Chemical Bonding and Compound Formation: Ionic, Covalent, and Mixtures

Elements on Opposite Sides of the Periodic Table: Tend to do opposite things chemically.
- Examples: A metal and a nonmetal will react by one losing electrons and the other gaining electrons.
Elements on the Same Side of the Periodic Table: Tend to do more of the same chemically.
- Two metals both want to lose electrons; they cannot both gain.
- Two nonmetals both want to gain electrons; they cannot both lose.

Compounds: Have a definite chemical formula, involving chemical bonds (gain/loss or sharing of electrons).
- We don't usually see metal compounds consisting only of metals or only of nonmetals in the way we see metal-nonmetal compounds.
Mixtures: Physical combinations where substances retain their individual properties.
- Alloys: Specific mixtures of metals, typically formed by melting metals down and mixing them together.
  - Examples:
    - Brass: A mixture of metals.
    - Bronze: Primarily copper, often mixed with tin. It may be less expensive than pure copper and desired for its color.
    - Pewter: A tin-based alloy.
    - Jewelry Gold: Pure gold (24 karat) is very soft. Lower karat gold (e.g., $10$ karat or $12$ karat) is mostly gold but contains other metals to provide stability and hardness. This is different from gold-plated items, where a thin layer of gold is applied and can rub off.
- In alloys, elements mix without gaining or losing electrons from each other because they both want to do the same thing (lose electrons).

Ionic Compounds: Formed between a metal and a nonmetal.
- Formation: Involves the transfer of electrons, leading to the formation of positive ions (cations, from metals) and negative ions (anions, from nonmetals).
- Characteristics: Involves a gain and a loss of electrons.
- Naming: Uses specific ionic rules, involves charges, and does not use covalent prefixes.
Covalent Compounds: Formed between two or more nonmetals.
- Formation: Involves the sharing of electrons because both nonmetals want to gain electrons but neither can lose.
- Characteristics: No ions present, no charges are used in the typical naming process (for binary covalent).
- Examples: Organic chemistry and biochemistry molecules are predominantly covalent.
- Binary Covalent Compounds: Covalent compounds containing only two different elements.
  - Octet Rule: Nonmetals obey the octet rule by sharing electrons. Predicting the exact number and source of shared electrons can be challenging (e.g., carbon needing $4$ electrons, oxygen needing $2$ ).

Identify Element Types: When given a compound name or formula, first determine if it contains:
- Two Nonmetals: It's a covalent compound.
  - No charges, no ions.
  - Do not use ionic rules.
  - Naming rules involve prefixes (e.g., mono, di, tri).
- A Metal with a Nonmetal: It's an ionic compound.
  - Ions are present, charges must be considered.
  - Use ionic rules.
  - Do not use covalent prefixes.

The difference in electronegativity dictates the type of bond formed.
General Trends for Tests: While specific electronegativity values can be used for precise classification, generally:
- Nonmetal with Nonmetal: Expected to be covalent (sharing).
- Metal with Nonmetal: Expected to be ionic (gain/loss).
Exceptions: A few elements might defy these general trends, but for exams, focus on the general rules unless specified.

Order: The positive ion (cation) always comes first in the name and formula.
Simple Ionic Compounds (Metal + Nonmetal):
1. Name the positive ion (metal).
2. Name the negative ion (nonmetal with an '-ide' ending).
3. The compound name is two words.
4. Do not use the word "ion" in the compound name.
- Example: Calcium phosphate, formula $Ca<em>3(PO</em>4)_2$ . (Note: The formula shows specific ratios, but the name doesn't initially indicate numbers).
Writing Formula from Name (Often Harder):
1. Know the symbols of the elements.
2. Determine the charges of the ions (from group number or Roman numeral).
3. Balance the charges to find the correct subscripts for the formula.
Polyatomic Ions (More Challenging):
- You must know the symbol, the number of oxygen atoms (if applicable), and crucially, the charge of the polyatomic ion.
- The name often does not tell you how many atoms are in the polyatomic ion nor its charge.
- Example: Determining subscripts for compounds involving polyatomic ions requires knowing their charges to balance with the cation.
- Visual Aids: Similar to flashcards used in lab for practice (e.g., sodium chloride ( $NaCl$ ) has a 1:1 ratio, but compounds with different charges will not always be 1:1).

Difficulty: Technically the hardest area of chemistry due to molecule size and complexity.
Nature of Molecules: These are large covalent molecules.
Naming: Often based on their discovery location in nature or their compound type, making formula learning more difficult.
Approach: We will start by recognizing certain groups and combinations rather than complex naming, especially for different biochemical types (e.g., carbohydrates).

Nonmetals obtain a stable electron configuration (octet) by sharing electrons.
Challenge in Prediction: While an element needs a specific number of electrons to complete its octet (e.g., carbon needs $4$ ), predicting how it will get them (which other atoms it will bond with, and how many of each) is more complex than for ionic compounds. There can be many different compounds for a small set of elements based on available atoms.