Atomic Structure

The Atom and Isotopes

The atom is the fundamental unit of matter, composed 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 or molecule with a net electric charge due to the 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 the orbitals of an atom. The Aufbau Principle states that electrons fill the lowest energy orbitals first.

• Aufbau Principle: Electrons occupy the lowest energy orbitals available.

• Condensed Electron Configuration: Uses the previous noble gas as a starting point.

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

Three Principles of Electron Configuration

• Aufbau Principle: Fill orbitals in order of increasing energy.

• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four 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 left to right and bottom to top.

• 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 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.

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.

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

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 outer atoms to satisfy the octet rule.

4. Place remaining electrons on the central atom.

5. If octet is not satisfied, form double or triple bonds.

6. Check formal charges.

Example: Draw the Lewis structure for COCl2.

Resonance Structures

Definition and Representation

Resonance structures are two or more valid Lewis structures for a molecule that differ only in the placement of electrons.

• Movement of electrons occurs in pi bonds or lone pairs.

• Double-sided arrows indicate resonance.

• The real structure is a resonance hybrid.

Example: Draw all resonance structures for the nitrate ion, NO3-.

Hybridization

Electron Groups and Hybridization Types

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 distribution of charge.

• Polar Molecule: Molecule with an asymmetric distribution of charge.

• Perfect Shape: Central atom has no lone pairs and all surrounding atoms are identical.

Example: Nitrogen trifluoride (NF3) is polar due to lone pairs on nitrogen.

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 given molecules and classify as hydrocarbons or organic compounds.

Additional info:

• Organic molecules contain both carbon and hydrogen.

• Hydrocarbons are organic molecules containing only carbon and hydrogen.

----------------------------------------