Atomic Structure

Atoms and Isotopes

The atom is the fundamental 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: Hydrogen (1 proton), Deuterium (1 proton, 1 neutron), 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 ion (loss of electrons).

• Anion: Negatively charged ion (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 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

Describes the distribution of electrons among the atom's orbitals in its lowest energy state.

• Aufbau Principle: Start from 1s, fill lower energy orbitals before higher energy ones.

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

Example: Phosphorus (Z = 15): Ground state: Condensed: [Ne]

Electronegativity

Definition and Periodic Trend

Electronegativity (EN): The ability of an atom 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 and I.

Octet Rule

Valence and Shared Electrons

The tendency of main group elements 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.

• Octet Rule: Atoms tend to form bonds until they are surrounded by eight valence electrons.

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 Rules

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

1. Determine total number of valence electrons.

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

3. Add electrons to complete octets (except H, which only needs 2).

4. Place remaining electrons on the central atom.

5. If octet is not achieved, form double or triple bonds.

6. Check formal charges to confirm the best structure.

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 true structure is a resonance hybrid, a composite of all resonance forms.

Example: Draw all resonance structures for 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 the central atom.

• Types:

  • 2 groups: Linear, sp

  • 3 groups: Trigonal planar, sp2

  • 4 groups: Tetrahedral, sp3

Example: HCN has linear geometry and 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

Definition and Classification

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

• Hydrocarbons: Alkanes, alkenes, alkynes, arenes (benzene ring).

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

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

Organic Chemistry Overview

Organic Molecules and Hydrocarbons

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: Identify organic molecules and hydrocarbons from given structures.

Additional info:

• Some content inferred for completeness, such as the full electron configuration for phosphorus and the classification of functional groups.

• Tables reconstructed from images and context.