Electrons in Atoms and the Periodic Table

The Nature of Light and Its Wave Properties

Light exhibits both wave-like and particle-like properties, which are fundamental to understanding atomic structure and electron behavior.

• Wavelength (λ): The distance between two consecutive peaks of a wave, typically measured in meters (m) or nanometers (nm).

• Frequency (ν): The number of wave cycles that pass a given point per second, measured in hertz (Hz).

• Speed of Light (c): All electromagnetic waves travel at the same speed in a vacuum: m/s.

• Energy of a Photon (E): The energy carried by a photon is related to its frequency and wavelength.

Key Equations:

• Relationship between speed, wavelength, and frequency:

• Energy of a photon: where J·s (Planck's constant)

Light and the Bohr Model of the Atom

The Bohr model explains atomic structure by proposing that electrons occupy specific energy levels. Light is emitted or absorbed when electrons transition between these levels, producing characteristic atomic spectra.

• Emission spectra of elements provided evidence for quantized energy levels.

• Bohr's postulates explained the stability of atoms and the discrete lines in atomic spectra.

Quantum Numbers and Atomic Orbitals

Quantum numbers describe the properties of atomic orbitals and the electrons within them.

• Principal Quantum Number (n): Indicates the main energy level or shell.

• Angular Momentum Quantum Number (l): Describes the shape of the orbital (s, p, d, f).

• Magnetic Quantum Number (ml): Specifies the orientation of the orbital in space.

• Spin Quantum Number (ms): Indicates the spin direction of the electron (+1/2 or -1/2).

Shapes of Atomic Orbitals

Atomic orbitals have characteristic shapes that influence chemical bonding and molecular geometry.

• s orbitals: Spherical shape.

• p orbitals: Dumbbell-shaped, oriented along x, y, or z axes.

• d orbitals: More complex shapes, often cloverleaf-like.

Electron Configurations and the Periodic Table

Electron configurations describe the arrangement of electrons in an atom. The noble gas core abbreviation simplifies notation by using the symbol of the nearest noble gas to represent filled inner shells.

• Example: Sodium (Na): [Ne] 3s1

• For ions, add or remove electrons according to the charge.

Periodic Trends

Periodic trends arise from the arrangement of electrons and the structure of the periodic table.

• Atomic Size: Increases down a group, decreases across a period.

• Ionization Energy: Energy required to remove an electron; increases across a period, decreases down a group.

• Metallic Character: Increases down a group, decreases across a period.

• Electron Affinity: Energy change when an atom gains an electron; generally becomes more negative across a period.

• Electronegativity: Tendency of an atom to attract electrons in a bond; increases across a period, decreases down a group.

Chemical Bonding and Molecular Structure

Lewis Structures

Lewis structures represent the arrangement of valence electrons in molecules and ions, showing how atoms are bonded together.

• Dots represent valence electrons; lines represent shared pairs (bonds).

• Used for elements, ionic compounds, ions, and molecular compounds.

Resonance Structures

Some molecules cannot be represented by a single Lewis structure. Resonance structures are multiple valid Lewis structures for the same molecule, differing only in the placement of electrons.

• Actual structure is a hybrid of all resonance forms.

Molecular Shapes (VSEPR Theory)

The shape of a molecule is determined by the repulsion between electron pairs around the central atom (Valence Shell Electron Pair Repulsion theory).

• Common shapes: linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral.

• Predict shape from Lewis structure by counting bonding and lone pairs.

Molecular Polarity

A molecule is polar if it has an uneven distribution of electron density, resulting in a dipole moment. Polarity depends on both the types of bonds and the molecular shape.

• Nonpolar molecules: Symmetrical shape and/or identical surrounding atoms.

• Polar molecules: Asymmetrical shape or different surrounding atoms.