Question

Write the formula for each ionic compound. a. copper(II) chloride b. copper(I) iodate c. lead(II) chromate d. calcium fluoride e. potassium hydroxide f. iron(II) phosphate

Short answer

a. CuCl2, b. CuIO3, c. PbCrO4, d. CaF2, e. KOH, f. Fe3(PO4)2

Step-by-step solution

Understanding Ionic Compound Formulas

An ionic compound formula consists of the cation (positive ion) and the anion (negative ion). The compound is electrically neutral, so the charges of the cations and anions must balance out. Use the Roman numeral in the metal's name to determine its charge.

Writing the Formula for Copper(II) Chloride

Copper(II) means Cu has a +2 charge, and chloride (Cl) has a -1 charge. To balance, you need two chloride ions for every copper ion. The formula is CuCl2.

Writing the Formula for Copper(I) Iodate

Copper(I) indicates Cu has a +1 charge, and the iodate (IO3) ion has a -1 charge. You need one iodate ion for every copper ion. The formula is CuIO3.

Writing the Formula for Lead(II) Chromate

Lead(II) suggests Pb has a +2 charge, and chromate (CrO4) has a -2 charge. You need one chromate ion for every lead ion. The formula is PbCrO4.

Writing the Formula for Calcium Fluoride

Calcium (Ca) has a +2 charge, and fluoride (F) has a -1 charge. Two fluoride ions are needed for every calcium ion. The formula is CaF2.

Writing the Formula for Potassium Hydroxide

Potassium (K) has a +1 charge, and hydroxide (OH) has a -1 charge. You need one hydroxide ion for every potassium ion. The formula is KOH.

Writing the Formula for Iron(II) Phosphate

Iron(II) indicates Fe has a +2 charge, and phosphate (PO4) has a -3 charge. To balance, combine three Fe2+ ions with two PO4-3 ions. The formula is Fe3(PO4)2.

Key concepts

Writing Chemical Formulas

Understanding how to write chemical formulas for ionic compounds is crucial in chemistry. It's like a language where you can convey a lot of information in a concise way. To start, identify the cation (the positive ion) and the anion (the negative ion) of the compound. Each cation and anion has a charge, and when they come together to form a compound, the total charge must equal zero. This means the number of positive charges must balance the number of negative charges.

For example, sodium ion (Na⁺) and chloride ion (Cl⁻) combine to form sodium chloride (NaCl) with a one-to-one ratio due to their charges. If the charges aren't the same, you must find the smallest ratio of ions that results in a net charge of zero. Use subscripts to show the number of ions needed for charge balance, always writing the cation first, followed by the anion.

Cation and Anion Balance

Achieving cation and anion balance in ionic compounds is the cornerstone of writing correct chemical formulas. The rule that the total charge must be neutral guides the process. When charges don't match, it's necessary to 'cross-over' the charges and use them as subscripts to balance the formula.

For instance, magnesium ion (Mg²⁺) and nitrate ion (NO₃⁻) combine as Mg(NO₃)₂, showing that two nitrate ions pair with one magnesium ion to achieve neutrality. Always make sure the subscripts are the smallest whole numbers possible and reduce them if they can be simplified. Remember, a polyatomic ion like nitrate must be enclosed in parentheses with its subscript outside if you need more than one of that ion.

Transition Metals with Roman Numerals

Transition metals often have multiple possible charges, making the use of Roman numerals in their names essential. The numeral indicates the charge of the metal in that compound. This information is vital for determining the correct formula.

Application in Formulas

For example, iron can have charges of +2 or +3, represented as iron(II) or iron(III), respectively. When creating the compound formula, this charge must be balanced by the corresponding number of negatively charged anions. In iron(III) oxide, where oxide has a charge of -2, the formula must reflect that each iron ion balances with one and a half (which is chemically impossible, so you double everything) oxide ions, resulting in Fe₂O₃ as the correct formula.

Polyatomic Ions

Polyatomic ions are charged particles consisting of two or more atoms bonded together. They act as a single unit when forming ionic compounds. Common examples include sulfate (SO₄²⁻), ammonium (NH₄⁺), nitrate (NO₃⁻), and phosphate (PO₄³⁻).

When writing formulas involving polyatomic ions, it's essential to use parentheses if more than one polyatomic ion is needed to balance the charges with another ion. For instance, aluminum sulfate requires two aluminum ions (Al³⁺) and three sulfate ions, leading to the formula Al₂(SO₄)₃ to achieve neutrality. Always remember to check for and include these groupings in your formulas to avoid any confusion.