Atomic Structure and Electron Configuration

Atomic Number, Mass Number, and Isotopes

The atom is the fundamental unit of matter. Each atom is characterized by its atomic number and mass number.

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

• Mass Number (A): The total number 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).

Ions

• Cation: Atom with more protons than electrons (positively charged).

• Anion: Atom with more electrons than protons (negatively charged).

Example: A hydrogen ion (proton) is H+; a hydride ion is H-.

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 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.

Example: The electron configuration of phosphorus (Z = 15): Ground state: Condensed:

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.

Example: In Group 7A, fluorine (F) is the most electronegative element.

Octet Rule and Valence Electrons

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.

Example: In H3COH, oxygen has 6 valence electrons and forms 2 shared (bonding) pairs, achieving an octet.

Formal Charge

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

• Formula:

• Only allowable formal charges: -1, 0, +1.

• The sum of all formal charges in a molecule equals the overall charge.

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 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.

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

Hybridization

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

• Electron Groups: Number of atoms bonded to the central atom plus lone pairs.

Example: HCN has a linear geometry and sp hybridization.

Molecular Polarity

Molecular polarity arises from the distribution of electron density in a molecule.

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

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

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

Functional Groups in Organic Chemistry

Functional Group: A specific group of atoms within a molecule responsible for characteristic chemical reactions.

• Examples: Alkane, Alkene, Alkyne, Benzene, Alcohol, Ether, Amine, Aldehyde, Ketone, Carboxylic Acid, Ester, Amide, Acid Chloride, Thiol.

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

Organic Molecules and Hydrocarbons

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

• Hydrocarbon: Molecule containing only carbon and hydrogen.

• Organic molecules may also contain oxygen, nitrogen, sulfur, etc.

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

Summary Table: Principles of Electron Configuration

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