Atomic Structure and Electron Configuration

Atomic Number, Mass Number, and Isotopes

The atom is the fundamental unit of matter, composed of protons, neutrons, and electrons.

• Atomic Number (Z): The number of protons in the nucleus of an atom. Determines the element's identity.

• Mass Number (A): The sum of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element (same Z) with different numbers of neutrons (different A).

Example: Hydrogen has three isotopes: protium (1H), deuterium (2H), and tritium (3H).

Subatomic Particles and Ions

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles in orbitals around the nucleus.

• Ions: Atoms with unequal numbers of protons and electrons.

  • Cations: Positively charged (fewer electrons than protons).

  • Anions: Negatively charged (more electrons than protons).

Example: The hydride ion (H-) has one proton and two electrons.

Electron Configuration Principles

Electrons occupy orbitals according to three main principles:

• Aufbau Principle: Electrons fill the lowest energy orbitals first.

• Pauli Exclusion Principle: No two electrons in the same atom can have the same set of four quantum numbers; each orbital holds a maximum of two electrons with opposite spins.

• Hund's Rule: Electrons fill degenerate (equal energy) orbitals singly before pairing up.

Electron Configuration Notation: Lists the occupied orbitals and the number of electrons in each (e.g., 1s2 2s2 2p6).

Condensed Electron Configuration: Uses the previous noble gas in brackets to simplify notation (e.g., [Ne] 3s2 3p3 for phosphorus).

Periodic Trends

Electronegativity

Electronegativity (EN): A measure of an atom's ability to attract electrons in a chemical bond.

• Periodic Trend: Electronegativity increases from left to right across a period and increases going up a group.

• Most Electronegative Element: Fluorine (F).

Bonding and the Octet Rule

Octet Rule

Most main group elements tend to achieve eight valence electrons (an octet) through chemical bonding.

• Valence Electrons: Electrons in the outermost shell, involved in bonding.

• Shared Electrons: Electrons shared between atoms in a covalent bond.

• Octet Calculation: Octet electrons = valence electrons + shared electrons.

Example: In H3COH, oxygen has 6 valence electrons and 2 shared electrons, totaling 8 octet electrons.

Formal Charge

Formal charge helps determine the most stable Lewis structure for a molecule.

• Formula:

• Sum of formal charges in a molecule equals the overall charge.

• Acceptable formal charges are typically -1, 0, or +1.

Lewis Dot Structures

Lewis structures represent the arrangement of valence electrons among atoms in a molecule.

1. Count total valence electrons.

2. Place the least electronegative atom in the center (except hydrogen).

3. Connect atoms with single bonds.

4. Distribute remaining electrons to complete octets (except for hydrogen, which only needs 2 electrons).

5. Use double or triple bonds if necessary to satisfy the octet rule.

6. Check formal charges to ensure the best structure.

Example: Drawing the Lewis structure for COCl2.

Resonance Structures

Some molecules have more than one valid Lewis structure, called resonance structures.

• Resonance structures differ only in the placement of electrons, not atoms.

• Double-headed arrows (↔) indicate resonance.

• The actual structure is a resonance hybrid, a blend of all resonance forms.

Example: The nitrate ion (NO3-) has three resonance structures.

Hybridization and Molecular Geometry

Hybridization

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals for bonding.

• Electron Groups: Number of atoms and lone pairs around a central atom.

Example: HCN has a linear geometry and sp hybridization.

Molecular Polarity

Polarity of Molecules

Molecular polarity arises from the distribution of electron density and molecular shape.

• Nonpolar Molecule: Has a symmetrical shape and even charge distribution.

• Polar Molecule: Has an asymmetrical shape or uneven charge distribution.

• Perfect Shape: Central atom has no lone pairs and all surrounding atoms are identical.

Example: Nitrogen trifluoride (NF3) is polar due to its lone pair on nitrogen.

Functional Groups and Organic Molecules

Functional Groups

A functional group is a specific group of atoms within a molecule responsible for characteristic chemical reactions.

• Hydrocarbons: Alkanes, alkenes, alkynes, and aromatic compounds (benzene).

• With Carbonyls: Aldehydes, ketones, carboxylic acids, esters, amides, acid chlorides.

• Without Carbonyls: Alcohols, ethers, amines, alkyl halides, thiols.

Organic Molecules

Organic chemistry studies molecules containing carbon and hydrogen, often with other elements.

• Organic Molecule: Contains both carbon and hydrogen.

• Hydrocarbon: Contains only carbon and hydrogen.

Example: Identifying organic and hydrocarbon molecules from a set of structures.

Summary Table: Key Concepts

Additional info: Some explanations and examples were expanded for clarity and completeness, including the summary table and explicit definitions of key terms.