Chemistry - The orbital model of the atom - building on Bohr's energy
The orbital model of the atom - building on Bohr's energy levels
Bohr's model of electron structure provided a base for later advances.
These advances were to provide satisfactory explanations for:
* Bohr's energy levels
* the line spectra of many-electron atoms
* chemical bonding
* the accepted periodic behaviour of the elements
Bohr's sharply defined 'orbits' were too specific in describing electron
movements, especially for larger atoms. The currently accepted explanation
is that electrons occupy regions of space around a nucleus and that
electron movement within these regions creates a cloud of negative charge.
These regions are called
The influence of the nucleus on electrons is undeniable given the
application of the simple electrostatics principle that unlike charges
attract. However, given that electrons must be moving or they would cascade
in on the nucleus, the current model of the atom deals with the of an
electron being in a particular location, i.e. orbital.
On another level, atomic orbitals are mathematical descriptions of where
the electrons in an atom (or molecule) are most likely to be found. These
descriptions are obtained by solving an equation known as the Schrodinger
Shown below is a representation of the movement of the electron in a
Essentially what we now accept is a of the atom as far as the location
(arrangement) of electrons is concerned. This brings into play the idea of
subshells and orbitals as a more definitive way of describing how electrons
are located around the nucleus.
An orbital is the mathematically defined region of space where an electron
An orbital may contain zero, one or two electrons but never more than two.
This is known as the Pauli exclusion principle.
Orbitals of equivalent energy are grouped into subshells (of similar
energy), which are then grouped into shells.
It is important to keep in mind that orbitals, subshells and shells are
all i.e. locations around the atom of specific energy. These energy levels
are defined, but occupied by electrons, depending on whether the atom is
in its ground state or an excited state.
Click here to view a word document on shells, subshells and orbitals.
Each atom has a complete set of orbitals located within subshells within
shells. The distinction between orbitals and charge clouds is also
important. A charge cloud represents electron movement in an occupied
While each atom is presumed to have a complete set of orbitals, only
orbitals containing one or two electrons have an associated charge cloud.
The significance of the accepted order of subshell filling is that it
allows for a more detailed way of writing electron arrangements (electron
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