4.3 Electron Configuration and Energy Levels

Electron configuration and energy levels are key to understanding atomic structure. They explain how electrons are arranged around the nucleus and why elements behave the way they do. This knowledge forms the basis for predicting chemical reactions and bonding.

These concepts build on earlier ideas about atomic structure. By learning about electron arrangements and energy states, we can better grasp how atoms interact and form compounds. This sets the stage for understanding chemical behavior and properties.

Electron Arrangement Principles

Fundamental Principles of Electron Configuration

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• Aufbau principle dictates electrons fill orbitals from lowest to highest energy
  • Follows the order 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p
• Pauli exclusion principle states no two electrons in an atom can have the same set of quantum numbers
  • Limits each orbital to a maximum of two electrons with opposite spins
• Hund's rule requires electrons to occupy orbitals of equal energy individually before pairing
  • Maximizes the number of unpaired electrons in degenerate orbitals
• Electron configuration notation represents the arrangement of electrons in an atom
  • Uses numbers, letters, and superscripts to show electron distribution (1s² 2s² 2p⁶)

Applications and Exceptions

• Noble gas configuration serves as a shorthand for inner shell electrons
  • Simplifies notation for elements with many electrons ([Ar] 4s² 3d¹⁰)
• Exceptions to Aufbau principle occur in transition metals and lanthanides
  • Chromium (Cr) has [Ar] 4s¹ 3d⁵ instead of [Ar] 4s² 3d⁴
  • Copper (Cu) has [Ar] 4s¹ 3d¹⁰ instead of [Ar] 4s² 3d⁹
• Orbital diagrams visually represent electron configurations
  • Use boxes to represent orbitals and arrows for electrons (↑↓)

Electron Energy Levels

Structure of Atomic Energy Levels

• Energy levels represent distinct electron shells around the nucleus
  • Numbered from 1 (closest to nucleus) outward (1, 2, 3, etc.)
• Electron shells consist of orbitals with similar energy
  • K shell (n=1), L shell (n=2), M shell (n=3), and so on
• Orbitals describe the probability of finding an electron in a specific region
  • s orbitals are spherical, p orbitals are dumbbell-shaped
  • d orbitals have more complex shapes (cloverleaf, doughnut)
• Sublevels within each energy level have specific shapes and orientations
  • s sublevel (1 orbital), p sublevel (3 orbitals), d sublevel (5 orbitals), f sublevel (7 orbitals)

Electron Behavior and Energy States

• Principal quantum number (n) determines the main energy level
  • Ranges from 1 to 7 for naturally occurring elements
• Excited states occur when electrons absorb energy and move to higher orbitals
  • Electrons emit photons when returning to ground state
• Quantum mechanical model describes electron behavior probabilistically
  • Electrons exist in "electron clouds" rather than fixed orbits
• Energy of orbitals increases as n increases and l increases
  • 4s has lower energy than 3d due to electron-electron repulsion

Electron Types

Valence Electrons and Chemical Behavior

• Valence electrons occupy the outermost shell of an atom
  • Determine chemical properties and reactivity of elements
• Number of valence electrons relates to an element's group in the periodic table
  • Group 1 (alkali metals) have 1 valence electron
  • Group 18 (noble gases) have 8 valence electrons (except helium with 2)
• Valence electron configuration influences bonding behavior
  • Elements tend to gain, lose, or share electrons to achieve noble gas configuration
• Electron dot diagrams represent valence electrons visually
  • Dots around element symbol show number and arrangement of valence electrons

Core Electrons and Atomic Properties

• Core electrons occupy inner shells closer to the nucleus
  • Less involved in chemical reactions but affect atomic size and ionization energy
• Effective nuclear charge increases across a period due to core electrons
  • Results in smaller atomic radii and higher ionization energies
• Shielding effect of inner electrons reduces the pull of the nucleus on outer electrons
  • Causes atomic radius to increase down a group
• Core electrons contribute to an atom's overall electron configuration
  • Influence the energy levels and orbitals available for valence electrons