Ions: Monatomic and Polyatomic

• Ions result from atoms or molecules gaining or losing electrons, leading to a net electrical charge.
• Monatomic ions are formed from a single atom.
• Polyatomic ions consist of two or more covalently bonded atoms with an overall charge.
• Memorizing common polyatomic and monatomic ions aids in predicting compound formulas and understanding chemical reactions.

Monatomic Ions

• Formed when a single atom either gains or loses electrons.
• Metals typically lose electrons to become positive ions, known as cations.
• Nonmetals typically gain electrons to become negative ions, known as anions.
• An element's position on the periodic table can often help predict the charge of its monatomic ion.

Common Monatomic Cations

• Group 1A elements, or alkali metals, form +1 ions, examples include $Na^+$ and $K^+$.
• Group 2A elements, or alkaline earth metals, form +2 ions, examples include $Mg^{2+}$ and $Ca^{2+}$.
• Aluminum commonly forms a +3 ion: $Al^{3+}$.
• Transition metals can form multiple cations with differing charges; examples include $Fe^{2+}$ and $Fe^{3+}$, but some common ones include $Ag^+$ and $Zn^{2+}$.

Common Monatomic Anions

• Group 7A elements, or halogens, form -1 ions, examples include $Cl^-$ and $Br^-$.
• Group 6A elements often form -2 ions, for example, $O^{2-}$, although sulfur can vary.
• Group 5A elements can form -3 ions: e.g., $N^{3-}$.
• Hydrogen can form $H^+$ or $H^-$ (hydride).

Polyatomic Ions

• These ions are composed of two or more atoms covalently bonded together and possess an overall charge.
• Knowing common polyatomic ions is critical because of their prevalence in chemical compounds.

Common Polyatomic Cations

• Ammonium: $NH_4^+$
• Hydronium: $H_3O^+$
• Mercury(I): $Hg_2^{2+}$

Common Polyatomic Anions

• Acetate: $C_2H_3O_2^-$ or $CH_3COO^-$
• Carbonate: $CO_3^{2-}$
• Hydrogen Carbonate (Bicarbonate): $HCO_3^-$
• Hypochlorite: $ClO^-$
• Chlorite: $ClO_2^-$
• Chlorate: $ClO_3^-$
• Perchlorate: $ClO_4^-$
• Chromate: $CrO_4^{2-}$
• Dichromate: $Cr_2O_7^{2-}$
• Cyanide: $CN^-$
• Hydroxide: $OH^-$
• Nitrate: $NO_3^-$
• Nitrite: $NO_2^-$
• Permanganate: $MnO_4^-$
• Phosphate: $PO_4^{3-}$
• Hydrogen Phosphate: $HPO_4^{2-}$
• Dihydrogen Phosphate: $H_2PO_4^-$
• Sulfate: $SO_4^{2-}$
• Hydrogen Sulfate (Bisulfate): $HSO_4^-$
• Sulfite: $SO_3^{2-}$
• Thiosulfate: $S_2O_3^{2-}$

Naming Ionic Compounds

• Begin by stating the cation, followed by the anion when naming ionic compounds.
• Monatomic cations are named the same as the element, e.g., $Na^+$ is named sodium.
• Monatomic anions are named by adding the suffix "-ide" to the root name of the element, e.g., $Cl^-$ is named chloride.
• Indicate the charge of a metal with multiple potential cations using Roman numerals in parentheses (e.g., $Fe^{2+}$ is iron(II), $Fe^{3+}$ is iron(III)).
• Polyatomic ions are named using their memorized name (e.g., $NH_4^+$ is ammonium, $SO_4^{2-}$ is sulfate).

Common Acids

• Acids release $H^+$ ions in water.
• Acid naming depends on the anion.
• For anions ending in "-ide", name the acid hydro- + root name of anion + -ic acid (e.g., $HCl$ is hydrochloric acid).
• For anions ending in "-ate", name the acid root name of anion + -ic acid (e.g., $H_2SO_4$ is sulfuric acid, from sulfate).
• For anions ending in "-ite", name the acid root name of anion + -ous acid (e.g., $HNO_2$ is nitrous acid, from nitrite).

Ions are atoms or molecules with a net electrical charge due to loss or gain of electrons. Monatomic ions are formed from a single atom, while polyatomic ions consist of covalently bonded atoms with an overall charge. Memorizing common ions is essential for predicting compound formulas.