Atomic Structure and Electron Configuration Fundamentals

Basic Atomic Structure

• Proton: A particle with a positive charge located in the nucleus. Mass = 1 AMU.
• Neutron: An atomic particle with no charge and a mass approximately equal to a proton. It is a structural component of the atomic nucleus. Mass = 1 AMU.
• Electron: A particle with a negative charge that orbits the nucleus.
• Nucleus: The positively charged center of an atom.
• Atom: The basic unit of a chemical element.
• Atomic Mass Unit (AMU): Used to express atomic and molecular weight.
• Atomic Number: The number of protons in an atom of a chemical element.
• Isotope: Atoms of an element containing an equal number of protons but a different number of neutrons in their nuclei.

Isotopes

• All atoms of an element do not weigh the same amount.
• The number of neutrons in an atom can vary; as long as the protons remain the same, it is the same element.
• Isotopes are named by their mass. For example, Mg-24, Mg-25, and Mg-26 are written with the mass number on top and the atomic number on the bottom.

Electron Cloud Model

• Electrons do not travel in simple circular orbits; they occupy regions with complex shapes.
• Each shell (energy level) is split into smaller parts called subshells (or orbitals).
• Shell: Contains orbitals of the same shape and can hold a specific maximum number of electrons.

Subshell and Shell Capacity

Note: Shell 5 is larger but never fully fills in standard configurations.

Order of Subshell Filling

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d.

Key Formulas

• Atomic Number: # of protons = # of electrons.
• Mass Number: Protons + Neutrons.
• Neutrons: Mass Number - Protons.
• Core Electrons: Total Protons - Valence Electrons.

Examples

Electron Configuration Example:

• Element: Nitrogen | Configuration: 1s² 2s² 2p³ | Valence Electrons: 5 (2s, 2p)

Isotope Data Example:

• Isotope: Carbon-14 | Symbol: C | Atomic #: 6 | Mass #: 14 | Protons: 6 | Neutrons: 8

Configuration Example:

• Element Symbol: Kr | Electron Configuration: [Ar] 4s² 3d¹⁰ 4p⁶ | Valence Electrons: 8