Question

Write formulas for the following. (a) ammonium sulfide (b) chromium(III) sulfate hexahydrate (c) silicon tetrafluoride (d) molybdenum(IV) sulfide (e) tin(IV) chloride (f) hydrogen selenide (g) tetraphosphorus heptasulfide

Short answer

(a) (NH4)2S, (b) Cr2(SO4)3·6H2O, (c) SiF4, (d) MoS2, (e) SnCl4, (f) H2Se, (g) P4S7.

Step-by-step solution

- Write the Formula for Ammonium Sulfide

Ammonium is a polyatomic ion with a chemical formula of NH4+ and sulfide is an anion with a formula of S2-. To write the formula, combine the two ions in a ratio that makes the compound electrically neutral: (NH4)2S.

- Write the Formula for Chromium(III) Sulfate Hexahydrate

Chromium(III) has a 3+ charge, Cr3+, and sulfate is a polyatomic ion with a 2- charge, SO4^2-. To balance the charges, you need two sulfate ions for every three chromium ions. Hexahydrate indicates six water molecules associated with the compound: Cr2(SO4)3·6H2O.

- Write the Formula for Silicon Tetrafluoride

Silicon has a 4+ charge, Si4+, and fluoride has a 1- charge, F-. You need four fluoride ions to balance out the charge of one silicon ion: SiF4.

- Write the Formula for Molybdenum(IV) Sulfide

Molybdenum(IV) means Mo has a 4+ charge, and sulfide has a 2- charge. To make the compound electrically neutral, you need two sulfide ions for every molybdenum ion: MoS2.

- Write the Formula for Tin(IV) Chloride

Tin(IV) means Sn has a 4+ charge and chloride has a 1- charge. Therefore, you need four chloride ions to balance one tin ion: SnCl4.

- Write the Formula for Hydrogen Selenide

Hydrogen generally has a 1+ charge, and selenide corresponds to the Se2- ion. To form a neutral compound, two hydrogen ions are needed for one selenide ion: H2Se.

- Write the Formula for Tetraphosphorus Heptasulfide

The prefix 'tetra' means four, so there are four phosphorus atoms, P4, and the prefix 'hepta' means seven, indicating seven sulfur atoms: P4S7.

Key concepts

Polyatomic Ions

Polyatomic ions are charged entities composed of two or more atoms covalently bonded or of a metal complex that can be considered to be acting as a single unit. The charge on a polyatomic ion is the result of the difference between the number of protons and electrons in the atoms making up that ion.

Understanding polyatomic ions is essential in writing chemical formulas. For instance, in the exercise, ammonium (NH₄⁺) and sulfate (SO₄^2-) are both polyatomic ions. To name a few, others include hydroxide (OH^-), nitrate (NO₃^-), and phosphate (PO₄^3-).

To balance the formula of a compound containing polyatomic ions, one often has to use parentheses to keep the polyatomic ion together as a group if multiple copies of the ion are needed. For example, to represent the ammonium sulfutide, we use (NH₄)₂S, wherein two ammonium ions pair with a single sulfide ion to create an electrically neutral compound.

Ionic Charge Balance

The balance of ionic charges is a fundamental concept when writing chemical formulas for ionic compounds. A stable ionic compound is electrically neutral, meaning the total positive charge must equal the total negative charge.

When creating a chemical formula, one must determine how the charges of the cations (positive ions) and anions (negative ions) cancel each other out. This is evident in the exercises provided. For example, in the case of chromium(III) sulfate hexahydrate, Cr₂(SO₄)₃·6H₂O, one must understand that there are two trivalent chromium ions (each with a 3+ charge) and three sulfate ions (each with a 2- charge) to achieve a neutral compound. Additionally, six molecules of water are added as part of the hydrate but do not affect the overall charge of the compound as water is neutral.

Chemical Nomenclature

Chemical nomenclature, the naming system for compounds, is not only a set of rules but also an orderly method to provide unique and unambiguous names for chemical substances. It includes the use of prefixes and suffixes to inform the composition and structure of compounds.

The rules for naming inorganic compounds, for instance, require the representation of the oxidation state of the cation if it can have more than one oxidation state, e.g., chromium(III). Similarly, polyatomic ionic compounds with a specific number of water molecules are named hydrous compounds and include terms like 'hydrate' in their names, as seen in chromium(III) sulfate hexahydrate.

For molecular compounds, prefixes are also utilized to denote the number of atoms of each element present in the compound, as in silicon tetrafluoride (SiF₄) and tetraphosphorus heptasulfide (P₄S₇). In nomenclature, learning and applying these systematic rules will significantly aid in writing and deciphering chemical formulas.