Atomic Structure and Electron Configuration

Atomic Structure

The atom is the fundamental unit of matter, composed of protons, neutrons, and electrons. Understanding atomic structure is essential for predicting chemical behavior.

• 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).

• Ions: Atoms or molecules that have gained or lost electrons, resulting in a net charge.

• Cations: Positively charged ions (loss of electrons).

• Anions: Negatively charged ions (gain of electrons).

Example: Hydrogen Isotopes

• Protium (¹H): 1 proton, 0 neutrons

• Deuterium (²H): 1 proton, 1 neutron

• Tritium (³H): 1 proton, 2 neutrons

Electron Configuration

Electron configuration describes the distribution of electrons in atomic orbitals. It is governed by 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 occupy degenerate orbitals singly before pairing up.

General Electron Configuration Notation: For example, the ground state configuration for Phosphorus (Z = 15):

• Full: 1s2 2s2 2p6 3s2 3p3

• Condensed: [Ne] 3s2 3p3

Periodic Table Blocks: The periodic table is divided into s, p, d, and f blocks, which correspond to the type of atomic orbital being filled.

Electronegativity and Periodic Trends

Electronegativity (EN)

Electronegativity is 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 from bottom to top within a group.

• Most Electronegative Element: Fluorine (F) is the most electronegative element.

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

Octet Rule and Valence Electrons

Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons to achieve eight electrons in their valence shell, resembling the electron configuration of noble gases.

• Valence Electrons: Electrons in the outermost shell, involved in bonding.

• Shared Electrons: Electrons shared between atoms in a covalent bond.

• Octet Calculation: Octet electrons = valence electrons + shared electrons.

Example: In H3COH (methanol), oxygen has 6 valence electrons and forms 2 shared (bonding) pairs, achieving an octet.

Formal Charge

Definition and Calculation

Formal charge is used to determine the most likely Lewis structure for a molecule or ion.

• Formula:

• Only allowable formal charges: -1, 0, +1

• The sum of all formal charges in a molecule equals the overall charge.

Example: For the thiocyanate ion (NCS-), calculate the formal charge for each atom using the formula above.

Lewis Dot Structures

Drawing Lewis Structures

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 hydrogen).

3. Connect atoms with single bonds.

4. Complete octets for outer atoms, then for the central atom.

5. If octets are incomplete, form double or triple bonds as needed.

6. Check formal charges to ensure the most stable structure.

Example: Draw the Lewis structure for COCl2 (phosgene).

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 (not atoms).

• 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 weighted average of all resonance forms.

• Resonance is indicated by drawing a dashed line where delocalized electrons are shared.

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

Hybridization

Electron Groups and Hybridization

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

• Electron Groups: Number of bonds and lone pairs around a central atom.

Example: HCN has 2 electron groups around the central carbon, so it is sp hybridized.

Molecular Polarity

Polarity of Molecules

Molecular polarity arises from the uneven distribution of electrons, resulting in a dipole moment.

• Nonpolar Molecule: Molecule with a symmetrical shape and even charge distribution.

• Polar Molecule: Molecule with an asymmetrical shape or uneven charge distribution.

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

Example: Nitrogen trifluoride (NF3) is polar due to the presence of a lone pair on nitrogen.

Functional Groups in Organic Chemistry

Definition and Classification

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

• Hydrocarbons: Compounds containing only carbon and hydrogen (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 a given molecule, such as alcohol (-OH), amine (-NH2), or carboxylic acid (-COOH).

Organic Chemistry Overview

Definition and Importance

Organic chemistry is the study of carbon-containing compounds, especially those found in living organisms. Organic molecules must contain both carbon and hydrogen. Hydrocarbons are organic molecules containing only carbon and hydrogen.

Example: Identify which molecules are organic and which are hydrocarbons from a given set.

Summary Table: Principles of Electron Configuration

Practice: Given electron diagrams, determine which principle is being violated.

Additional info: Some explanations and examples have been expanded for clarity and completeness, as the original slides contained fill-in-the-blank prompts and partial tables.