Question

Write the chemical formula for each of the following compounds, and indicate the oxidation state of the group 16 elements in each: (a) selenious acid, (b) sulphur trioxide, \((\mathbf{c})\) selenium dichloride, \((\mathbf{d})\) aluminium selenide, (e) iron(II) sulfate, (f) tellurium trioxide.

Short answer

The chemical formulas and oxidation states of group 16 elements for the given compounds are: a) Selenious Acid: H2SeO3; Se oxidation state: +4 b) Sulphur Trioxide: SO3; S oxidation state: +6 c) Selenium Dichloride: SeCl2; Se oxidation state: +2 d) Aluminium Selenide: Al2Se3; Se oxidation state: -2 e) Iron(II) Sulfate: FeSO4; S oxidation state: +6 f) Tellurium Trioxide: TeO3; Te oxidation state: +6

Step-by-step solution

a) Selenious Acid

Selenious Acid is an oxyacid of selenium with the -OUS ending, which means it has one oxygen less than the -IC acid (Selenic Acid). Selenic acid is represented by the formula H2SeO4 (similar to sulfuric acid H2SO4), so selenious acid can be represented by the formula H2SeO3. The oxidation state of selenium can be determined using the overall charge of the compound. In this case, the compound is neutral, and since hydrogen has an oxidation state of +1 and oxygen of -2, we have: 2(+1) + Se + 3(-2) = 0 Se = +4 The oxidation state of selenium is +4.

b) Sulphur Trioxide

Sulphur trioxide is represented by the formula SO3. To determine the oxidation state of sulfur, keep in mind that the overall charge is zero and that oxygen has an oxidation state of -2: S + 3(-2) = 0 S = +6 The oxidation state of sulfur is +6.

c) Selenium Dichloride

Selenium dichloride is a binary compound formed by selenium and chlorine. It has one selenium atom and two chlorine atoms. Chlorine has an oxidation state of -1 in this compound, and since the overall charge is zero, the formula is SeCl2. The oxidation state of selenium can be determined: Se + 2(-1) = 0 Se = +2 The oxidation state of selenium is +2.

d) Aluminium Selenide

Aluminium selenide is a binary ionic compound composed of aluminium and selenium ions. The formula for aluminium ion is Al^3+ and for selenium ion is Se^2-. Since we need an electrically neutral compound, we must balance the charges. The formula for this compound is Al2Se3. The oxidation state of selenium is -2.

e) Iron(II) Sulfate

Iron(II) sulfate indicates that iron has an oxidation state of +2. It is a compound formed by iron ions (Fe^2+) and sulfate ions (SO4^2-). Since the charges are equal and opposite, one iron ion combines with one sulfate ion: FeSO4 The oxidation state of sulfur in the sulfate ion is calculated as follows: S + 4(-2) = -2 (charge on sulfate ion) S = +6 The oxidation state of sulfur is +6.

f) Tellurium Trioxide

Tellurium trioxide is represented by the formula TeO3. To determine the oxidation state of tellurium, remember that the overall charge is zero and that oxygen has an oxidation state of -2: Te + 3(-2) = 0 Te = +6 The oxidation state of tellurium is +6.

Key concepts

Oxidation States

Oxidation states are an essential concept in chemistry, helping us understand how elements bond within a compound. This numerical indicator shows us the charge of an atom if the compound was made up of ions. It helps determine the electron distribution among atoms.

To calculate an oxidation state, consider the standard rules:

• Free elements have an oxidation state of zero.
• Simple monoatomic ions hold an oxidation state equal to their charge.
• Hydrogen typically has an oxidation state of +1, except in metal hydrides where it's -1.
• Oxygen usually adopts an oxidation state of -2, although there are exceptions such as in peroxides.

By applying these rules, you can figure out the oxidation state of any element in a compound by making sure that the sum of oxidation states in a neutral compound equals zero, and in ions equals the ion's charge.

Group 16 Elements

Group 16 in the periodic table includes the elements oxygen, sulfur, selenium, tellurium, and polonium, all commonly referred to as the chalcogens. They are known for forming compounds that gain two electrons to fill their outer shell, hence typically exhibiting a -2 oxidation state.

Chalcogens play a crucial role in numerous chemical reactions:

• Oxygen is essential for combustion and respiration.
• Sulfur is used extensively in industry, for example, in sulfuric acid production.
• Selenium and tellurium have specialized roles in electronics and materials science.

The tendency of these elements to form various compounds with metals and nonmetals alike makes them indispensable in chemistry.

Binary Compounds

Binary compounds consist of two different elements. They can be either ionic or covalent in nature, depending on the types of atoms involved and the nature of their bonding.

When naming binary compounds, the more electropositive element (usually a metal) is named first, followed by the more electronegative element (usually a nonmetal) with its ending changed to '-ide'. For example, selenium dichloride, a binary compound, follows this structure:

• Selenium is less electronegative than chlorine, so it comes first.
• Chlorine becomes 'chloride' in the compound's name.

The composition of binary compounds is straightforward, involving formulas that reflect the needed balance of charges to achieve a neutral compound.

Ionic Compounds

Ionic compounds form from the electrostatic attraction between positively charged cations and negatively charged anions. These compounds are common in the world of chemistry due to the ability of ions to donate or accept electrons fully during bond formation.

The formula for an ionic compound is established based on charge neutrality, where the total positive and negative charges must balance. This is particularly evident in compounds like aluminium selenide, where two aluminium ions balance three selenium ions’ charges, leading to the formula \( \text{Al}_2\text{Se}_3 \).

• These compounds often have high melting and boiling points due to strong ionic bonds.
• They can conduct electricity when dissolved in water or melted, as ions are free to move.

Ionic compounds are ubiquitous in everyday life, from the salt you sprinkle on your food to the components in batteries driving technology.