Naming Ionic and Covalent Compounds

Naming Ionic and Covalent Compounds

High School

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Learning Objectives

• Distinguish between ionic and covalent compounds based on their composition and properties.

• Write chemical formulas for ionic compounds with polyatomic ions and transition metals.

• Correctly name ionic and covalent compounds from their chemical formulas.

• Write chemical formulas for covalent compounds using the appropriate Greek prefixes.

Key Vocabulary

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Cation

A positively charged ion formed when a metal atom loses one or more of its electrons.

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Anion

A negatively charged ion that is formed when a non-metal atom gains one or more electrons.

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Ionic Bonds

A chemical bond formed between a metal and a nonmetal through the transfer of electrons.

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Oxidation Numbers

The charge on an ion, representing the number of electrons an atom has gained or lost.

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Polyatomic Ion

A group of bonded atoms that acts as a single unit with an overall ionic charge.

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Ionic vs. Covalent Compounds

Ionic Compounds

• Ionic compounds form between metal and nonmetal atoms.

• Electrons are completely transferred from the metal to the nonmetal.

• They have high melting points and are typically solids.

Covalent Compounds

• Covalent compounds form when two nonmetal atoms bond together.

• Electrons are shared between atoms, resulting in no charges.

• They have low melting points and exist as solids, liquids, or gases.

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Solved Example 1
What is the correct chemical formula for an ionic compound formed between aluminum, which forms a +3 ion, and oxygen, which forms a -2 ion?

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Step 1: Analyze and Sketch the Problem

• Goal: To write the neutral chemical formula for a compound made of aluminum and oxygen.

• Knowns: The aluminum ion has a charge of +3 (Al³⁺). The oxygen ion (oxide) has a charge of -2 (O^2-).

• Unknown: The subscripts for each element in the chemical formula.

• Formula: The total positive charge must balance the total negative charge to equal zero.

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Solved Example 1
What is the correct chemical formula for an ionic compound formed between aluminum, which forms a +3 ion, and oxygen, which forms a -2 ion?

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Step 2: Solve for the Unknown

• Use the criss-cross method, where the magnitude of the charge of each ion becomes the subscript for the other ion.

• The '3' from Al³⁺ crosses over to become the subscript for O.

• The '2' from O^2- crosses over to become the subscript for Al.

• This gives the formula: Al₂O₃.

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Solved Example 1
What is the correct chemical formula for an ionic compound formed between aluminum, which forms a +3 ion, and oxygen, which forms a -2 ion?

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Step 3: Evaluate the Answer

• The total positive charge from two Al³⁺ ions is 2 × (+3) = +6, and the total negative charge from three O^2- ions is 3 × (-2) = -6.

• The sum of the charges is (+6) + (-6) = 0, which means the compound is neutral and the formula Al₂O₃ is correct.

Multiple Choice

A newly discovered solid substance has a very high melting point and is formed between a metal and a nonmetal. What type of compound is it most likely to be?

• Polar

• Ionic

• Covalent

• Metallic

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Fundamentals of Ionic Compounds

Forming Ions

• Cations are positive ions that form when metal atoms lose one or more of their electrons.

• Anions are negative ions that form when nonmetal atoms gain one or more electrons from other atoms.

• An ionic bond is the strong electrostatic attraction that occurs between a cation and an anion.

Predicting Ion Charge

• The oxidation number is the charge on an ion after it has gained or lost electrons.

• Elements in Group 1 have a ⁺¹ charge, while elements in Group 2 have a ⁺² charge.

• Elements in Group 15 have a ^-3 charge, while elements in Group 17 have a ^-1 charge.

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Solved Example 2
What is the correct chemical formula for an ionic compound formed between magnesium (Mg), a group 2 element, and nitrogen (N), a group 15 element?

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Step 1: Analyze and Sketch the Problem

• Goal: To find the chemical formula for the ionic compound made of magnesium and nitrogen.

• Knowns: Magnesium (Mg) is in Group 2 and forms a cation with a +2 charge (Mg²⁺). Nitrogen (N) is in Group 15 and forms an anion with a -3 charge (N^3-).

• Unknown: The subscripts for Mg and N in the chemical formula.

• Formula: The total charge of an ionic compound must be zero. The criss-cross method can be used to find the subscripts.

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Solved Example 2
What is the correct chemical formula for an ionic compound formed between magnesium (Mg), a group 2 element, and nitrogen (N), a group 15 element?

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Step 2: Solve for the Unknown

• Using the criss-cross method, the numerical value of the charge of the cation (Mg²⁺) becomes the subscript for the anion (N).

• The numerical value of the charge of the anion (N^3-) becomes the subscript for the cation (Mg), resulting in the final formula Mg₃N₂.

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Solved Example 2
What is the correct chemical formula for an ionic compound formed between magnesium (Mg), a group 2 element, and nitrogen (N), a group 15 element?

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Step 3: Evaluate the Answer

Multiple Choice

Based on its position in the periodic table, what is the oxidation number for an element in Group 2?

• +1

• +2

• -2

• -1

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Writing Formulas for Ionic Compounds

Balancing Charges Method

• The goal is to write a formula that results in a net charge of zero.

• For Mg²⁺ and Cl^-, you need two Cl^- ions to balance the +2 charge.

• This gives the final formula MgCl₂, where the charges are perfectly balanced.

Criss-Cross Rule

• The charge of each ion becomes the subscript for the other, like X^m+ and Y^n- gives X_nY_m.

• Always reduce the subscripts to the simplest whole-number ratio for the correct final formula.

• Use parentheses for polyatomic ions with a subscript greater than 1, like in Ca(OH)₂.

Multiple Choice

Using the criss-cross rule, what is the correct formula for a compound formed between Calcium (Ca²⁺) and the Nitrate ion (NO₃^-)?

• Ca(NO₃)₂

• Ca(NO₃)₃

• CaNO₃

• Ca₂(NO₃)

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Naming Ionic Compounds

• Follow this three-step process for naming simple ionic compounds.

• ​First, always name the cation (positive ion) by its full element name.

• Next, name the anion, changing the ending to –ide for single elements.

• For polyatomic anions like SO₄^2-, use the ion’s specific name, such as sulfate.

Multiple Choice

What is the correct name for the ionic compound with the formula Na₂S?

• Sodium sulfide

• Disodium sulfide

• Sodium(II) sulfide

• Sodium sulfate

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Ionic Compounds with Transition Metals

• Transition metals can form ions with different charges, like Fe²⁺ and Fe³⁺.

• ​A Roman numeral in the name is used to specify the charge.

• Iron (II) chloride indicates an Fe²⁺ ion, giving the formula FeCl₂.

• From FeCl₂, two Cl^- ions mean the iron must be Fe²⁺.

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Solved Example 5
The compound CuO consists of copper and oxygen. Given that the charge of an oxide ion (O) is -2, what is the chemical name for CuO?

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the correct chemical name for the ionic compound CuO.

• Knowns: The compound contains one copper (Cu) ion and one oxide (O) ion. The charge of the oxide ion is -2. The overall charge of the compound is 0.

• Unknown: The charge of the copper ion.

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Solved Example 5
The compound CuO consists of copper and oxygen. Given that the charge of an oxide ion (O) is -2, what is the chemical name for CuO?

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Step 2: Solve for the Unknown

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Solved Example 5
The compound CuO consists of copper and oxygen. Given that the charge of an oxide ion (O) is -2, what is the chemical name for CuO?

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Step 3: Evaluate the Answer

Multiple Choice

What is the correct name for the compound CuO, given that Copper (Cu) is a transition metal and Oxide (O) has a -2 charge?

• Copper (II) Oxide

• Monocopper Monoxide

• Copper (I) Oxide

• Copper Oxide

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Rules for Naming Covalent Compounds

• Use Greek prefixes like mono-, di-, and tri- for the number of atoms.

• Name the first element; use a prefix only if there is more than one atom.

• Name the second element with a prefix and change its ending to -ide.

• For example, N₂O₅ is dinitrogen pentoxide and SiF₄ is silicon tetrafluoride.

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Solved Example 6
What is the chemical formula for the covalent compound sulfur hexafluoride?

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Step 1: Analyze and Sketch the Problem

• Goal: Write the chemical formula for sulfur hexafluoride.

• Knowns: The compound is covalent, containing sulfur (S) and fluoride (F). The prefix "hexa-" means 6.

• Unknown: The subscripts for S and F in the chemical formula.

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Solved Example 6
What is the chemical formula for the covalent compound sulfur hexafluoride?

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Step 2: Solve for the Unknown

• Identify the symbol for each element: Sulfur is S, and fluoride refers to fluorine (F).

• Determine the subscript for each element from the prefixes. "Sulfur" has no prefix, so its subscript is 1. "Hexafluoride" means there are 6 fluorine atoms, so its subscript is 6.

• Combine the symbols and subscripts to write the formula: SF₆.

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Solved Example 6
What is the chemical formula for the covalent compound sulfur hexafluoride?

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Step 3: Evaluate the Answer

• The name "sulfur hexafluoride" indicates one sulfur atom and six fluorine atoms, and the formula SF₆ correctly represents this.

• The answer is reasonable and correctly follows the naming rules for covalent compounds.

Multiple Choice

What is the correct name for the covalent compound PBr₅?

• Phosphorus bromide

• Phosphorus(V) bromide

• Monophosphorus pentabromide

• Phosphorus pentabromide

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Common Misconceptions

Misconception	Correction
All compounds are named using prefixes.	Prefixes are only for covalent compounds, not ionic ones.
The charge of a transition metal is always the same.	Transition metals can have multiple charges, shown by a Roman numeral.
Polyatomic ion names are changed to end in ‘-ide’.	The names of polyatomic ions are kept the same.

Multiple Choice

Based on the rules for ionic compounds, what would be the correct chemical formula for the compound formed between Aluminum (Al³⁺) and the Sulfate ion (SO₄^2-)?

• Al₃(SO₄)₂

• AlSO₄

• Al(SO₄)

• Al₂(SO₄)₃

Multiple Choice

Explain why the compound CO₂ is named carbon dioxide, while the compound CaCl₂ is named calcium chloride, focusing on the differences in their bonding and naming conventions.

• CO₂ is ionic and uses prefixes; CaCl₂ is covalent and balances charges.

• Both are covalent, but CaCl₂ is a special exception.

• CO₂ is covalent and uses prefixes; CaCl₂ is ionic and balances charges.

• Both are ionic, but CO₂ uses prefixes for clarity.

Multiple Choice

A compound has the formula XF₂, where X is an unknown metal. Given that Fluoride (F) has a -1 charge, what would be the predicted chemical formula for the compound formed between metal X and the Carbonate ion (CO₃^2-)?

• XCO₃

• X₂CO₃

• X(CO₃)₂

• X₂(CO₃)₃

Multiple Choice

Analyze the name 'Iron (III) Oxide'. From this name, determine the type of bonding, the charge of the iron ion, the charge of the oxide ion, and deduce the correct chemical formula.

• Covalent, Fe³⁺, O^2-, Formula: Fe₂O₃

• Ionic, Fe²⁺, O^2-, Formula: FeO

• Ionic, Fe³⁺, O^2-, Formula: FeO

• Ionic, Fe³⁺, O^2-, Formula: Fe₂O₃

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Summary

• Ionic compounds are a metal and nonmetal; use the criss-cross rule for formulas.

• Covalent compounds are two nonmetals; use Greek prefixes to show atom counts.

• Use Roman numerals for transition metal charges; polyatomic ions act as single units.

• For binary compounds, change the second element's ending to -ide.

Poll

On a scale of 1-4, how confident are you with naming and writing formulas for ionic and covalent compounds?

• 1

• 2

• 3

• 4