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 Isotopes

• Hydrogen (1H): 1 proton, 0 neutrons

• Deuterium (2H): 1 proton, 1 neutron

• Tritium (3H): 1 proton, 2 neutrons

Ions

Atoms can gain or lose electrons to form ions.

• Cation: Positively charged atom (loss of electrons)

• Anion: Negatively charged atom (gain of electrons)

Example: Hydrogen Ions

• Proton (H+): 1 proton, 0 electrons

• Hydride (H-): 1 proton, 2 electrons

Electron Configuration 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 occupy degenerate orbitals singly before pairing.

Example: Electron configuration diagrams can be used to identify which principle is being violated.

Electron Configuration

Ground State Electron Configuration

Describes the distribution of electrons among the orbitals of an atom using the Aufbau Principle.

• Electrons fill from lowest to highest energy orbitals: ...

Condensed Electron Configuration

Uses the previous noble gas as a shorthand for inner electrons.

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

Periodic Table Blocks

The periodic table is divided into s-block, p-block, d-block, and f-block elements based on electron configuration.

Electronegativity

Definition and Periodic Trend

Electronegativity (EN): A measure of an atom's ability to attract electrons in a chemical bond.

• 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

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 of an atom.

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

• Example: In H3COH, oxygen has 6 valence electrons and 2 shared electrons, achieving an octet.

Formal Charge

Definition and Calculation

Formal charge is used to check the accuracy of Lewis Dot Structures.

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

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

Formal Charge Formula:

Example: Calculate formal charges for atoms in the thiocyanate ion (NCS-).

Lewis Dot Structures

Drawing Rules

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 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 accuracy.

Example: Draw the Lewis Dot Structure for COCl2.

Resonance Structures

Definition and Representation

Resonance structures are two or more valid Lewis Dot 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.

• Dashed lines are used to show delocalized electrons in the resonance hybrid.

• 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 2 electron groups, so the central atom is sp hybridized.

Molecular Polarity

Polar and Nonpolar Molecules

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

• Nonpolar Molecule: Hydrocarbons or molecules with a perfect shape 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 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

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

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

Organic Chemistry Basics

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 definition of resonance hybrid.

• Tables reconstructed for periodic table blocks, hybridization, and molecular polarity.