Electromagnetism, Atomic Models, and Theories
Electromagnetic Waves
Electromagnetic waves carry electromagnetic energy as they propagate. This energy can take various forms, such as X-rays and ultraviolet radiation. Key wave characteristics include:
- Wavelength (λ): The minimum distance between two points in phase. Unit: meters (m).
- Period (T): The time it takes for a wave to travel a distance equal to its wavelength. Unit: seconds (s).
- Frequency (f): The number of wavelengths passing through a given point per second. Unit: Hertz (Hz = 1/s).
Different types of electromagnetic radiation are distinguished by their wavelengths and frequencies.
Quantum Mechanical Theory
This theory rests on two principles:
- De Broglie’s Principle: Energy has wave-like properties.
- Heisenberg’s Uncertainty Principle: It is impossible to simultaneously determine the exact position and momentum of an electron.
Current Atomic Models
An orbital is a region around the nucleus where there is a high probability of finding an electron. Each orbital is defined by three quantum numbers:
- Principal quantum number (n): Determines the orbital’s size.
- Azimuthal quantum number (l): Determines the orbital’s shape.
- Magnetic quantum number (ml): Determines the orbital’s orientation.
Electronic Configurations
Electronic configurations describe how electrons are distributed within an atom’s orbitals. They adhere to the following rules:
- Pauli Exclusion Principle: No two electrons in an atom can have the same four quantum numbers.
- Aufbau Principle: Electrons fill orbitals starting with the lowest energy levels.
- Hund’s Rule: Electrons occupy orbitals individually before pairing up.
Atomic Spectra
Atomic spectra represent the decomposition of light into its constituent colors, each corresponding to a specific electromagnetic radiation.
- Continuous Spectrum: An unbroken range of overlapping radiation.
- Discontinuous Spectrum: Specific, distinct lines of radiation emitted by a particular light source.
- Absorption Spectrum: A spectrum where certain wavelengths are absorbed by a substance, resulting in dark lines.
Bohr Model
In 1913, Niels Bohr proposed a model for the hydrogen atom based on Rutherford’s model and three postulates:
Initial Considerations:
- The atom’s nucleus is small, containing most of the atom’s mass and a positive charge.
Postulates:
- Electrons orbit the nucleus in defined circular paths without absorbing or emitting energy.
- Only specific orbits with certain energy levels are allowed.
- Energy is absorbed when an electron jumps to a higher orbit and emitted as electromagnetic radiation when it drops to a lower orbit.
Early Atomic Theories
Democritus’ Atomic Theory
Democritus, a Greek philosopher, proposed the existence of indivisible particles called atoms. His theory included:
- Atoms move in a void, colliding with each other.
- Bodies are formed by the combination of atoms.
- Differences between bodies are due to the shape, arrangement, and size of atoms.
Dalton’s Atomic Theory
In the early 19th century, John Dalton proposed an atomic theory based on existing chemical laws:
- Atoms of an element are identical in mass and properties.
- Matter is made of atoms.
- Compounds are formed by the union of atoms in fixed ratios.
Early Atomic Models
The discovery of the electron in 1897 led to the development of early atomic models, including those by Kelvin, Thomson, and Nagaoka.
Rutherford’s Atomic Model
Based on experiments with alpha particles, Rutherford proposed:
- Most of the atom’s mass is concentrated in a small, positively charged nucleus.
- Electrons orbit the nucleus.
- Most of the atom is empty space.