Structure of the Atom and Valence Electrons

Atomic Structure and Electron Cloud

The atom consists of a central nucleus containing protons and neutrons, surrounded by an electron cloud. The arrangement of electrons, especially the valence electrons, determines the chemical properties of an element.

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles occupying energy levels around the nucleus.

• Valence Electrons: Electrons in the highest energy level, crucial for chemical reactions.

Energy Levels are numbered (n = 1, 2, 3, ...), with higher numbers indicating greater energy and distance from the nucleus. Valence electrons are found in the outermost energy level.

Lewis Symbols

Lewis (electron-dot) symbols represent valence electrons as dots around the element symbol, with no more than two dots per side.

• Example: Calcium (Ca) has two dots, Carbon (C) has four, Sulfur (S) has six.

Ions and Ionic Compounds

Formation of Ions

Atoms gain or lose electrons to achieve a stable electron configuration, often an octet (eight valence electrons).

• Cations: Formed by metals losing electrons; positive charge.

• Anions: Formed by nonmetals gaining electrons; negative charge.

• Octet Rule: Atoms tend to gain, lose, or share electrons to reach eight in their valence shell (except H, Li, Be, which follow the duet rule).

Ion Charges and Naming

• Group 1A metals: +1 charge

• Group 2A metals: +2 charge

• Group 3A metals: +3 charge

• Group 5A, 6A, 7A nonmetals: -3, -2, -1 charges respectively

• Ion charges are shown as superscripts (e.g., Ca2+, O2−)

Ionic Compounds

Ionic compounds consist of cations and anions held together by ionic bonds. The formula shows the cation first, followed by the anion, with subscripts indicating the number of each ion to ensure a net charge of zero.

• High melting and boiling points

• Solid at room temperature

Writing and Naming Ionic Compounds

• Formula: Cation symbol (with charge) + Anion symbol (with charge)

• Balance charges by adjusting subscripts

• Names: Cation name (with Roman numeral if variable charge) + Anion name (root + "-ide" for monatomic anions)

Periodic Table and Predictable Charges

Some metal cations have predictable charges based on their group number. Others require Roman numerals to indicate their charge.

States of Matter and Changes of State

Solids, Liquids, and Gases

Matter exists in three primary states: solid, liquid, and gas. Each state has distinct properties regarding shape, volume, and particle arrangement.

• Solids: Definite shape and volume; particles in fixed arrangement, moving slowly.

• Liquids: Indefinite shape, definite volume; particles close together but mobile.

• Gases: Indefinite shape and volume; particles far apart, moving rapidly.

Temperature and Temperature Scales

Temperature measures the average kinetic energy of particles and determines the direction of heat flow. Common scales are Celsius (°C), Fahrenheit (°F), and Kelvin (K).

• Celsius: Water freezes at 0°C, boils at 100°C

• Fahrenheit: Water freezes at 32°F, boils at 212°F

• Kelvin: Absolute zero is 0 K; water freezes at 273 K, boils at 373 K

Temperature Conversion Formulas

• To convert Celsius to Fahrenheit:

• To convert Fahrenheit to Celsius:

• To convert Celsius to Kelvin:

Physical Changes and Phase Changes

Physical changes alter the state, shape, or size of a substance without changing its identity. Phase changes are physical changes that require or release heat.

• Endothermic: Melting, vaporization, sublimation (heat absorbed)

• Exothermic: Freezing, condensation, deposition (heat released)

Heat of Fusion and Heat of Vaporization

• Heat of Fusion: Amount of heat required to melt 1 g of a solid or released when 1 g of liquid freezes.

• For water: or

• Heat of Vaporization: Amount of heat required to vaporize 1 g of liquid or released when 1 g of gas condenses.

• For water: or

Chemical Changes and Reactions

Chemical Changes

Chemical changes involve the formation of new substances with different compositions and properties. Chemical reactions break old bonds and form new ones, rearranging atoms.

• Heat may be released (exothermic) or required (endothermic).

Examples of Physical vs. Chemical Changes

Polyatomic Ions

Definition and Examples

A polyatomic ion is a group of atoms bonded together with an overall ionic charge. Most are anions, except ammonium (NH4+).

• Examples: Nitrate (NO3−), Sulfate (SO42−), Phosphate (PO43−), Hydroxide (OH−), Ammonium (NH4+)

Writing Formulas with Polyatomic Ions

• Cation is written first, followed by the polyatomic ion.

• Parentheses are used if more than one polyatomic ion is present (e.g., Mg(NO3)2).

Naming Compounds with Polyatomic Ions

• Name the cation first, then the polyatomic anion.

• Examples: NaNO3 (sodium nitrate), K2SO4 (potassium sulfate), Fe(HCO3)3 (iron(III) bicarbonate)

Covalent (Molecular) Compounds

Covalent Bonds and Molecules

Covalent bonds form between two nonmetal atoms by sharing valence electrons to complete octets (or duets for hydrogen).

• Examples: H2, N2, CO2, SO42−

• Electron pairs not shared are called lone pairs.

Naming Covalent Compounds

• First nonmetal is named fully; second uses root + "-ide"

• Prefixes indicate the number of atoms: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-

• Example: SO3 is sulfur trioxide; P4S3 is tetraphosphorus trisulfide

Writing Covalent Compound Formulas

• List element symbols in order given by the name

• Use subscripts according to prefixes

• Do not reduce subscripts

Identifying Ionic vs. Covalent Compounds

• Ionic: Formula starts with a metal or NH4+

• Covalent: Formula starts with a nonmetal

Electronegativity and Types of Bonds

Electronegativity Trends

Electronegativity measures an atom's attraction for shared electrons. It increases across a period and decreases down a group. Nonmetals have high electronegativity; metals have low.

Bond Types Based on Electronegativity Difference

• Nonpolar Covalent: Difference ≤ 0.4; equal sharing

• Polar Covalent: Difference > 0.4 and ≤ 1.8; unequal sharing, partial charges (δ+ and δ−)

• Ionic: Difference > 1.8; electron transfer, full charges

Partial Charges: In polar covalent bonds, the more electronegative atom has a partial negative charge (δ−), and the less electronegative atom has a partial positive charge (δ+).