Atomic Structure

Atoms and Isotopes

An 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: Hydrogen (1 proton), Deuterium (1 proton, 1 neutron), Tritium (1 proton, 2 neutrons).

• Ions: Atoms that have gained or lost electrons.

• Cations: Positively charged ions (lost electrons).

• Anions: Negatively charged ions (gained electrons).

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

Electron Configuration

Ground State Electron Configuration

Describes the distribution of electrons in atomic orbitals using the Aufbau Principle, Pauli Exclusion Principle, and Hund's Rule.

• 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.

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

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

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

Periodic Table Blocks

• s-block: Groups 1A and 2A

• p-block: Groups 3A to 8A

• d-block: Transition metals

• f-block: Lanthanides and actinides

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

Octet Rule

Valence 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 (from bonds).

Formal Charge

Calculation and Application

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

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

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

Formula:

Example: For the thiocyanate ion (NCS-), calculate formal charges for N, C, and S using their group numbers.

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 complete octets (except H, which only needs 2).

4. Place remaining electrons on the central atom.

5. If atoms lack octets, form double/triple bonds as needed.

6. Check formal charges for stability.

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 actual structure is a resonance hybrid of all possible 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 the central atom.

Example: HCN has a linear geometry and sp hybridization.

Molecular Polarity

Polar and Nonpolar Molecules

Molecular Polarity refers to the distribution of electrical charge over the atoms 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: Nitrogen trifluoride (NF3) is polar due to lone pairs and asymmetry.

Functional Groups

Definition and Classification

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

• Hydrocarbons: Alkane, Alkene, Alkyne, Benzene

• 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 heteroatoms.

Organic Chemistry Basics

Definition and Applications

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

• Organic molecules contain both carbon and hydrogen.

• Hydrocarbons contain only carbon and hydrogen.

Example: Identify organic molecules and hydrocarbons from a list of compounds.

Summary Table: Principles of Electron Configuration

Additional info: Some content inferred for completeness, such as specific examples and expanded definitions.