Question

Assign oxidation numbers to the atoms in each substance. a) \(\mathrm{C}_{6} \mathrm{H}_{6}\) b) \(\mathrm{B}(\mathrm{OH})_{3}\) C) \(\mathrm{Li}_{2} \mathrm{~S}\) d) \(\mathrm{Au}\)

Short answer

a) C: -1; H: +1. b) B: +3; O: -2; H: +1. c) Li: +1; S: -2. d) Au: 0.

Step-by-step solution

Understanding Oxidation Numbers

Oxidation numbers are assigned to elements in a molecule to indicate the distribution of electrons among the atoms. Each element in a compound typically follows certain rules to assign these numbers. For example, hydrogen is +1 (except in metal hydrides), oxygen is -2, and the sum of oxidation numbers in a neutral molecule is zero.

Assign Oxidation Numbers to \(\mathrm{C}_{6} \mathrm{H}_{6}\)

In benzene \(\mathrm{C}_{6} \mathrm{H}_{6}\), hydrogen has an oxidation number of +1. Since there are six hydrogen atoms, the total contribution is +6. The molecule is neutral so the total oxidation state is 0. Let the oxidation number of carbon be \(x\). This gives the equation:\[6x + 6(1) = 0\]Solving for \(x\) gives:\[6x = -6\]\[x = -1\]Therefore, each carbon atom in benzene has an oxidation number of -1.

Assign Oxidation Numbers to \(\mathrm{B}(\mathrm{OH})_{3}\)

In boric acid, the hydroxide, \(\mathrm{OH}^{-}\), has a total oxidation number of -1 (with oxygen being -2 and hydrogen +1). Since there are three \(\mathrm{OH}\) groups, the total contribution is -3. Let the oxidation number of boron be \(y\). The compound is neutral, so:\[y - 3 = 0\]Solving for \(y\) gives:\[y = +3\]Therefore, the oxidation number of boron is +3.

Assign Oxidation Numbers to \(\mathrm{Li}_{2} \mathrm{~S}\)

Lithium, \(\mathrm{Li}\), as an alkali metal, typically has an oxidation number of +1. With two lithium atoms, the total is +2. Let sulfur's oxidation number be \(z\). The compound is neutral, so:\[2(+1) + z = 0\]Solving for \(z\) gives:\[z = -2\]Therefore, the oxidation number of sulfur is -2.

Assign Oxidation Numbers to \(\mathrm{Au}\)

In its elemental form, gold (\(\mathrm{Au}\)) has an oxidation number of 0 because it is not combined with any other element.

Key concepts

Chemical Compounds

Chemical compounds are substances consisting of two or more different types of atoms bonded together. These atoms are held together by chemical bonds forming a stable unit.
Chemical compounds can be further classified as either ionic or covalent based on the nature of the bonds they contain. In ionic compounds, atoms transfer electrons, leading to the formation of ions with positive and negative charges attracting each other.

• Example: Sodium chloride (NaCl) is an ionic compound, where sodium (Na) gives up an electron to chlorine (Cl).

Covalent compounds, on the other hand, involve the sharing of electrons between atoms to achieve stability. This sharing typically occurs between non-metals.

• Example: Water (H₂O) is a covalent compound where oxygen shares electrons with hydrogen atoms.

Additionally, compounds have a chemical formula which provides important information such as the type and number of each atom present. Knowing how to interpret these formulas helps in determining the compound's composition and understanding its chemical behavior.

Bonding Rules

Bonding rules are the principles that govern how atoms combine to form molecules. These rules are based on achieving a more stable electronic arrangement, often resembling a noble gas configuration.
The octet rule is a common guiding principle, especially for main-group elements. It suggests that atoms tend to form bonds until they are surrounded by eight electrons, achieving a stable electronic arrangement.

• This rule is applicable in covalent bonding as seen in molecules like methane (CH₄).

There are several specific rules to assign oxidation numbers, which are crucial in understanding redox reactions and balancing chemical equations.
Some key rules include:

• The oxidation number of an atom in its elemental form is always zero (e.g., O₂, H₂).
• For ions composed of only one atom, the oxidation number equals the charge of the ion (e.g., Na⁺ = +1).
• In compounds, hydrogen is typically +1 and oxygen is -2, although there are exceptions.

Understanding these rules aids in predicting the reactivity of compounds and analyzing chemical reactions.

Chemical Equations

Chemical equations are representations of chemical reactions where the reactants are transformed into products. These equations show the conservation of mass, implying that the number of each type of atom is the same on both sides of the equation.

• This is known as balancing chemical equations.

To balance an equation, we adjust the coefficients (numbers before the compounds/molecules) to ensure the same number of each type of atom appears on both sides. This practice is essential for accurately describing chemical reactions and for quantitative analysis in chemistry.
Moreover, chemical equations may include symbols and notations that provide additional information.

• For example, (s) indicates a solid, (l) a liquid, (g) a gas, and (aq) an aqueous solution.
• Arrows symbolize the direction of the reaction, sometimes with double-headed arrows for reversible reactions.

Utilizing chemical equations effectively requires an understanding of stoichiometry, which helps us calculate the quantities of reactants and products involved. Mastery of these concepts enables chemists to predict reaction outcomes and optimize industrial chemical processes.