Atomic Structure

Atoms and Isotopes

Atoms are the fundamental units of matter, consisting of protons, neutrons, and electrons. The atomic number (Z) is the number of protons in the nucleus, while the mass number (A) 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 with the same number of protons but different numbers of neutrons.

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

• Ions: Atoms that have gained or lost electrons. Cations are positively charged (loss of electrons), and anions are negatively charged (gain of electrons).

Example: D+ (deuterium ion) and 14C2- (carbon ion).

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 up.

The Electron Configuration

Ground State Electron Configuration

The ground state electron configuration describes the distribution of electrons among the orbitals (1s, 2s, 2p, etc.) using the Aufbau Principle.

• Aufbau Principle: Electrons fill from lower to higher energy orbitals.

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

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

Periodic Table Blocks

• s-block: Groups 1A and 2A

• p-block: Groups 3A to 8A

• d-block: Transition metals (Groups 3B to 2B)

• 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 or I.

Octet Rule

Valence and Shared Electrons

The octet rule states that 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, achieving an octet.

Formal Charge

Definition and Calculation

Formal charge is used to 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 structures represent the arrangement of valence electrons among atoms in a molecule.

1. Count total valence electrons.

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

3. Add electrons to outer atoms to complete octets.

4. Place remaining electrons on the central atom.

5. If needed, form double or triple bonds to satisfy the octet rule.

6. Check formal charges to ensure 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.

• Resonance involves the movement of pi electrons or lone pairs.

• Double-headed 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 Hybrid Orbitals

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

• Electron Groups: Number of atoms bonded to the central atom plus lone pairs.

Example: HCN has a 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: Has a symmetrical (perfect) shape and no net dipole moment.

• Polar Molecule: Has an asymmetrical shape or contains polar bonds that do not cancel out.

Example: Nitrogen trifluoride (NF3) is polar due to its lone pair and asymmetrical shape.

Functional Groups

Definition and Types

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

• Hydrocarbons: Alkanes, alkenes, alkynes, and aromatic compounds (benzene).

• With Carbonyls: Aldehydes, ketones, carboxylic acids, esters, amides, acid chlorides.

• Without Carbonyls: Alcohols, ethers, amines, thiols, alkyl halides.

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

Organic Chemistry Overview

Definition and Examples

Organic chemistry is the study of carbon-containing compounds, especially those found in living organisms. Organic molecules contain both carbon and hydrogen, and may include other elements such as oxygen, nitrogen, sulfur, and halogens.

• Hydrocarbons: Molecules containing only carbon and hydrogen.

• Organic Molecules: Any molecule with both carbon and hydrogen.

Example: Identify which molecules are organic and which are hydrocarbons.

Additional info:

• Some context and explanations were expanded for clarity and completeness.

• Tables were recreated in HTML for hybridization and molecular polarity.

• Examples were inferred and expanded based on standard General Chemistry curriculum.