electrons. For example, in calcium oxide (CaO), calcium loses two electrons to form the calcium ion \( \mathrm{Ca^{2+}} \). The positive charge results from having more protons than electrons.
On the other hand, anions are negatively charged ions, formed when nonmetals gain electrons. In the same compound, oxygen gains two electrons to become the oxide ion \( \mathrm{O^{2-}} \). The negative charge means there are more electrons than protons.
Understanding the formation and charges of cations and anions helps predict the properties of ionic compounds, such as their solubility and electrical conductivity.

Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, that together carry a charge. Unlike single-atom ions, polyatomic ions contain multiple atoms that can be either positively or negatively charged - though more commonly negative.
For instance, the sulfate ion \( \mathrm{SO_4^{2-}} \) in sodium sulfate \((\mathrm{Na_2SO_4})\) consists of sulfur and oxygen atoms bonded together and carrying an overall charge of -2. Another example is the perchlorate ion \( \mathrm{ClO_4^{-}} \), found in potassium perchlorate \((\mathrm{KClO_4})\), with a -1 charge.
Polyatomic ions are crucial in determining the composition and properties of compounds in which they occur. Recognizing and memorizing them enhances one's ability to name and write chemical formulas for compounds accurately.

Chemical Formulas

Chemical formulas represent the combination of different atoms in a compound. In ionic compounds, formulas express the ratio of cations and anions needed to balance the overall charge.
For example, in calcium oxide \((\mathrm{CaO})\), one calcium ion \( \mathrm{Ca^{2+}} \) pairs with one oxide ion \( \mathrm{O^{2-}} \), resulting in a neutral compound. In more complex formulas like iron(II) nitrate \((\mathrm{Fe(NO_3)_2})\), one iron ion \( \mathrm{Fe^{2+}} \) combines with two nitrate ions \( \mathrm{NO_3^{-}} \), again maintaining charge neutrality.
The subscripts in formulas indicate the number of each type of ion present. Understanding and writing chemical formulas is essential for identifying the components and proportions of substances, and for describing chemical reactions and properties.