lec 23 ionic and molecular compounds

Bonds

Definition of a Bond
- A bond is defined as a strong force that holds atoms together in a compound or a molecule.
- It pertains to the attractive forces between atoms that form compounds.

Nature of Ionic Bonds
- Electron Transfer: Ionic bonds occur through the transfer of electrons between atoms.
- Example:
  - Atom A transfers an electron to Atom B.
  - After transfer:
  - Atom A becomes positively charged due to loss of an electron.
  - Atom B becomes negatively charged due to gaining an electron.
- Principle: Opposite charges attract, so the positively charged ion (cation) and the negatively charged ion (anion) are attracted to each other like magnets.

Nature of Covalent Bonds
- Covalent bonds involve the sharing of electrons between two atoms.
- Example: Two atoms each share one of their electrons to form a bond.
- Emphasis: Both ionic and covalent bonds revolve around the behavior of electrons.

Definition: Ionic compounds are formed from the transfer of electrons, consisting of cations (positively charged ions) and anions (negatively charged ions).

Metals:
- Tend to lose electrons and form cations.
- Example: Sodium (Na) loses one electron to become Na⁺.
Nonmetals:
- Tend to gain electrons and form anions.
- Example: Chlorine (Cl) gains an electron to become Cl⁻.
Remembering Cation and Anion:
- Mnemonic: A cation can be remembered by associating it with a 'cat' (cations are positive).

Ionic compounds form between metals and nonmetals.
- Recognizing Compounds: If a compound consists of a metal and a nonmetal, it is classified as ionic.
- Example 1: Sodium Chloride (NaCl)
Step 1: Identify the ions formed.
- Sodium (Na): Na⁺
- Chlorine (Cl): Cl⁻
Step 2: Combine to form a neutral compound.
- Na⁺ + Cl⁻ → NaCl (Neutral compound)
- Example 2: Calcium Chloride (CaCl₂)
Step 1: Identify the ions formed.
- Calcium (Ca): Ca²⁺
- Chlorine (Cl): Cl⁻
Step 2: Balance the charges to form a neutral compound.
- Need two Cl⁻ to balance Ca²⁺: Ca²⁺ + 2Cl⁻ → CaCl₂

Ionic compounds can also include polyatomic ions.
Example: Calcium Phosphate
Step 1: Identify the charges.
- Calcium (Ca): Ca²⁺
- Phosphate (PO₄): PO₄³⁻
Step 2: Balance the charges.
- Requires multiple ions to neutralize:
  - 3 Ca²⁺ (6+) + 2 PO₄³⁻ (6-) → Ca₃(PO₄)₂
- Note: Parentheses denote multiple units of the polyatomic ion.

Crisscross Method
- For ionic compounds, to determine the quantities from the charges, you may crisscross the magnitude of the charge from one ion to become the subscript of the other.
- Example: Ca²⁺ and PO₄³⁻ become Ca₃(PO₄)₂ after applying the crisscross method.

General Steps:
1. Split the compound into its constituent ions.
2. Identify charge of each ion using the periodic table.
3. Not all methods work universally; verify charges for compounds with transition metals or polyatomic ions.

Step 1: Identify ions.
- Lithium (Li): Li⁺ (one plus)
- Oxygen (O): O²⁻ (two minus)

The positively charged ion is typically monatomic unless it's ammonium.
Example: Potassium Carbonate (K₂CO₃)
- Potassium (K): K⁺
- Carbonate (CO₃): CO₃²⁻
- This results in a neutral compound since K⁺ and CO₃²⁻ balance each other.

Ionic compounds are neutral and must balance charges.
Formula indicates ratios of cations and anions.
Properties:
- Typically form solid structures.
- Exhibit high melting and boiling points (often referred to as salts).
- Are nonconductive in solid form but conductive in molten form.

Definition: Formed through covalent bonds.
Occur between nonmetals or metalloids.
Characterized by sharing of electrons rather than transfer.
Often exist as gases, low boiling liquids, or low melting solids.
Example: Water (H₂O) and ethanol.
Contrast with Ionic Compounds:
- Ionic compounds involve transfer and are formed between metals and nonmetals, while molecular compounds involve sharing and occur typically between nonmetals.

Future topics will include naming ionic compounds and further exploration of related chemistry concepts.