Physical and Chemical Changes, Ions, and Compounds

Introduction

This unit covers the fundamental concepts of ions, ionic and molecular compounds, and changes of state. Understanding these topics is essential for grasping the basics of General, Organic, and Biological (GOB) Chemistry.

Chemistry and Valence Electrons

Structure of the Atom

• Atoms consist of a nucleus (containing protons and neutrons) surrounded by electrons.

• Protons (p+): Positively charged particles in the nucleus.

• Neutrons (n0): Neutral particles in the nucleus.

• Electrons (e-): Negatively charged particles in orbitals around the nucleus.

The arrangement of electrons, especially the valence electrons (outermost electrons), determines the chemical properties of an element.

Valence Electrons and Energy Levels

• Valence electrons are found in the highest energy level (shell) and are farthest from the nucleus.

• They are responsible for chemical bonding and reactivity.

• The number of valence electrons can be determined from the group number for main group elements.

Electron Dot Symbols (Lewis Symbols)

• Valence electrons are represented as dots around the element symbol.

• Each dot represents one valence electron.

• Example: The Lewis symbol for sodium (Na) is Na·, for chlorine (Cl) is Cl·······.

Ions and Ionic Charges

Formation of Ions

• Cations: Positively charged ions formed when atoms lose electrons (usually metals).

• Anions: Negatively charged ions formed when atoms gain electrons (usually nonmetals).

• The charge of an ion is shown as a superscript after the element symbol (e.g., Na+, Cl-).

Predicting Ionic Charges

• Group 1A metals form +1 cations, Group 2A metals form +2 cations.

• Group 7A nonmetals form -1 anions, Group 6A nonmetals form -2 anions.

• The charge can often be predicted from the group number.

Octet Rule

• Atoms tend to gain, lose, or share electrons to achieve a full set of eight valence electrons (an octet), similar to noble gases.

• Helium is stable with two valence electrons (a duet).

Examples

• Na (sodium) loses one electron to form Na+.

• Cl (chlorine) gains one electron to form Cl-.

Ion Symbols and Names

Ion Symbols

• For cations, the ion symbol and Lewis dot symbol are the same (e.g., Na+).

• For anions, the ion symbol shows the element symbol and the gained electrons as dots (e.g., Cl- with 8 dots).

Naming Ions

• Cations: Named after the element (e.g., Mg2+ is magnesium ion).

• Anions: Named by taking the root of the element name and adding -ide (e.g., Cl- is chloride ion).

Charges with Roman Numerals

• Some metals can form more than one cation with different charges. The charge is indicated with a Roman numeral in parentheses (e.g., Fe2+ is iron(II) ion, Fe3+ is iron(III) ion).

Ionic Compounds

Properties of Ionic Compounds

• Contain cations and anions held together by electrostatic attraction (ionic bonds).

• Have high melting points and are usually solid at room temperature.

• Are electrically neutral (total positive charge equals total negative charge).

Formulas of Ionic Compounds

• The formula shows the simplest ratio of cations to anions.

• Subscripts indicate the number of each ion in the compound.

• Example: MgCl2 contains one Mg2+ and two Cl- ions.

Writing Ionic Compound Formulas

1. Write the symbol for the cation first, then the anion.

2. Balance the charges by adjusting subscripts as needed.

3. Do not show charges in the final formula.

Table: Metal Ions with Predictable Charges

Changes of State and Temperature Scales

States of Matter

• Solids: Definite shape and volume; particles are closely packed and vibrate slowly.

• Liquids: Definite volume but no definite shape; particles are close but can move past each other.

• Gases: No definite shape or volume; particles are far apart and move rapidly.

Three States of Matter for Water

• Ice (solid), liquid water, and water vapor (gas) are the three physical states of water.

Changes of State

• Melting: Solid to liquid

• Freezing: Liquid to solid

• Vaporization: Liquid to gas

• Condensation: Gas to liquid

• Sublimation: Solid to gas

• Deposition: Gas to solid

Temperature Scales

• Celsius (°C): Water freezes at 0°C and boils at 100°C.

• Fahrenheit (°F): Water freezes at 32°F and boils at 212°F.

• Kelvin (K): Absolute temperature scale; 0 K is absolute zero.

Conversion formulas:

Summary Table: Naming Ionic Compounds

Practice Problems and Learning Checks

• Identify the number of valence electrons for various elements.

• Predict the charge of ions formed by main group elements.

• Write formulas for ionic compounds given the ions or names.

• Name ions and ionic compounds using the rules provided.

Example: Writing Formulas for Ionic Compounds

• Magnesium and fluorine: Mg2+ and F- combine to form MgF2.

• Calcium and oxygen: Ca2+ and O2- combine to form CaO.

Example: Naming Ionic Compounds with Roman Numerals

• FeCl2: iron(II) chloride

• CuO: copper(II) oxide

Additional info: These notes are based on standard GOB Chemistry curriculum and include expanded explanations and examples for clarity and completeness.