Atomic Structure

Atoms and Isotopes

The atom is the fundamental unit of matter, consisting 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 with a net charge due to 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

Ground State Electron Configuration

Electron configuration describes the distribution of electrons among atomic orbitals. The Aufbau Principle states that electrons fill the lowest energy orbitals first.

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

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

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

Principles of Electron Configuration

• 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.

Electronegativity

Definition and Periodic Trend

Electronegativity (EN) is a measure of an atom's ability to attract electrons in a chemical bond. It increases from left to right across a period and from bottom to top within a group.

• Periodic Trend: Electronegativity increases across a period and up a group.

• Most electronegative element: Fluorine (F).

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

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, 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, totaling 8 octet 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: For the thiocyanate ion (NCS-), calculate formal charges for N, C, and S using the formula above.

Lewis Dot Structures

Drawing Lewis 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 structure for COCl2.

Resonance Structures

Definition and Representation

Resonance structures are multiple valid Lewis structures for a molecule with delocalized 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 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 hybridization

  • 3 groups: Trigonal planar, sp2 hybridization

  • 4 groups: Tetrahedral, sp3 hybridization

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: 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 heteroatoms.

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

Additional info:

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

• Tables reconstructed for clarity and academic context.