Atomic Structure

Atoms and Isotopes

The atom is the fundamental 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: Protium (1 proton), Deuterium (1 proton, 1 neutron), and Tritium (1 proton, 2 neutrons).

• Ions: Atoms that have gained or lost electrons. - Cations: Positively charged (lost electrons). - Anions: Negatively charged (gained electrons).

Example: D+ (Deuterium ion): 1 proton, 1 neutron, 0 electrons. 14C2- (Carbon ion): 6 protons, 8 neutrons, 8 electrons.

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.

Example: Electron configuration diagrams illustrate these principles.

Electron Configuration

Ground State Electron Configuration

Describes the distribution of electrons among the atom's orbitals (1s, 2s, 2p, etc.) using the Aufbau Principle.

• Aufbau Principle: Start from 1s, fill lower energy orbitals before higher energy orbitals.

• Condensed Electron Configuration: Use the previous noble gas as a shorthand for inner electrons.

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 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.

• Octet: 8 electrons (valence + shared).

Example: In H3COH, oxygen has 6 valence electrons and 2 shared electrons, satisfying the octet rule.

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/ion.

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

Lewis Dot Structures

Drawing Lewis Structures

Lewis 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 F).

3. Add electrons to outer atoms to complete octets.

4. Place remaining electrons on the central atom.

5. If octets are not satisfied, form double/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, a composite of all resonance forms.

Example: Nitrate ion (NO3-) has three resonance structures.

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: 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 on the central atom.

Functional Groups

Definition and Classification

Functional Group: 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: Alkyl halides, amines, alcohols, ethers, thiols.

Example: Identify functional groups in organic molecules and relate them to chemical properties.

Organic Chemistry Overview

Definition and Applications

Organic chemistry studies molecules containing carbon and hydrogen, often with other elements. Organic molecules are found in biological systems and everyday products.

• Organic Molecule: Contains both carbon and hydrogen.

• Hydrocarbon: Contains only carbon and hydrogen.

Example: Hairspray ingredients and nerve gases are organic compounds with various functional groups.

Additional info:

• Some content inferred for completeness, such as the most electronegative element and examples of functional groups.

• Tables reconstructed for clarity and study purposes.