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Ionic and Molecular Compounds: Structure, Naming, and Properties

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  • Ionic and Molecular Compounds

Introduction

This chapter explores the formation, structure, and properties of ionic and molecular compounds, focusing on the transfer and sharing of electrons, the octet rule, and the classification and naming of compounds.

Transfer of Electrons and the Octet Rule

Formation of Bonds

  • Ionic bonds are formed when atoms lose or gain valence electrons to achieve a stable octet configuration, following the octet rule.

  • Ionic bonds occur when valence electrons from a metal are transferred to a nonmetal.

  • Covalent bonds form when nonmetal atoms share electrons to achieve an octet.

Formation of Ions

Loss of Electrons: Cations

  • Atoms with low ionization energy (typically metals) readily lose valence electrons, forming cations (positively charged ions).

  • Upon losing electrons, the atom attains the electron configuration of the nearest noble gas. Example: Sodium (Na) loses one electron to become Na+, with an electron arrangement of 2,8 (like Ne).

Gain of Electrons: Anions

  • Nonmetals have high ionization energy and tend to gain electrons, forming anions (negatively charged ions).

  • Gaining electrons allows nonmetals to achieve a noble gas configuration. Example: Chlorine (Cl) gains one electron to become Cl-, with an electron arrangement similar to Argon.

Predicting Charges Based on Group Number

Group Trends and Ion Formation

  • Group 1A elements lose 1 electron to form +1 cations.

  • Group 2A elements lose 2 electrons to form +2 cations.

  • Group 3A elements lose 3 electrons to form +3 cations.

  • Group 4A elements typically do not lose electrons, except Pb and Sn, which can lose 4.

  • Group 5A elements gain 3 electrons to form -3 anions.

Noble Gas

1A (1)

2A (2)

He

Li+

Ne

Na+

Mg2+

Ar

K+

Ca2+

Kr

Rb+

Sr2+

Xe

Cs+

Ba2+

Balancing Ionic Compounds

Stoichiometry and Charge Balance

  • Subscripts in chemical formulas indicate the number of each ion present in a compound.

  • For example, in NaCl, sodium gives up one electron to chlorine, resulting in a 1:1 ratio.

  • In MgCl2, magnesium gives up two electrons, each accepted by two chloride ions.

Ions with Variable Charges

Transition Metals and Multiple Oxidation States

  • Transition metals (Groups 3-12) can have variable charges, forming ions with different oxidation states.

  • Examples: Fe2+ (iron(II)), Fe3+ (iron(III)), Cu+ (copper(I)), Cu2+ (copper(II)).

  • Roman numerals are used to denote the charge in the compound's name.

Common Polyatomic Ions

Table of Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, carrying a net charge.

Name

Ion

Name

Ion

Ammonium

NH4+

Nitrite

NO2-

Hydroxide

OH-

Nitrate

NO3-

Cyanide

CN-

Hypochlorite

ClO-

Acetate

CH3COO-

Chlorite

ClO2-

Sulfite

SO32-

Chlorate

ClO3-

Sulfate

SO42-

Perchlorate

ClO4-

Hydrogen sulfate (bisulfate)

HSO4-

Carbonate

CO32-

Thiosulfate

S2O32-

Hydrogen carbonate (bicarbonate)

HCO3-

Phosphate

PO43-

Oxalate

C2O42-

Hydrogen phosphate

HPO42-

Dichromate

Cr2O72-

Dihydrogen phosphate

H2PO4-

Chromate

CrO42-

Thiocyanate

SCN-

Permanganate

MnO4-

Cyanate

OCN-

Naming and Writing Chemical Formulas

Classification of Compounds

  • Covalent compounds contain only nonmetals and are named using prefixes to indicate the number of each element.

  • Ionic compounds contain a metal and a nonmetal or a metal and a polyatomic ion. They are not named using prefixes.

  • Transition metal compounds are named with the cation charge in Roman numerals.

  • Acids contain H+ with either a nonmetal or a polyatomic ion.

Writing Formulas for Ionic Compounds

Charge Balance and Formula Construction

  • Ionic compounds always contain positive (cations) and negative (anions) ions.

  • The sum of the charges of the cations must equal the sum of the charges of the anions.

  • The chemical formula reflects the smallest whole-number ratio of ions that results in charge neutrality.

  • Example: For Al3+ and O2-, the formula is Al2O3 because 2(3+) + 3(2-) = 0.

Naming Ionic Compounds

Rules for Naming

  • Metal cations are named for the metal (e.g., Na+ is sodium, Mg2+ is magnesium).

  • Transition metals with variable charges use Roman numerals (e.g., Fe2+ is iron(II), Fe3+ is iron(III)).

  • Anions are named by taking the stem of the element and adding the suffix -ide (e.g., Cl- is chloride).

Additional info:

  • Polyatomic ions retain their specific names in compounds (e.g., NaNO3 is sodium nitrate).

  • For compounds with transition metals, always specify the charge using Roman numerals in the name.

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