chapter 2 lec 2

Atoms, Molecules, and Ions

Ions

An ion is an atom or a group of atoms that carries a positive or negative charge due to the loss or gain of electrons. The number of protons in the nucleus remains unchanged, determining the element's identity.

• Cations: Positively charged ions formed by the loss of electrons. Examples include Cu+ (copper(I)), Cu2+ (copper(II)), Pb2+ (lead(II)), Pb4+ (lead(IV)), Cd2+ (cadmium), Fe2+ (iron(II)), Fe3+ (iron(III)), Mn2+ (manganese(II)).

• Anions: Negatively charged ions formed by the gain of electrons. Examples include F- (fluoride), O2- (oxide), N3- (nitride), Br- (bromide).

Monatomic ions are classified by their group in the periodic table:

Group 4A	Group 5A	Group 6A	Group 7A
Carbide (C4-)	Nitride (N3-)	Oxide (O2-)	Fluoride (F-)
Silicide (Si4-)	Phosphide (P3-)	Sulfide (S2-)	Chloride (Cl-)
		Selenide (Se2-)	Bromide (Br-)
			Iodide (I-)

Example: Calcium (Ca) in Group 2A has 20 protons and 20 electrons (neutral atom). It loses 2 electrons to form Ca2+ (calcium ion), which has 20 protons and 18 electrons.

Example: Sulfur (S) in Group 6A has 16 protons and 16 electrons (neutral atom). It gains 2 electrons to form S2- (sulfide ion), which has 16 protons and 18 electrons.

Ionic compounds are formed from cations and anions, resulting in a neutral compound. For example, CaS is formed from Ca2+ and S2-, and its total charge is zero.

• Examples of ionic compounds: sodium phosphide, barium carbide, barium iodide, barium oxide, barium nitride, copper(I) fluoride, copper(II) fluoride, iron(II) oxide, iron(III) oxide, iron(II) bromide, lead(IV) selenide.

Polyatomic Ions

Polyatomic ions consist of two or more atoms covalently bonded, carrying a net charge. Most are anions, but some are cations. These ions remain intact during chemical reactions.

Name	Formula/Charge	Name	Formula/Charge
Ammonium	NH4+	Hydronium	H3O+
Mercury(I)	Hg22+	Mercury(II)	Hg2+
Azide	N3-	Perchlorate	ClO4-
Chlorate	ClO3-	Chlorite	ClO2-
Hypochlorite	ClO-	Cyanide	CN-
Dichromate	Cr2O72-	Dihydrogen Phosphate	H2PO4-
Hydrogen Carbonate	HCO3-	Hydrogen Phosphate	HPO42-
Carbonate	CO32-	Hydroxide	OH-
Chromate	CrO42-	Nitrate	NO3-
Nitrite	NO2-	Oxalate	C2O42-
Hydrogen Sulphate	HSO4-	Permanganate	MnO4-
Peroxide	O22-	Phosphate	PO43-
Phosphite	PO33-	Sulfate	SO42-
Sulfite	SO32-	Thiocyanate	SCN-

Note: Polyatomic ions must be memorized as they are commonly used in naming compounds.

Molecular and Ionic Compounds

Molecular Formulas

Molecular formulas represent the exact number of atoms of each element in a molecule. Examples include Cl2, CH4, CCl4, CHCl3, CH2Cl2, NH3, C6H12O3.

Empirical Formulas

Empirical formulas show the simplest whole-number ratio of atoms in a compound. For example, hydrogen peroxide has a molecular formula of H2O2 and an empirical formula of HO. Hydrazine is N2H4 (molecular), NH2 (empirical).

• For many compounds, the empirical and molecular formulas are identical (e.g., H2O, CO, CO2, C8H18, NH3).

• Some compounds have different empirical and molecular formulas, determined experimentally.

Example: CH3O2 is the empirical formula, but the molecular formula is C3H9O6.

Naming Oxoanions and Oxoacids

Oxoacids contain hydrogen, oxygen, and another central atom. Examples include:

• H2CO3 (carbonic acid)

• HClO3 (chloric acid)

• HNO3 (nitric acid)

• H3PO4 (phosphoric acid)

• H2SO4 (sulfuric acid)

Rules for naming oxoacids and their anions:

1. "Per" prefix: An acid with one more oxygen than the base acid uses "per". E.g., HClO4 is perchloric acid (one more O than HClO3).

2. "-ous" suffix: An acid with one less oxygen than the base acid changes "-ic" to "-ous". E.g., H2SO3 is sulfurous acid, HNO2 is nitrous acid.

3. "Hypo" prefix: Removing two O atoms from the "-ic" acid adds "hypo" to the "-ous" acid. E.g., HClO is hypochlorous acid.

Rules for naming oxoanion forms:

1. Removing all H+ from an "-ic" acid gives an anion ending in "-ate". E.g., PO43- is phosphate (from H3PO4).

2. Removing all H+ from an "-ous" acid gives an anion ending in "-ite". E.g., ClO2- is chlorite (from HClO2).

3. Anions with remaining H+ indicate the number of hydrogens. E.g., H2PO4- is dihydrogen phosphate.

Prefix/Suffix	Anion Example	Acid Example
hypo...-ite	ClO- (hypochlorite)	HClO (hypochlorous acid)
...-ite	ClO2- (chlorite)	HClO2 (chlorous acid)
...-ate	ClO3- (chlorate)	HClO3 (chloric acid)
per...-ate	ClO4- (perchlorate)	HClO4 (perchloric acid)

Similar rules apply for bromine and iodine oxoacids and oxoanions.

• NO2- = nitrite, HNO2 = nitrous acid

• NO3- = nitrate, HNO3 = nitric acid

• SO32- = sulfite, H2SO3 = sulfurous acid

• SO42- = sulfate, H2SO4 = sulfuric acid

Polyprotic Acids and Their Ions

Polyprotic acids contain more than one ionizable hydrogen atom. Their anions are named according to the number of hydrogens remaining.

Ion/Acid	Formula	Name
Carbonate	CO32-	carbonate ion
Hydrogencarbonate	HCO3-	hydrogencarbonate ion
Carbonic acid	H2CO3	carbonic acid
Sulfite	SO32-	sulfite ion
Hydrogensulfite	HSO3-	hydrogensulfite ion
Sulfurous acid	H2SO3	sulfurous acid
Sulfate	SO42-	sulfate ion
Hydrogensulfate	HSO4-	hydrogensulfate ion
Sulfuric acid	H2SO4	sulfuric acid
Phosphite	PO33-	phosphite ion
Hydrogenphosphite	HPO32-	hydrogenphosphite ion
Dihydrogenphosphite	H2PO3-	dihydrogenphosphite ion
Phosphorous acid	H3PO3	phosphorous acid
Phosphate	PO43-	phosphate ion
Hydrogenphosphate	HPO42-	hydrogenphosphate ion
Dihydrogenphosphate	H2PO4-	dihydrogenphosphate ion
Phosphoric acid	H3PO4	phosphoric acid

Key Formulas and Equations

• Charge of an ion: $\text{Charge} = \text{Number of protons} - \text{Number of electrons}$

• Empirical formula: $\text{Empirical formula} = \text{Simplest whole-number ratio of atoms}$

• Molecular formula: $\text{Molecular formula} = \text{Actual number of atoms in a molecule}$

Summary Table: Naming Oxoanions and Oxoacids

Acid	Anion	Suffix/Prefix
HClO	ClO-	hypochlorous/hypochlorite
HClO2	ClO2-	chlorous/chlorite
HClO3	ClO3-	chloric/chlorate
HClO4	ClO4-	perchloric/perchlorate

Additional info: The rules for naming oxoacids and oxoanions apply similarly to other elements such as bromine and iodine, following the same pattern of prefixes and suffixes.