Question

Write formulas for the following. (a) dialuminum hexachloride (b) tetraarsenic decaoxide (c) magnesium hydroxide (d) copper(II) bisulfate (e) ammonium thiocyanate (f) potassium thiosulfate (g) diiodine pentaoxide

Short answer

The chemical formulas are (a) Al2Cl6, (b) As4O10, (c) Mg(OH)2, (d) Cu(HSO4)2, (e) (NH4)SCN, (f) K2S2O3, (g) I2O5.

Step-by-step solution

Identify the Elements

Firstly, identify the elements present in each compound. Dialuminum suggests the presence of Aluminum (Al), hexachloride suggests Chlorine (Cl). Tetraarsenic indicates Arsenic (As) and decaoxide suggests Oxygen (O). Magnesium hydroxide contains Magnesium (Mg) and Hydroxide (OH). Copper(II) bisulfate contains Copper (Cu) with a +2 charge, Bisulfate (HSO4). Ammonium thiocyanate contains Ammonium (NH4) and Thiocyanate (SCN). Potassium thiosulfate consists of Potassium (K) and Thiosulfate (S2O3). Diiodine pentaoxide consists of Iodine (I) and Oxygen (O).

Determine the Prefixes

The prefixes in the names indicate the number of atoms present. 'Di' stands for two, 'tetra' for four, 'hexa' for six, 'deca' for ten, 'penta' means five. Use these prefixes to determine the subscript numbers for each element in the compounds.

Write the Chemical Formulas

Using the prefixes and the valency of elements, write down the chemical formulas: (a) Al2Cl6 for dialuminum hexachloride, (b) As4O10 for tetraarsenic decaoxide, (c) Mg(OH)2 for magnesium hydroxide by recognizing the hydroxide as a polyatomic ion, (d) Cu(HSO4)2 for copper(II) bisulfate indicating copper has a +2 charge, (e) (NH4)SCN for ammonium thiocyanate as ammonium and thiocyanate are both polyatomic ions, (f) K2S2O3 for potassium thiosulfate, and (g) I2O5 for diiodine pentaoxide.

Key concepts

Chemical Nomenclature

Understanding chemical nomenclature is crucial for students as it is the system used for naming chemical compounds. It ensures that each compound has a unique name that describes its composition, allowing chemists to communicate precisely and avoid confusion. In chemical nomenclature, specific prefixes, like 'di-', 'tetra-', 'penta-', and 'hexa-', indicate the number of atoms of an element present in a compound. For instance, 'dialuminum' indicates the presence of two aluminum atoms. Similarly, 'hexachloride' signifies six chlorine atoms, resulting in Al₂Cl₆ for dialuminum hexachloride.

The system also accommodates the naming of compounds with varying charges, particularly transition metals. For example, copper can have different valencies, and in 'copper(II) bisulfate', the (II) indicates that copper has a +2 oxidation state. Therefore, its formula, Cu(HSO₄)₂, contains two bisulfate ions to balance the charge of the copper ion.

In writing chemical formulas, it is especially important to have a clear understanding of these conventions, as they directly impact the subscript numbers that denote the quantity of each ion or atom in the compound. Catering to compound naming rules will enable students to both correctly interpret the names of compounds and to write their formulas based on the nomenclature.

Polyatomic Ions

Polyatomic ions are ions consisting of two or more atoms covalently bonded together, carrying an overall charge. Recognizing and understanding these ions is vital for correctly writing the chemical formulas of compounds. In some cases, polyatomic ions can be thought of as a single unit when determining the formula of a compound. For example, hydroxide is a common polyatomic ion comprised of one oxygen and one hydrogen with a charge of -1; as seen in magnesium hydroxide, Mg(OH)₂.

Chemical nomenclature for compounds with polyatomic ions follows the standard rules but also requires knowledge of the specific ions. Ammonium (NH₄⁺) and thiocyanate (SCN^-) are also polyatomic ions, so the chemical formula for ammonium thiocyanate is written as (NH₄)SCN. Familiarity with common polyatomic ions is instrumental for correctly writing chemical formulas and understanding their properties.

Often, students may struggle with memorizing these ions, but utilizing flashcards or mnemonics can make this task more manageable. Moreover, being aware of polyatomic ions can help to demystify otherwise complex-sounding compound names and promote a more intuitive grasp of chemical reactions and equations where these ions are present.

Valency of Elements

The valency of an element is a measure of its ability to combine with other atoms, often considered as the number of electrons an atom can lose, gain, or share to become stable. This concept is integral to forming chemical compounds and writing their formulas. For instance, aluminum typically has a valency of +3 and chlorine has a -1 valency. In the compound dialuminum hexachloride, Al₂Cl₆, two aluminum ions with a valency of +3 each balance the charge of six chloride ions with a valency of -1 each.

Understanding the valency of elements helps in deducing the correct proportions of each element in a compound. Transition metals can have more than one possible valency, complicating matters. As seen in the exercise, copper(II) bisulfate, the (II) highlights the +2 valency of copper, affecting the subscript number for the bisulfate ion. To ensure the compound is electrically neutral, the formula becomes Cu(HSO₄)₂.

When students grasp the concept of valency, they are more equipped to tackle chemical equations and predict the outcome of reactions. Concepts like valency are foundations upon which further understanding of chemistry is built; they should be made as clear as possible to support students in developing a solid foundation in the subject.