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 total number 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.

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus.

• Electron: Negatively charged particle in orbitals around the nucleus.

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 several 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:

• Example: $1s^2 2s^2 2p^6 3s^2 3p^3$ (for Phosphorus, Z = 15)

Condensed Electron Configuration: Uses the previous noble gas as a starting point. For Phosphorus: [Ne] $3s^2 3p^3$

Periodic Table Blocks:

• s-block: Groups 1A and 2A

• p-block: Groups 3A to 8A

• d-block: Transition metals

• f-block: Lanthanides and actinides

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 going up a group.

• Most Electronegative Element: Fluorine (F)

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: 8 valence electrons (except for hydrogen, which seeks 2).

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

Formal Charge

Definition and Calculation

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

• Formula:

$\text{Formal Charge} = \text{Valence Electrons} - (\text{Nonbonding Electrons} + \text{Bonding Electrons}/2)$

• 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 central atom.

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

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

Exceptions: Hydrogen only forms one bond; halogens usually form one bond as terminal atoms; carbon is rarely a central atom unless in organic molecules.

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 (not atoms).

• Resonance involves the movement of pi electrons or lone pairs.

• Double-headed arrows ($\leftrightarrow$) are used between resonance structures.

• The actual structure is a resonance hybrid, a composite 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 atoms bonded to the central atom plus lone pairs.

Example: For HCN, the central carbon is sp hybridized with 2 unhybridized p orbitals.

Molecular Polarity

Definition and Determination

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

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

• Polar Molecule: Has 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 Types

A functional group is a specific group of atoms within a molecule responsible for its 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: Alcohols contain an -OH group; carboxylic acids contain a -COOH group.

Organic Chemistry Overview

Definition and Applications

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.

• Applications: Pharmaceuticals, plastics, fuels, biological molecules

Example: Identifying organic molecules and hydrocarbons from a list of structures.

Summary Table: Principles of Electron Configuration

Practice: Identify which principle is violated in given electron configuration diagrams.

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.