Pauli Exclusion Principle and Hund's Rule 

ChemistryExplain provide notes about Atomic structure and the periodic table today we discuss “Pauli Exclusion Principle and Hund's Rule” inInorganic chemistry jobs, Inorganic Chemistry, Inorganic chemistry examples.

Pauli Exclusion Principle

Three quantum: numbers n, I, and m are needed to define an orbital. Each orbital may hold up to two electrons, provided they have opposite spihs. An extra ·quantum number is required to define the spin of an electron in an orbital. Thus four quantum numbers are needed to define the energy of an electron in an atom. The Pauli exclusion principle states that no two electrons in one atom can have all four quantum numbers the ~ame. Ry permutating the quantum numbers, the maximum number of electrons that can be contained in each main energy level can be calculated (see Figure 1.12).
For More Chegg Questions

• Inorganic Chemistry
• Organic Chemistry
• Physical Chemistry

Hund's Rule

When atoms are in their ground state, the electrons occupy the lowest possible energy levels.
The simplest element, hydrogen, has one electron, which occupies the ls level; this level has the principal quantum number n = 1, and the subsidiary quantum number I = 0.
Helium has two electrons. The second electron also occupies the level . This is possible because the two electrons have opposite spins. This level is now full.
The next atom lithium has three electrons. The third electron occupies the next lowest level. This is the 2s level, which has the principal quantum number n = 2 and subsidiary quantum number I = 0.

For More Chemistry Notes and Helpful Content Subscribe Our YouTube Chanel - ChemistryExplain
The fourth electron in beryllium also occupies the 2s level. Boron must have its fifth electron in the 2p level as the 2s level is full. The sixth electron in carbon is also in the 2p level. Hund's rule states . that the number of unpaired electrons in a given energy level is a maximum. Thus in the ground state the two p electrons in carbon are unpaired. They occupy separate p orbitals and have parallel spins. Similarly in nitrogen the three p electrons are unpaired and have parallel spins.
To show the positions of the electrons in an atom, the symbols Is, 2s, 2p, etc. are used to denote the main energy level and sub-level. A superscript indicates the number of electrons in each set of orbitals. Thus for 1 hydrogen, the Is orbital contains one electron, and this is shown as Is • For 2 helium the ls orbital contains two electrotls, denoted ls . The electronic structures of the first few 'atoms in the periodic table tnay be written:

An alternative way of showing the electroniC structure of an atom is .to draw boxes for orbitals, and arrows for the electrons.

Sequence of energy levels

It is important to know the sequence in which the energy levels are filled. Figure 1.13 is a useful aid. From this it can be seen that the order of filling of energy levels is: ls, 2s, 2p, 3s, 3p, 4s, 3d, 4p, Ss, 4d, Sp, 6s, 4/, Sd, 6p, 7s, etc.
After the ls, 2s, '{.p, 3s and 3p levels have been filled at argon, the next two electrons go into the 4s level. This gives the elements potassium and calcium. Once the 4s level is full the 3d level is the next lowest in energy, not the 3p level. Thus tl:ie 3d starts to fill at scandium. The elements from scandium to copper have two electrons in the 4s level and an incomplete 3d level, and all behave in a similar manner chemically. Such a series of atoms is known as a tninsition series.
A second transition series starts after the 5s orbital has been tilled, at strontium , because in the next element, yttrium, the 4d level begins to fill up. A third transition series starts at- lanthanum where the electrons start to fill the Sd level after the 6d level has been filled with two electrons.
A further complication arises here because after lanthanum, which has one electron in the 5d level, the 4/ level begins to fill, giving the elements from cerium to lutetium with from one to 14/ electrons. These are sometimes called the inner transition elements, but are usually known as the lanthanides or rare earth metals.