modern+atomic+theory

The Bohr model of the atom worked very well in predicting the emission spectrum of hydrogen, the simplest element. However, it did not explain the complex spectra of larger elements, hence a better model was needed.

In 1924, Louis De Broglie theorized a wave-particle theory that could be applied to electrons. Basically, electrons orbit the nucleus not in concentric rings but as "standing waves" (imagine taking a picture after tossing a rock in a calm pond):



De Broglie related the wavelength of an electron (recall the electromagnetic spectrum) to its mass and frequency.

In 1929, Erwin Schrodinger used mathematical equations (called //wave functions//) to describe the regions electrons travel in about the atom. These regions are quantized, which means electrons are only allowed to travel in certain regions, at very specific energy levels, around the nucleus.

The Bohr atomic model and atomic emission spectra gave evidence of this quantized energy level concept:



Quantization can also be explained by thinking of climbing a ladder. As you climb up the rungs (rung 1 being the first one at the bottom) you gain potential energy; you also cannot "hover" in between the rungs. The same concept applies to electrons in an atom - they can only exist in specific energy levels (rungs) and the higher the energy level, the higher the electron's energy.

Also consider the 'standing wave' image: electrons will be "trapped" in the troughs of standing waves of energy around the nucleus.

See how quantization applies to electron arrangements in atoms.



This mathematical description of electrons' regions is simply described as an **orbital**. An orbital is defined as the region of space about a nucleus where the probability of finding an electron is 90%, or 9 out of 10.
 * Each orbital can hold up to two electrons; a group of orbitals is called a sublevel.
 * There are four major types of sublevels, designated //s, p, d//, and //f.//
 * These sublevels range in energy (and therefore size) from energy levels 1 to 7.

The periodic table is arranged into these sublevels as shown:



These sublevels have distinct orbital shapes, which define a region where the electron spends 90% of its time: We won't be concerned with knowing these shapes; the point is that the modern electron model is very complex.

Watch this interactive video explaining probability and the shape of the 1s orbital.

See all kinds of probability density plots here.