Atomic Models, Electron Energy Levels, and Atomic Orbitals

Atomic Models and the Structure of the Atom

Historical Development of Atomic Models

The understanding of atomic structure has evolved through several key models, each contributing to our knowledge of how atoms are organized and how electrons behave within them.

• Dalton Model (1803): Atoms are small, indivisible spheres. This model did not account for internal structure.

• Thomson Model (1897): Known as the "Plum Pudding" model, it proposed that electrons are suspended in a positively charged gel.

• Rutherford Model (1911): Most of the atom is empty space, with a small, dense, positively charged nucleus at the center. Electrons surround the nucleus.

• Bohr Model (1913): Electrons travel in specific, quantized orbits (energy levels) around the nucleus. This model explained why elements have characteristic chemical properties and why they emit or absorb light at specific energies.

Electron Energy Levels and Quantum Theory

Energy Levels and Electron Transitions

Electrons in atoms are restricted to certain energy levels, also called shells or orbits. These levels are quantized, meaning electrons can only exist at specific energy values, not in between.

• Energy Levels: Electrons travel in orbits or paths at fixed distances from the nucleus, each with a specific amount of energy (E).

• Quantum: The minimum amount of energy required to move an electron from one energy level to another.

• Electron Transitions:

  • If an electron absorbs energy, it can jump to a higher energy level (excited state).

  • If an electron loses energy, it falls to a lower energy level, emitting light (photon) in the process.

• Quantized Energy: Energy is emitted or absorbed in discrete packets called quanta (singular: quantum), not in a continuous range.

Equation for Energy of a Photon:

$E = h\nu$

• Where E is the energy, h is Planck's constant, and \nu (nu) is the frequency of the emitted or absorbed light.

Ground State vs. Excited State:

• Ground State: The lowest energy state of an atom.

• Excited State: Any energy state higher than the ground state, achieved when an electron absorbs energy.

Atomic Orbitals and Quantum Numbers

Principal Quantum Number (n)

The principal quantum number, n, indicates the main energy level or shell of an electron in an atom. As n increases:

• The electron is, on average, farther from the nucleus.

• The energy of the electron increases.

• The number of possible sublevels and orbitals increases.

Sublevels and Types of Atomic Orbitals

Each principal energy level contains one or more sublevels, which are designated by the letters s, p, d, and f. Each sublevel contains a specific number of orbitals with characteristic shapes:

Sublevel	Number of Orbitals	Shape
s	1	Spherical
p	3	Peanut-shaped (dumbbell)
d	5	Double peanut-shaped
f	7	Flower-shaped

• Total number of orbitals in each energy level:

  • n = 1: 1 orbital (1s)

  • n = 2: 4 orbitals (1s, 3p)

  • n = 3: 9 orbitals (1s, 3p, 5d)

  • n = 4: 16 orbitals (1s, 3p, 5d, 7f)

• Each individual orbital can hold 2 electrons.

Electron Clouds and Probability

Modern atomic theory describes electrons as existing in "clouds" or regions of probability called orbitals, rather than fixed paths. The exact location of an electron cannot be determined, only the probability of finding it in a certain region.

• Orbital: A region in space where there is a high probability of finding an electron.

• Electron cloud: The visual representation of all possible locations for an electron in an atom.

Summary Table: Atomic Sublevels and Orbitals

Principal Quantum Number (n)	Sublevels	Total Orbitals	Maximum Electrons
1	1s	1	2
2	2s, 2p	4	8
3	3s, 3p, 3d	9	18
4	4s, 4p, 4d, 4f	16	32

Additional info: The notes also reference the concept of quantized energy levels, the emission and absorption of light by electrons, and the relationship between energy, frequency, and wavelength in the context of atomic spectra.