Atomic Structure

Atoms and Isotopes

The atom is the basic unit of matter, consisting of a nucleus (protons and neutrons) surrounded by 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 (1 proton), Deuterium (1 proton, 1 neutron), Tritium (1 proton, 2 neutrons).

• Ions: Atoms that have gained or lost electrons. - Cations: Positively charged (lost electrons). - Anions: Negatively charged (gained electrons).

Example: Proton (H+), Hydride (H-).

Electron Configuration

Principles and Notation

Electron configuration describes the arrangement of electrons in an atom's orbitals.

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

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

• Hund's Rule: Electrons occupy degenerate orbitals singly before pairing.

Notation: Use the format 1s2 2s2 2p6 etc.

• Condensed Electron Configuration: Start at the last noble gas before the element.

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

Periodic Table Blocks

The periodic table is divided into s-, p-, d-, and f-blocks, corresponding to the type of orbital being filled.

Electronegativity

Definition and Trends

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: In Group 7A, Cl is more electronegative than Br or I.

Octet Rule

Valence Electrons and Shared Electrons

The octet rule states that most main group elements tend to achieve eight electrons in their valence shell through chemical bonding.

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

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

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

Formal Charge

Calculation and Application

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

• Formula:

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

• The sum of formal charges equals the overall charge of the molecule or ion.

Example: Calculate formal charges for N, C, and S in the thiocyanate ion (NCS-).

Lewis Dot Structures

Drawing Rules

Lewis Dot Structures represent the arrangement of valence electrons in molecules.

1. Count total valence electrons.

2. Place the least electronegative atom in the center (except H and halogens).

3. Add electrons to outer atoms to complete octets.

4. Place remaining electrons on the central atom.

5. If atoms lack octets, form double or triple bonds.

6. Check formal charges for correctness.

Example: Draw the Lewis structure for COCl2.

Resonance Structures

Definition and Representation

Resonance structures are two or more valid Lewis structures for a molecule or ion that differ only in the placement of electrons.

• Movement of electrons occurs in pi bonds or lone pairs.

• Double-sided arrows indicate resonance between structures.

• The real structure is a resonance hybrid, a composite of all resonance forms.

Example: Draw all resonance structures for the nitrate ion, NO3-.

Hybridization

Electron Groups and Hybrid Orbitals

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

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

Example: HCN has sp hybridization.

Molecular Polarity

Polar and Nonpolar Molecules

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

• Nonpolar Molecule: Hydrocarbons or molecules with perfect symmetry and no lone pairs on the central atom.

• Polar Molecule: Molecules with an asymmetric shape or lone pairs on the central atom.

Example: NF3 is polar due to lone pairs on the central atom.

Functional Groups

Identification and Importance

Functional groups are specific groups of atoms within molecules responsible for characteristic chemical reactions.

• Hydrocarbons: Alkanes, alkenes, alkynes, aromatic rings.

• With Carbonyls: Aldehyde, ketone, acid chloride, amide, carboxylic acid, ester.

• Without Carbonyls: Alkyl halide, amine, alcohol, ether, thiol.

Example: Alcohols contain an -OH group; ketones contain a C=O group.

Organic Chemistry Overview

Definition and Applications

Organic chemistry is the study of molecules containing carbon, often found in biological systems.

• Organic Molecule: Contains both carbon and hydrogen.

• Hydrocarbon: Contains only carbon and hydrogen.

Example: Ethanol (C2H5OH) is organic; benzene (C6H6) is a hydrocarbon.

Summary Table: Principles of Electron Configuration

Additional info: Some content was expanded for clarity and completeness, including examples and definitions for key terms.