Atomic Structure and Electron Configuration

Atomic Structure

The atom is the fundamental unit of matter, composed of protons, neutrons, and electrons. Understanding atomic structure is essential for predicting chemical behavior.

• 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).

• Ions: Atoms or molecules that have gained or lost electrons, resulting in a net charge.

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

Electron Configuration

Electron configuration describes the distribution of electrons in atomic orbitals. It follows specific principles:

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

• Pauli Exclusion Principle: No two electrons in an 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 occupy degenerate orbitals singly before pairing up.

Condensed Electron Configuration: Uses the previous noble gas to simplify notation.

Example: Phosphorus (Z = 15): Ground state: 1s2 2s2 2p6 3s2 3p3 Condensed: [Ne] 3s2 3p3

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).

Example: Among Group 7A elements, Cl is more electronegative than Br or I.

Chemical Bonding and the Octet Rule

Octet Rule

The octet rule states that main group elements tend to form bonds until they are surrounded by eight valence electrons, achieving a noble gas configuration.

• 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.

• Only allowable formal charges are -1, 0, or +1 for most main group elements.

Example: For the thiocyanate ion (NCS-), calculate the formal charge for each atom using the formula above.

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. Complete octets for outer atoms, then for the central atom.

5. If octets are incomplete, form double or triple bonds as needed.

6. Check formal charges to ensure the best structure.

Example: Draw the Lewis structure for COCl2.

Resonance Structures

Some molecules have more than one valid Lewis structure, called resonance structures. The actual molecule is a resonance hybrid of these forms.

• Resonance: Delocalization of electrons across multiple atoms or bonds.

• Double-sided arrows (↔): Indicate resonance between structures.

• Resonance Hybrid: The true structure, a blend of all resonance forms.

Example: Nitrate ion (NO3-) has three resonance structures, each with a different N=O double bond.

Hybridization and Molecular Geometry

Hybridization

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

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

Example: HCN has 2 electron groups, so the central atom is sp hybridized.

Molecular Polarity

Molecular polarity depends on the shape and distribution of charge in a molecule.

• Nonpolar Molecule: Has a symmetrical (perfect) shape and no net dipole moment.

• Polar Molecule: Has an asymmetrical shape or uneven distribution of electrons, resulting in a net dipole moment.

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

Organic Chemistry Basics

Functional Groups

Functional Group: 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, thiols, alkyl halides.

Example: Alcohols contain an -OH group; carboxylic acids contain a -COOH group.

Organic Molecules and Hydrocarbons

Organic molecules contain both carbon and hydrogen. Hydrocarbons are organic molecules made solely of carbon and hydrogen.

• Example: Methane (CH4) is a hydrocarbon; ethanol (C2H5OH) is organic but not a hydrocarbon.

Summary Table: Principles of Electron Configuration

Additional info: These notes cover foundational concepts in atomic structure, periodic trends, bonding, molecular geometry, and basic organic chemistry, suitable for a General Chemistry college course.