Question

Write a chemical formula for each compound or ion, and indicate the oxidation state of the group 5 A element in each formula: (a) phosphate ion, (b) arsenous acid, (c) antimony(III) sulfide, (d) calcium dihydrogen phosphate, (e) potassium phosphide, (f) gallium arsenide.

Short answer

(a) Phosphate ion: \(PO_4^{3-}\), oxidation state of P: +5. (b) Arsenous acid: \(H_3AsO_3\), oxidation state of As: +3. (c) Antimony(III) sulfide: \(Sb_2S_3\), oxidation state of Sb: +3. (d) Calcium dihydrogen phosphate: \(Ca(H_2PO_4)_2\), oxidation state of P: +5. (e) Potassium phosphide: \(K_3P\), oxidation state of P: -3. (f) Gallium arsenide: \(GaAs\), oxidation state of As: -3.

Step-by-step solution

(a) Phosphate ion

To write the formula for the phosphate ion, we first need to identify the elements involved. Phosphate ion consists of a phosphorus (P) atom and four oxygen (O) atoms. The oxidation state of phosphorus in phosphate is +5. The chemical formula for the phosphate ion is PO4^3-.

(b) Arsenous acid

Arsenous acid consists of an arsenic (As) atom, three oxygen (O) atoms, and two hydrogen (H) atoms. The oxidation state of arsenic in arsenous acid is +3. The chemical formula for arsenous acid is H3AsO3.

(c) Antimony(III) sulfide

Antimony(III) sulfide consists of an antimony (Sb) atom and three sulfur (S) atoms. The oxidation state of antimony in this compound is +3, as indicated by the (III). The chemical formula for antimony(III) sulfide is Sb2S3.

(d) Calcium dihydrogen phosphate

Calcium dihydrogen phosphate consists of calcium (Ca) cation, two hydrogen (H) atoms, and a phosphate (PO4) ion. The oxidation state of phosphorus in this compound is +5. The chemical formula for calcium dihydrogen phosphate is Ca(H2PO4)2.

(e) Potassium phosphide

Potassium phosphide consists of potassium (K) cations and phosphorus (P) anions. The oxidation state of phosphorus in this compound is -3. The chemical formula for potassium phosphide is K3P.

(f) Gallium arsenide

Gallium arsenide consists of gallium (Ga) atoms and arsenic (As) atoms. The oxidation state of arsenic in this compound is -3. The chemical formula for gallium arsenide is GaAs.

Key concepts

Oxidation Numbers

Understanding the oxidation states or oxidation numbers in chemistry is crucial for knowing how electrons are distributed among atoms in a molecule.

An oxidation number is a positive or negative number assigned to an atom based on a set of rules. These numbers signify how many electrons an atom gains, loses, or shares when it forms chemical bonds.

• The oxidation state of an atom in its elemental form is always zero.
• For monoatomic ions, the oxidation state is equal to the charge of the ion.
• Oxygen typically has an oxidation state of -2, and hydrogen is usually +1.
• In compounds, the sum of oxidation states must equal the charge of the molecule or ion.

Take the phosphate ion (PO₄)^3- as an example. To maintain a charge balance within the ion, the oxidation state of phosphorus (P) is +5, while each oxygen (O) has an oxidation state of -2. When calculating oxidation numbers, it's essential to consider these rules to accurately determine the overall charge and distribution of electrons in a compound.

Inorganic Nomenclature

Naming inorganic compounds, also known as inorganic nomenclature, follows a set of conventions to ensure clarity and precision.

There are different rules for different types of compounds: ionic compounds, covalent compounds, acids, and polyatomic ions. For example, in ionic compounds like potassium phosphide (K₃P), the cation (potassium) is named first, followed by the name of the anion, which is derived from phosphorus, the non-metal element, by adding the suffix -ide to the root of the element's name.

Polyatomic Ions

Polyatomic ions have specific names, like phosphate in Ca(H₂PO₄)₂.

Acids are named based on their anions. For instance, arsenous acid (H₃AsO₃) is named for the arsenite ion (AsO₃)^3-. Oxidation states can be included in the name when an element can have more than one, as seen with antimony(III) sulfide (Sb₂S₃), indicating that antimony has an oxidation state of +3.

Polyatomic Ions

Polyatomic ions are charged entities composed of two or more atoms covalently bonded together, acting as a single unit with a net charge.

They can be cations (positively charged) or anions (negatively charged). Common polyatomic ions include sulfate (SO₄)^2-, nitrate (NO₃)^-, and as mentioned earlier, phosphate (PO₄)^3-. Knowing the formula and charge of these ions is essential when constructing chemical formulas for compounds that contain them.

• When a polyatomic ion is part of a compound, its name remains the same as when it is alone.
• Some polyatomic ions can gain or lose hydrogen ions (H⁺) to form different ions, like dihydrogen phosphate (H₂PO₄)^- in calcium dihydrogen phosphate (Ca(H₂PO₄)₂).
• Understanding polyatomic ions is key to writing formulas and naming compounds accurately.

With practice, recognizing these ions within compounds becomes second nature, simplifying the process of deducing chemical formulas and oxidation states.