Atomic Structure

Atoms and Isotopes

Atoms are the basic units of matter, consisting of a nucleus (protons and neutrons) surrounded by electrons. The atomic number is the number of protons in the nucleus, which defines the element. 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 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.

Electron Configuration

Ground State Electron Configuration

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

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

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

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

Electronegativity

Definition and Periodic Trend

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 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 shares 2 electrons in bonds, achieving an octet.

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 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 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.

• Types:

  Electron Groups	Geometry	Hybridization	Example
  2	Linear	sp	CO2
  3	Trigonal Planar	sp2	BF3
  4	Tetrahedral	sp3	CH4

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: Molecule with a symmetric distribution of charge (perfect shape, no lone pairs on central atom).

• Polar Molecule: Molecule with an asymmetric distribution of charge (lone pairs or different atoms attached).

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, aromatic compounds.

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

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

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

Organic Chemistry Introduction

Organic Molecules and Hydrocarbons

Organic chemistry studies molecules containing carbon and hydrogen, often with other elements such as oxygen, nitrogen, or 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 context and examples were inferred to provide complete explanations and tables.

• All equations are provided in LaTeX format as required.