CHE-103 CHAPTER 3

General, Organic, and Biological Chemistry: Ionic Compounds and Chemical Bonding

Fundamental Electronic Structure Review

• The chemical identity, valence electron count, and total electron count of a neutral atom can be determined directly from its ground-state electronic configuration:

  • $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2$:

  • Total number of electrons: $38$

  • Number of valence electrons: $2$ (from the outermost $5s^2$ orbital)

  • Identity of element: Strontium ($Sr$

  • $1s^2 2s^2 2p^6 3s^2 3p^4$:

  • Total number of electrons: $16$

  • Number of valence electrons: $6$ (from the outermost shell orbitals $3s^2 3p^4$

  • Identity of element: Sulfur ($S$

  • $1s^2 2s^2 2p^6 3s^1$:

  • Total number of electrons: $11$

  • Number of valence electrons: $1$ (from the outermost $3s^1$ orbital)

  • Identity of element: Sodium ($Na$

  • $[Ne] 3s^2 3p^5$:

  • Total number of electrons: $17$ ($10$ core electrons from neon plus $7$ valence electrons)

  • Number of valence electrons: $7$ (from the outermost shell orbitals $3s^2 3p^5$

  • Identity of element: Chlorine ($Cl$

• Analysis of the phosphorus ($P$) atom ($Z = 15$, atomic mass $\approx 31$):

  • Number of neutrons: $16$ (calculated as $\text{mass number} - \text{atomic number} = 31 - 15 = 16$

  • Total occupied electron shells: $3$ shells ($n = 1, 2, 3$

  • Full electron configuration: $1s^2 2s^2 2p^6 3s^2 3p^3$

  • Noble gas notation: $[Ne] 3s^2 3p^3$

  • Number of valence electrons: $5$ (from the highest principal energy level, $n = 3$

Introduction to Chemical Bonding

• Chemical bonding represents the joining of two atoms in a stable arrangement.

• Driving force of bonding: Elements gain, lose, or share electrons in order to attain the electron configuration of the noble gas closest to them in the periodic table (a stable valence octet or duet).

• Primary classifications of chemical bonds:

  • Ionic Bonds: Result from the complete transfer of electrons from one atom to another. These typically form between:

  • A metal located on the left side of the periodic table.

  • A nonmetal located on the right side of the periodic table.

  • Covalent Bonds: Result from the sharing of electrons between two atoms. These form when:

  • Two nonmetals combine with each other.

  • A metalloid bonds to a nonmetal.

• A molecule is defined as a discrete compound or element containing two or more atoms bound together through covalent bonds.

Ions and the Octet Rule

• Ions are charged species formed when an atom gains or loses electrons, resulting in an unequal number of protons and electrons.

• Ionic compounds consist of oppositely charged ions held together by strong electrostatic attractions.

Cations and Anions

• Cations: Positively charged ions containing fewer electrons than protons.

  • Formed when neutral metal atoms lose one, two, or three electrons to achieve a completely filled outer shell of electrons.

  • Sodium Cation Formation:

  • Neutral sodium atom ($Na$): $11$ protons, $11$ electrons ($1s^2 2s^2 2p^6 3s^1$

  • Loss of $1$ electron yields the sodium cation ($Na^+$): $11$ protons, $10$ electrons ($1s^2 2s^2 2p^6$

• Magnesium Cation Formation:

  • Neutral magnesium atom ($Mg$): $12$ protons, $12$ electrons ($1s^2 2s^2 2p^6 3s^2$

  • Loss of $2$ electrons yields the magnesium cation ($Mg^{2+}$): $12$ protons, $10$ electrons ($1s^2 2s^2 2p^6$

• Anions: Negatively charged ions containing more electrons than protons.

  • Formed when neutral nonmetal atoms gain one, two, or three electrons to obtain a completely filled outer electron shell.

  • Chlorine Anion Formation:

  • Neutral chlorine atom ($Cl$): $17$ protons, $17$ electrons ($1s^2 2s^2 2p^6 3s^2 3p^5$

  • Gain of $1$ electron yields the chloride anion ($Cl^-$): $17$ protons, $18$ electrons ($1s^2 2s^2 2p^6 3s^2 3p^6$

• The Octet Rule: Main group elements undergo chemical reactions to achieve maximum stability by acquiring an octet ($8$ valence electrons) in their valence shell, matching the electronic configuration of a noble gas.

Predicting Ionic Charges from Group Numbers

• For main group metals (Groups 1A–3A), neutral atoms lose valence electrons to form cations:

  • $\text{Cation Charge} = +\text{Group Number}$

  • Group 1A metals ($Li, Na, K, Rb, Cs$): Have $1$ valence electron and lose $1 e^-$ to form $+1$ cations ($M^+$).

• Group 2A metals ($Be, Mg, Ca, Sr, Ba$): Have $2$ valence electrons and lose $2 e^-$ to form $+2$ cations ($M^{2+}$).

• Group 3A metals ($Al, Ga, In, Tl$): Have $3$ valence electrons and lose $3 e^-$ to form $+3$ cations ($M^{3+}$).

• For main group nonmetals (Groups 5A–7A), neutral atoms gain electrons to form anions:

  • $\text{Anion Charge} = -(8 - \text{Group Number})$

  • Group 5A nonmetals ($N, P$): Have $5$ valence electrons and gain $3 e^-$ to form $-3$ anions ($N^{3-}, P^{3-}$).

  • Group 6A nonmetals ($O, S, Se$): Have $6$ valence electrons and gain $2 e^-$ to form $-2$ anions ($X^{2-}$ where $X = O, S, Se$).

• Group 7A nonmetals / Halogens ($F, Cl, Br, I$): Have $7$ valence electrons and gain $1 e^-$ to form $-1$ anions ($X^-$ where $X = F, Cl, Br, I$).

Transition Metals and Metals with Variable Charge

• Transition metals and larger p-block metals (Group 4A) can lose variable numbers of electrons, forming cations with different positive charges:

  • Chromium: Chromium(II) ($Cr^{2+}$), Chromium(III) ($Cr^{3+}$)

  • Manganese: Manganese(II) ($Mn^{2+}$)

  • Iron: Iron(II) ($Fe^{2+}$), Iron(III) ($Fe^{3+}$)

  • Cobalt: Cobalt(II) ($Co^{2+}$), Cobalt(III) ($Co^{3+}$)

  • Nickel: Nickel(II) ($Ni^{2+}$)

  • Copper: Copper(I) ($Cu^+$), Copper(II) ($Cu^{2+}$)

  • Zinc: Zinc ($Zn^{2+}$)

  • Silver: Silver ($Ag^+$)

  • Cadmium: Cadmium ($Cd^{2+}$)

  • Tin: Tin(II) ($Sn^{2+}$), Tin(IV) ($Sn^{4+}$)

  • Gold: Gold(I) ($Au^+$), Gold(III) ($Au^{3+}$)

  • Mercury: Mercury(I) ($Hg_2^{2+}$), Mercury(II) ($Hg^{2+}$)

  • Lead: Lead(II) ($Pb^{2+}$), Lead(IV) ($Pb^{4+}$)

Biological Functions of Essential Ions

• Specific cations and anions perform critical biological roles throughout human organs and biological fluids:

  • Calcium ($Ca^{2+}$): Structural component found abundantly in teeth and bones.

  • Sodium ($Na^+$) and Potassium ($K^+$): Major biological cations present in all cellular and extracellular body fluids.

  • Iron ($Fe^{2+}$): Essential active component present in the hemoglobin of red blood cells for oxygen transport.

  • Chloride ($Cl^-$): Major biological anion present in gastric juices (as $HCl$) of the stomach and other physiological fluids.

  • Magnesium ($Mg^{2+}$): Enzymatic cofactor required for nerve impulse transmission and muscle contraction control.

Ionic Compounds and Formula Writing

• An ionic bond is created through the full transfer of one or more valence electrons from a metallic element to a nonmetallic element.

• Principle of Electrical Neutrality: The total positive charge of all cations must exactly balance the total negative charge of all anions, yielding a net charge of zero for any ionic compound.

Proportions and Charge Neutrality Examples

• Sodium Chloride ($NaCl$):

  • Contains $1$ $Na^+$ ion ($+1$ charge) and $1$ $Cl^-$ ion ($-1$ charge).

  • Charge sum: $+1 + (-1) = 0$

• Calcium Fluoride ($CaF_2$):

  • Contains $1$ $Ca^{2+}$ ion ($+2$ charge) and $2$ $F^-$ ions ($-1$ charge each).

  • Charge sum: $+2 + 2(-1) = 0$

• Lithium Oxide ($Li_2O$):

  • Contains $2$ $Li^+$ ions ($+1$ charge each) and $1$ $O^{2-}$ ion ($-2$ charge).

  • Charge sum: $2(+1) + (-2) = 0$

• Barium Iodide ($BaI_2$):

  • Contains $1$ $Ba^{2+}$ ion ($+2$ charge) and $2$ $I^-$ ions ($-1$ charge each).

  • Charge sum: $+2 + 2(-1) = 0$

• Aluminum Oxide ($Al_2O_3$):

  • Contains $2$ $Al^{3+}$ ions ($+3$ charge each) and $3$ $O^{2-}$ ions ($-2$ charge each).

  • Charge sum: $2(+3) + 3(-2) = +6 - 6 = 0$

Step-by-Step Method to Write Ionic Chemical Formulas

• Step 1: Identify Cation and Anion Charges

  • Determine which element is the metal (cation) and which is the nonmetal (anion).

  • Use the main group number to assign ionic charges ($+1, +2, +3$ for metals; $-3, -2, -1$ for nonmetals).

• Step 2: Balance Charges to Zero

  • Determine the stoichiometric ratio of cations to anions needed to achieve a overall net charge of zero.

  • When cation and anion charges possess equal magnitudes ($+1$ and $-1$, or $+2$ and $-2$), exactly one of each ion is required:

  • $K^+ + Cl^- \rightarrow KCl$

  • $Ca^{2+} + O^{2-} \rightarrow CaO$

  • When charges have different magnitudes, use the charge values as cross-subscripts to establish net neutrality:

  • $Ca^{2+} + 2 Cl^- \rightarrow CaCl_2$

• Step 3: Write the Formula

  • Write the cation symbol first, followed by the anion symbol.

  • Omit ionic charges in the final formula.

  • Place numerical subscripts to specify the ratio of each ion present. If a subscript is $1$, it is omitted.