Question

Write formulas for the following. (a) sodium monohydrogen phosphate (b) lithium selenide (c) chromium(III) acetate (d) disulfur decafluoride (e) nickel(II) cyanide (f) iron(III) oxide (g) antimony pentafluoride

Short answer

The formulas are: (a) Na2HPO4, (b) Li2Se, (c) Cr(C2H3O2)3, (d) S2F10, (e) Ni(CN)2, (f) Fe2O3, (g) SbF5.

Step-by-step solution

Identifying the Components

The first step in writing the formula for a compound involves identifying the cation and the anion as well as their respective charges. Ions of elements from groups 1 and 2 and aluminum have a fixed charge, while transition metals can have variable charges. Non-metals form anions with negative charges.

Writing Formulas for Each Compound

For ionic compounds, write the symbol and charge for the cation followed by the symbol and charge for the anion. Transition metals require Roman numerals to indicate their charge. For molecular compounds, use prefixes to indicate the number of each type of atom present.

(a) Sodium Monohydrogen Phosphate

Sodium has a +1 charge (Na+). Monohydrogen phosphate is the anion with the formula HPO4 with a charge of -2. To balance the charges, you need two sodium ions for every monohydrogen phosphate ion. The formula is Na2HPO4.

(b) Lithium Selenide

Lithium has a +1 charge (Li+). Selenide is the anion with the formula Se with a charge of -2. To balance the charges, you need two lithium ions for every selenide ion. The formula is Li2Se.

(c) Chromium(III) Acetate

Chromium(III) indicates a chromium ion with a +3 charge (Cr3+). Acetate is the anion with the formula C2H3O2 with a charge of -1. To balance the charges, you need three acetate ions for every chromium ion. The formula is Cr(C2H3O2)3.

(d) Disulfur Decafluoride

This is a covalent compound. Disulfur means two sulfur atoms (S2) and decafluoride means ten fluorine atoms. The formula is S2F10.

(e) Nickel(II) Cyanide

Nickel(II) indicates a nickel ion with a +2 charge (Ni2+). Cyanide is the anion with the formula CN with a charge of -1. To balance the charges, you need two cyanide ions for every nickel ion. The formula is Ni(CN)2.

(f) Iron(III) Oxide

Iron(III) indicates an iron ion with a +3 charge (Fe3+). Oxide is the anion with the formula O with a charge of -2. To balance the charges, you need two iron ions for every three oxide ions. The formula is Fe2O3.

(g) Antimony Pentafluoride

Antimony forms a cation with a +5 charge (Sb5+). Pentafluoride refers to five fluoride anions (F-). Since the antimony has a +5 charge and each fluoride has -1 charge, one ion of each is needed. The formula is SbF5.

Key concepts

Ionic Compounds

Ionic compounds are composed of positively charged ions known as cations and negatively charged ions called anions. These ions come together to form a neutral compound through ionic bonding, which involves the electrostatic attraction between oppositely charged ions. In sodium monohydrogen phosphate, for instance, we observe the pairing of the sodium cation (Na+) with the monohydrogen phosphate anion (HPO4^2-), leading to the balanced formula Na₂HPO₄.

Understanding the charges of various ions, such as knowing lithium (Li⁺) pairs with selenide (Se^2-) to create lithium selenide (Li₂Se), is key in determining the correct stoichiometric ratios. Adequate charge balance is essential, as in iron(III) oxide, where the iron cation (Fe³⁺) and oxide anion (O^2-) combine to form Fe₂O₃, with the charges satisfying the requirement for electrical neutrality.

Molecular Compounds

Molecular compounds, or covalent compounds, are formed when atoms share electrons to achieve stability, resulting in molecules with non-ionic bonds. Prefixes are often used to denote the number of atoms present in these compounds. For example, the compound disulfur decafluoride consists of two sulfur atoms and ten fluorine atoms, hence the name S₂F₁₀.

Prefixes in Molecular Compounds

Prefixes like mono-, di-, tri-, tetra-, and so on, permit us to understand the composition of molecular compounds definitively. They precisely convey the composition, making it essential for writing and reading chemical formulas of such compounds.

Transition Metals

Transition metals can be more challenging due to their ability to adopt multiple varying charges. These metals, located in the d-block of the periodic table, exhibit this variable valency which is indicated by Roman numerals in the compound's name. This is crucial in compounds like chromium(III) acetate and nickel(II) cyanide, where the Roman numeral tells us the oxidation state of the metal.

In chromium(III) acetate, for example, chromium has a charge of +3, denoted by the (III) next to its name. This dictates that three acetate ions (C₂H₃O₂^-) are needed to balance the charge, leading to the formula Cr(C₂H₃O₂)₃. Recognizing the oxidation state is vital for writing accurate formulas with transition metals.

Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded together, behaving as a single charged entity. These ions can either have a positive or negative charge. An example is the acetate ion (C₂H₃O₂^-), a common anion in many compounds like chromium(III) acetate.

Becoming familiar with common polyatomic ions, such as cyanide (CN^-), monohydrogen phosphate (HPO₄^2-), and sulfate (SO₄^2-), is immensely helpful. In nickel(II) cyanide, the correct pairing of one nickel ion (Ni²⁺) with two cyanide ions is required to create the neutral compound Ni(CN)₂. Remembering the composition and charge of polyatomic ions streamlines the process of writing chemical formulas.