Atomic Structure

The Atom and Isotopes

The atom is the fundamental unit of matter, composed of protons, neutrons, and electrons. Each element is defined by its atomic number, which equals the number of protons in its nucleus. The mass number is the sum of protons and neutrons.

• Atomic Number (Z): Number of protons in the nucleus.

• Mass Number (A): Number of protons plus neutrons.

• Isotopes: Atoms of the same element with different numbers of neutrons.

Example: Hydrogen has three isotopes: Hydrogen (1 proton), Deuterium (1 proton, 1 neutron), and Tritium (1 proton, 2 neutrons).

• Ion: An atom or molecule with a net electric charge due to the loss or gain of electrons.

• Cation: Positively charged atom (loss of electrons).

• Anion: Negatively charged atom (gain of electrons).

Example: Proton (H+), Hydride (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 fill degenerate orbitals singly before pairing.

Example: Electron configuration diagrams illustrate these principles.

Electron Configuration

Ground State Electron Configuration

The ground state electron configuration describes the distribution of electrons among the atom's orbitals using the Aufbau Principle. Electrons fill from lower to higher energy orbitals.

• Aufbau Principle: Start from 1s, fill orbitals in order of increasing energy.

• Condensed Electron Configuration: Use the previous noble gas to abbreviate the configuration.

Example: Phosphorus (Z = 15): Ground state: Condensed:

Periodic Table Blocks

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

• s-block: Groups 1A and 2A

• p-block: Groups 3A to 8A

• d-block: Transition metals

• f-block: Lanthanides and actinides

Electronegativity

Definition and Periodic Trend

Electronegativity (EN) is a measure of an atom's ability to attract electrons in a chemical bond. It varies across the periodic table.

• 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.

Octet Rule

Valence Electrons and Shared Electrons

The Octet Rule states that atoms tend to gain, lose, or share electrons to achieve eight valence electrons, similar to noble gases.

• 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

Calculating 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 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 Lewis Dot Structures

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

1. Determine total valence electrons.

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

3. Add electrons to surrounding atoms to satisfy the octet rule.

4. Place remaining electrons on the central atom.

5. If atoms lack an octet, form double or triple bonds.

6. Check formal charges to confirm the best structure.

Example: Draw the Lewis Dot Structure for COCl2.

Resonance Structures

Definition and Representation

Resonance structures are two or more valid Lewis structures for a molecule 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 actual 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 Hybridization Types

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

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

Example: HCN has sp hybridization and 2 unhybridized p orbitals.

Molecular Polarity

Polar and Nonpolar Molecules

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

• Nonpolar Molecule: Molecule with a symmetric shape and no lone pairs on the central atom.

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

Example: Nitrogen trifluoride (NF3) is polar due to lone pairs on the central atom.

Functional Groups

Definition and Classification

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

• Hydrocarbons: Alkanes, alkenes, alkynes, aromatic compounds

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

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

Example: Identify functional groups in organic molecules and classify them as hydrocarbons or containing carbonyls.

Organic Chemistry Overview

Definition and Applications

Organic chemistry is the study of molecules containing carbon and hydrogen, often found in biological systems. Organic molecules may also contain other elements such as oxygen, nitrogen, sulfur, and halogens.

• Organic Molecule: Contains both carbon and hydrogen.

• Hydrocarbon: Contains only carbon and hydrogen.

Example: Identifying organic molecules and hydrocarbons from chemical structures and product labels.

Additional info:

• Some content inferred for completeness, such as the most electronegative element (Fluorine) and the condensed electron configuration example.

• Tables reconstructed for hybridization, molecular polarity, and functional groups for clarity.