Question

Write the formula for each of the following: a. magnesium chloride b. sodium sulfide c. copper(I) oxide d. zinc phosphide e. gold(III) nitride f. chromium(II) chloride

Short answer

a. MgCl\textsubscript{2}, b. Na\textsubscript{2}S, c. Cu\textsubscript{2}O, d. Zn\textsubscript{3}P\textsubscript{2}, e. AuN, f. CrCl\textsubscript{2}

Step-by-step solution

Identify the elements and their charges

Determine the elements involved and their oxidation states. Metals typically form positive ions (cations) and non-metals form negative ions (anions).

Write the charges of the ions

Using periodic table information or knowledge of ionic compounds, write down the charges of the cations and anions. For instance, magnesium (Mg) typically has a +2 charge, while chloride (Cl) has a -1 charge.

Balance the charges to get the formula

Balance the total positive and negative charges to get a neutral compound. For example, for magnesium chloride, you need two chloride ions to balance one magnesium ion: \(\text{Mg}^{2+} + 2\text{Cl}^{-} = \text{MgCl}_2\).

Repeat for each compound

Apply the same process to determine the formulas for the other compounds listed.

Key concepts

ionic compounds

Ionic compounds are formed through the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). This bond creates a neutral compound.
These compounds usually consist of a metal and a non-metal.
For example, magnesium chloride (MgCl2) is an ionic compound where magnesium (a metal) donates electrons and forms a cation (Mg²⁺), while chlorine (a non-metal) gains electrons to form an anion (Cl^-).
The strength of ionic bonds contributes to the high melting and boiling points of ionic compounds.
Such compounds also tend to dissolve in water, creating solutions that conduct electricity due to the movement of ions.

oxidation states

Oxidation states (or oxidation numbers) indicate the degree of oxidation or reduction of an element in a compound.
For ionic compounds, the oxidation state corresponds to the charge of the ion. For example, sodium (Na) has an oxidation state of +1, while sulfide (S) in sodium sulfide (Na2S) has an oxidation state of -2.
Determining oxidation states helps in predicting how elements combine to form compounds. It also aids in understanding the formula of the compound. For example, in copper(I) oxide (Cu2O), copper has an oxidation state of +1, which means each copper ion (Cu⁺) combines with a single oxygen ion (O^2-), resulting in two copper atoms for every oxygen atom.

Common oxidation states to remember include:

• Alkali metals (Group 1) always have an oxidation state of +1.
• Alkaline earth metals (Group 2) always have an oxidation state of +2.
• Halogens (Group 17) usually have an oxidation state of -1.

charge balancing

Charge balancing is crucial when forming neutral ionic compounds.
The total positive charge must equal the total negative charge to achieve a neutral compound. For instance, when combining Mg²⁺ and Cl^-, the formula for magnesium chloride becomes MgCl2, since two chloride ions are needed to balance the charge of one magnesium ion.
To balance charges:

• Identify the charges of each ion involved.
• Determine the ratio of ions needed to balance overall charge.
• Write the formula that represents that ratio. For example, in zinc phosphide (Zn3P2), zinc has a +2 charge (Zn²⁺), and phosphide has a -3 charge (P^3-). Balancing these, three zinc ions are required for every two phosphide ions, making the formula Zn3P2.

chemical formulas

Chemical formulas represent the composition of a compound.
They provide the ratio and types of atoms involved in the compound. In ionic compounds, this includes both cations and anions.
To write a chemical formula, first identify the elements and their oxidation states. Then balance the ions to ensure a neutral charge.
For example:

• Magnesium chloride (MgCl2): Magnesium (Mg) has a +2 charge, and chloride (Cl) has a -1 charge. The formula is MgCl2.
• Sodium sulfide (Na2S): Sodium (Na) has a +1 charge, and sulfide (S) has a -2 charge. The formula is Na2S.
• Copper(I) oxide (Cu2O): Copper (Cu) has a +1 charge, and oxide (O) has a -2 charge. The formula is Cu2O.
• Zinc phosphide (Zn3P2): Zinc (Zn) has a +2 charge, and phosphide (P) has a -3 charge. The formula is Zn3P2.
• Gold(III) nitride (AuN): Gold (Au) has a +3 charge, and nitride (N) has a -3 charge. The formula is AuN.
• Chromium(II) chloride (CrCl2): Chromium (Cr) has a +2 charge, and chloride (Cl) has a -1 charge. The formula is CrCl2.

Always verify the charge balance when writing the chemical formula to ensure the final compound is neutral.