The aufbau principle is the theoretical approach to the construction
of the periodic table of the elements. The structures of the atomic
orbitals are considered and an extra electron is added to these
orbitals for each succeeding element in the periodic table, in such
a manner as to allow prediction of the chemical properties of the
elements from their electronic configuration.
The chemical properties of the elements are explained by the
distribution of the electrons in the orbitals about the nuclei of
the atoms of that element. The atoms of each element have an
electronic configuration which is unique to that element.
The aufbau principle involves the construction of the sequence of
elements by the assembly of the atoms of each element in turn from
atomic particles. Considers the bare nucleus of an element,
consisting of the correct number of protons and neutrons for atoms
of that element. In order to have electrically neutral atoms, as
many electrons will be required for the orbitals about the nucleus
as there are protons in the nucleus. Now, if we start adding the
electrons, one at a time, to the orbitals about the nucleus, each
electron enters the lowest energy level sub-orbital that is
available, and complies with the requirements of Hund's Rule
and the Pauli exclusion principle.
The sequence in which orbitals are filled depends on the energy
levels of the sub-orbitals, which is not exactly the same sequence
as that of the main orbitals. For example, the 4s sub-orbital has
lower energy level than the 3d sub-orbital, and thus the 4s
sub-orbital is filled before the 3d sub-orbital.
The order of increasing energy among the sub-orbitals and thus
the order of filling can be represented in the following way :
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d,
This sequence of energy levels of the sub-orbitals explains in
great detail the chemical properties of the elements and the
structure of the periodic table, including the existence of
the transition elements and the inner transition elements.
Avogadro's number is the number of atoms present in one mole of an
element or the number of molecules present in one mole of a substance.
Count Amedeo Avogadro published his
hypothesis in 1811 AD
An azeotrope (also called a constant boiling mixture) is a liquid
mixture that has the peculiar property of giving a vapour which
has the same composition as the liquid.
The azimuthal quantum number, l, describes the shape of the
sub-orbitals. The main shells, described by the
principal quantum number are divided into a number of sub-orbitals.
Within each main level, each sub-orbital is defined by a different
value for the subsidiary quantum number, l. As this subsidiary
quantum number increases, the complexity of the sub-orbital
Azimuthal quantum number Electron Capacity of Sub-Orbital
type of sub-orbital
1 s 2
2 p 6
3 d 10
4 f 14
- l = 0
- This is the simplest type of sub-orbital, which is spherical in shape. It is designated as
the s sub-level, and can hold two electrons. There is only one s sub-orbital in each main
- l = 1
- This is the next sub-level and it has a dumb-bell shape. It is designated as the p
sub-orbital, and can hold two electrons. There are three p sub-orbitals, all mutually
perpendicular to each other in space. These are assigned the labels px, py and pz to
indicate their directional characteristics. Since each can hold two electrons, the total electron
capacity of the p sub-level is six.
- l = 2
- This is the next sub-level and it has a double dumb-bell shape. It is designated as the
d sub-orbital, and can hold two electrons. There are five d sub-orbitals, and thus, this
sublevel can hold a maximum of ten electrons. The shapes of d sub-orbitals are complex and
cannot be easily be represented in two dimensions.
- l = 3
- This is the next sub-level and it has a complex double dumb-bell shape. It is
designated as the f sub-orbital, and can hold two electrons. There are seven f sub-orbitals,
and thus, this sub-level can hold a maximum of 14 electrons.
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