chapter four: molecules & compounds

What are the two major types of chemical bonds?

Ionic bonds and covalent bonds.

What is an ionic bond?

The attraction between oppositely charged ions, usually formed when electrons are transferred.

What is a covalent bond?

A bond in which atoms share electrons.

What is a molecular compound?

A compound made of nonmetals joined by covalent bonds.

What is an ionic compound?

A compound made of positive and negative ions arranged in a lattice.

What is a cation?

A positively charged ion.

What is an anion?

A negatively charged ion.

What is a formula unit?

The simplest whole-number ratio of ions in an ionic compound.

What is a molecule?

A group of atoms held together by covalent bonds.

What is the difference between a molecular formula and an empirical formula?

The molecular formula gives the actual number of each atom; the empirical formula gives the simplest whole-number ratio.

What are valence electrons?

Electrons in an atom's outermost occupied energy level that are involved in bonding.

What is a Lewis symbol?

An element symbol surrounded by dots representing its valence electrons.

What is the octet rule?

Atoms tend to gain, lose, or share electrons to achieve 8 valence electrons.

What is a bonding pair?

A pair of electrons shared between two atoms.

What is a lone pair?

A pair of valence electrons that is not used in a bond.

What does a single bond represent?

One shared pair of electrons.

What does a double bond represent?

Two shared pairs of electrons.

What does a triple bond represent?

Three shared pairs of electrons.

Which seven elements naturally occur as diatomic molecules?

H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂.

Why does N₂ have a triple bond?

Each nitrogen needs three more electrons for an octet, so the atoms share three pairs of electrons.

What is an ionic lattice?

A repeating three-dimensional arrangement of positive and negative ions.

Why do ionic compounds have high melting points?

Strong attractions between ions require a lot of energy to overcome.

Why don't solid ionic compounds conduct electricity well?

Their ions are locked in place and cannot move.

How do you write the formula of an ionic compound?

Write the ions and their charges, then choose subscripts so the total positive and negative charges equal zero.

What is charge balance?

The total positive charge and total negative charge in an ionic compound must cancel to zero.

What is a variable-charge metal?

A metal that can form ions with different charges.

Why are Roman numerals used in names such as iron(III) chloride?

The Roman numeral tells you the charge of the metal ion.

How do you name a binary ionic compound?

Name the metal first, then the nonmetal with an -ide ending.

How do you name an ionic compound containing a polyatomic ion?

Name the cation followed by the name of the polyatomic ion.

What is a polyatomic ion?

A group of covalently bonded atoms that has an overall charge.

What are the formulas and charges of the most important polyatomic ions?

Acetate C₂H₃O₂⁻; nitrate NO₃⁻; nitrite NO₂⁻; sulfate SO₄²⁻; sulfite SO₃²⁻; phosphate PO₄³⁻; hydroxide OH⁻; carbonate CO₃²⁻; chromate CrO₄²⁻; permanganate MnO₄⁻; perchlorate ClO₄⁻; ammonium NH₄⁺.

What is the difference between -ate and -ite oxyanions?

For the common pair, -ate has more oxygen and -ite has fewer oxygen atoms.

What do the prefixes hypo- and per- mean for oxyanions?

Hypo- means fewer oxygen atoms than -ite; per- means more oxygen atoms than -ate.

Why are parentheses used around a polyatomic ion in a formula?

They show that the subscript applies to the entire polyatomic ion.

What is the formula for calcium sulfide?

CaS.

What is the formula for strontium iodide?

SrI₂.

What is the name of SrI₂?

Strontium iodide.

What is the formula for manganese(IV) oxide?

MnO₂.

What is the name of Pb(C₂H₃O₂)₂?

Lead(II) acetate.

What is the name of P₂I₄?

Diphosphorus tetraiodide.

What is the correct Lewis symbol for sulfur?

Sulfur has 6 valence-electron dots around S.

What is a hydrate?

An ionic compound containing water molecules within its crystal structure.

How is a hydrate written?

The ionic compound is followed by a dot and the number of water molecules, such as CuSO₄·5H₂O.

How do you name a binary molecular compound?

Use prefixes to indicate the number of atoms and change the second element to an -ide ending.

What are the molecular prefixes 1–10?

Mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.

When is mono- usually omitted?

It is usually omitted for the first element in a binary molecular compound.

What is the name of CO?

Carbon monoxide.

What is the name of NI₃?

Nitrogen triiodide.

What is the name of SiCl₄?

Silicon tetrachloride.

What is the name of N₄Se₄?

Tetranitrogen tetraselenide.

What is the formula for phosphorus trichloride?

PCl₃.

What is the formula for dichlorine monoxide?

Cl₂O.

What is the name of BrF₃?

Bromine trifluoride.

What is the formula for nitrogen trifluoride?

NF₃.

What is the difference between naming ionic and molecular compounds?

Ionic compounds use ion names and may require Roman numerals; molecular compounds use prefixes to show atom numbers.

What is formula mass?

The sum of the atomic masses of all atoms represented in a chemical formula.

What is molar mass of a compound?

The mass of one mole of the compound in g/mol.

What is the representative particle of an ionic compound?

A formula unit.

What is the representative particle of a molecular compound?

A molecule.

What is the basic pathway for converting grams to molecules?

Grams → moles → molecules.

What is the basic pathway for converting molecules to grams?

Molecules → moles → grams.

How many CH₂Cl₂ molecules are in 25.0 g CH₂Cl₂?

1.77 × 10²³ molecules.

What is mass percent composition?

The percentage of a compound's total mass contributed by a particular element.

How do you calculate mass percent of an element?

Mass percent = (mass of the element in 1 mole of compound ÷ molar mass of compound) × 100%.

In CF₂Cl₂, what is the order of the elements from greatest to smallest mass percent?

Cl > F > C.

How do you find the mass of an element in a given mass of compound?

Find the element's mass fraction and multiply it by the total sample mass.

What mass of potassium is in 35.5 g KBr?

17.4 g K.

What is a mole ratio within a compound?

The ratio of the numbers of different atoms shown by the subscripts in the chemical formula.

What is the empirical formula of C₁₀H₈?

C₅H₄.

How do you find an empirical formula from experimental masses?

Convert each element's mass to moles, divide by the smallest mole value, and convert the ratios to the smallest whole numbers.

How do you find an empirical formula from percentages?

Treat the percentages as grams in a 100-g sample, convert to moles, divide by the smallest value, and obtain whole-number ratios.

What should you do if an empirical-formula ratio is not a whole number?

Multiply all ratios by a small whole number until they become whole numbers.

What is the empirical formula of a compound containing 52.14% C, 13.13% H, and 34.73% O?

C₂H₆O.

How do you find a molecular formula from an empirical formula?

Divide the molecular molar mass by the empirical-formula mass, then multiply every empirical-formula subscript by that whole-number multiplier.

A compound has empirical formula CH₂O and formula mass 120.10 amu. What is its molecular formula?

C₄H₈O₄.

What is combustion analysis?

A method for determining a compound's elemental composition by analyzing the products formed when it burns.

In combustion analysis, what does CO₂ tell you?

The amount of carbon in the original compound.

In combustion analysis, what does H₂O tell you?

The amount of hydrogen in the original compound.

How can oxygen be determined in a C/H/O compound after combustion?

Find the masses of carbon and hydrogen and subtract them from the original sample mass; the remainder is oxygen.

What is the empirical formula for the combustion problem with 30.42 g sample, 35.21 g CO₂, and 14.42 g H₂O?

C₂H₄O₃.

What is an organic compound?

A compound primarily based on carbon; organic compounds commonly contain carbon and hydrogen and may also contain elements such as O, N, or S.

What is a hydrocarbon?

A compound containing only carbon and hydrogen.

What makes carbon especially important in organic chemistry?

Carbon forms four bonds and can bond to other carbon atoms, allowing chains, branches, rings, double bonds, and triple bonds.

Is C₄H₈ a hydrocarbon?

Yes.

Is C₄H₆O₆ a hydrocarbon?

No, because it contains oxygen.

Is H₃C–CH₂OH a hydrocarbon?

No, because it contains oxygen.

Is H₃C–CH₃ a hydrocarbon?

Yes.

Which substance is an ionic compound: SrI₂, N₂O₄, He, or CCl₄?

SrI₂.

What is the correct formula for the compound formed between calcium and sulfur?

CaS.

What is the formula for manganese(IV) oxide?

MnO₂.

What is the name of Pb(C₂H₃O₂)₂?

Lead(II) acetate.

What is the name of P₂I₄?

Diphosphorus tetraiodide.

How many CH₂Cl₂ molecules are in 25.0 g CH₂Cl₂?

1.77 × 10²³ molecules.

What is the empirical formula of a compound that is 52.14% C, 13.13% H, and 34.73% O?

C₂H₆O.

What is the molecular formula when the empirical formula is CH₂O and the formula mass is 120.10 amu?

C₄H₈O₄.

What is the empirical formula from combustion of 30.42 g of a compound producing 35.21 g CO₂ and 14.42 g H₂O?

C₂H₄O₃.

What is the formula for potassium chromate?

K₂CrO₄.

What is the formula for lead(II) phosphate?

Pb₃(PO₄)₂.

What is the formula for cobalt(II) bromide?

CoBr₂.

What is the formula for phosphorus pentachloride?

PCl₅.