Electronic configuration Aufbau principle.pdf

Fundamentals for chemistry

1. Bohr's Model and Electron distribution or
Electronic configuration
Different scientists have suggested various atom models. These have led to a better understanding of atomic
structure. Among these models, Rutherford proposed that electrons revolve in well-defined orbits. If that was the
case, there is an issue.
The motion of the electrons in Rutherford's model is unstable. As electrons revolve in orbit, they accelerate and
lose energy. After that, they fall into the nucleus resulting in the atom which is highly unstable.
To overcome these objections, NeilsBohr proposed a new atomic model.
NeilsBohr got theNobel prize for his work on the structure of the atom in 1922.

2. Postulate of Bohr’s model:
•The electrons revolve around the nucleus in a specific orbit, and these
orbits are associated with definite energies called shells or energy
levels.
•The electrons do not emit energy when revolving in specific orbits.
•These shells or energy levels or orbits are represented by the letters
K, L, M, N or by the numbers 1, 2, 3, 4.
Bohr's atom model
Distribution of electrons in orbits or shells:
Bohr and Bury proposed the distribution of electrons in orbits.
“ The definite distribution of electrons around the
nucleus is called electronic configuration.”
To achieve the electronic configuration, it follows a certain set of
rules:
The formula 2n
2
defines the total number of electrons in a shell.
Where, n is energy level or orbit number. n=1,2,3,4, etc.
Therefore, the maximum number of electrons in different
shells are as follows:
This implies that the first shell (K shell) can have a
maximum of twoelectrons, the second shell (L shell) can have a
maximum of eight electrons and so on.

3. Example:
Sodium atom with energy level (2,8,1) not (2,9).
Unless the inner shells are filled, electrons cannot
fill in a given shell.
In other words, the shells are gradually filled.
Example:
Incorrect and correct filling of electrons in sodium
According to Bohr, the energy of the shell is proportional to its size. The greater the size, the greater the energy.
Since the first shell is the smallest, it has the lowest energy, and it gets filled first.
Hence, the energy level or size of the shells are given by: K < L < M < N
The atomic structure of the first eighteen elements is shown schematically.
Some elements and their electronic configuration

4. Limitation Of Bhor-Bury atomic
structure(Electronic Configuration)
Failed to explain formation of molecules from atoms by chemical bonding; hence could not explain the
geometry of molecules.
Only hydrogen and hydrogen-like ions were included in this model (example, He
+
, Li
2+,
Be
3+
and so on). It
was not possible to apply it to multi-electron nuclei.
Bohr's model of atoms failed to explain the Zeeman Effect (effect of magnetic field on spectra of atoms). It
also failed to explain the Stark effect (effect of electric field on spectra of atoms). It disobeys Heisenberg
Uncertainty Principle.
(The uncertainty principle states that we cannot know both the position and speed of a particle, such
as a photon or electron, with perfect accuracy; the more we nail down the particle's position, the less we know
about its speed and vice a versa...
The uncertainty principle is important because it helps physicists to understand how things work at the
subatomic scale. The study of tiny subatomic particles and how they interact is known as quantum mechanics).
According to Bohr, the circular orbits of electrons are plane. But modern researches reveal that an electron
moves around the nucleus in three dimensional space.
The pictorial jumping of electrons from one to another orbit is not justified.

5. ELECTRONIC CONFIGURATION: QUANTUM
Mechanics (THEORY )
Advanced studies in the field of atomic structure revealed that electron has dual nature; as a Particle and
as a Wave.
Therefore , it is not logical to assume that electrons move in well defined orbits. This lead to the
introduction of the concept of probability in describing the atomic systems and give birth to a new model
known as Quantum Mechanical Model of an Atom.
What is the AufbauPrinciple?
The Aufbauprinciple dictates the manner in which electrons are filled in the atomic orbitals of an
atom in its ground state.
It states that electrons are filled into atomic orbitals in the increasing order of orbital energy level.
According to the Aufbauprinciple, the available atomic orbitals with the lowest energy levels are
occupied before those with higher energy levels.
The word ‘Aufbau’ has German roots and can be roughly translated as ‘construct’ or ‘build up’. A
diagram illustrating the order in which atomic orbitals are filled is provided below. Here, ‘n’ refers to
the principal quantum number and ‘l’ is the azimuthal quantum number.
What are Quantum Numbers?
The set of numbers used to describe the position and energy of the electron in an atom are called
quantum numbers. There are four quantum numbers, namely, principal, azimuthal, magnetic and
spin quantum numbers.

6. Four quantum numbers can be used to completely
describe all the attributes of a given electron
belonging to an atom, these are:
Principal quantum number, denoted by n.
Orbital angular momentum quantum number (or
azimuthal quantum number), denoted by l.
Magnetic quantum number, denoted by ml.
The electron spin quantum number, denoted by m
s
.

7. In order to simplify the details of the four different quantum numbers that are related to atomic
physics, a tabular column detailing their names, symbols, meanings, and possible values is
provided below.
It is important to note that it is impossible for two electrons of
the same atom to have exactly the same quantum state or
exactly the same values of the set of quantum numbers, as per
Hund’srules.

8. The Aufbauprinciple can be used to understand the location of electrons in an atom and their corresponding energy
levels. For example, carbon has 6 electrons and its electronic configuration is 1s22s22p2.
It is important to note that each orbital can hold a maximum of two electrons (as per the Pauli exclusion principle). Also,
the manner in which electrons are filled into orbitals in a single subshell must follow Hund’srule, i.e. every orbital in a
given subshell must be singly occupied by electrons before any two electrons pair up in an orbital.
The Pauli exclusion principle states that in a single atom, no two electrons will have an identical set or the same
quantum numbers (n, l, ml, and ms). To put it in simple terms, every electron should have or be in its own unique
state (singlet state). There are two salient rules that the Pauli exclusion principle follows:
Only two electrons can occupy the same orbital.
The two electrons that are present in the same orbital must have opposite spins, or they should be antiparallel.

9. Aufbauprinciple tells us that the lowest energy orbitals get filled by electrons first. After the lower energy orbitals are
filled, the electrons move on to higher energy orbitals. The problem with this rule is that it does not tell about the three 2p
orbitals and the order that they will be filled in.
According to Hund’srule:
Before the double occupation of any orbital, every orbital in the sub level is singly occupied.
For the maximization of total spin, all electrons in a single occupancy orbital have the same spin.
An electron will not pair with another electron in a half-filled orbital as it has the ability to fill all its orbitals with similar
energy. Many unpaired electrons are present in atoms which are at the ground state. If two electrons come in contact they
would show the same behaviouras two magnets do. The electrons first try to get as far away from each other as possible
before they have to pair up.
HundsRule of Maximum Multiplicity
HundsRule of Maximum Multiplicity rule states that for a given electron configuration, the term with maximum
multiplicity falls lowest in energy. According to this rule electron pairing in p, d and f orbitals cannot occur until each orbital
of a given subshell contains one electron each or is singly occupied.
State Hund’sRule
It states that:
1. In a sublevel, each orbital is singly occupied before it is doubly occupied.
2. The electrons present in singly occupied orbitals possess identical spin.

11. Salient Features of the AufbauPrinciple
According to the Aufbauprinciple, electrons first occupy those orbitals whose energy is the lowest. This implies that the
electrons enter the orbitals having higher energies only when orbitals with lower energies have been completely filled.
The order in which the energy of orbitals increases can be determined with the help of the (n+l) rule, where the sum of
the principal and azimuthal quantum numbers determines the energy level of the orbital.
Lower (n+l) values correspond to lower orbital energies. If two orbitals share equal (n+l) values, the orbital with the lower
n value is said to have lower energy associated with it.
The order in which the orbitals are filled with electrons is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s,5f, 6d,
7p, and so on.

12. Exceptions
The electron configuration of chromium is [Ar]3d54s1 and not [Ar]3d44s2 (as suggested by the Aufbauprinciple).
This exception is attributed to several factors such as the increased stability provided by half-filled subshells and the
relatively low energy gap between the 3d and the 4s subshells.
The energy gap between the different subshells is illustrated below.
Half filled subshells feature lower electron-electron repulsions in the
orbitals, thereby increasing the stability. Similarly, completely filled
subshells also increase the stability of the atom. Therefore, the
electron configurations of some atoms disobey the Aufbauprinciple
(depending on the energy gap between the orbitals).
For example, copper is another exception to this principle with an
electronic configuration corresponding to [Ar]3d104s1. This can be
explained by the stability provided by a completely filled 3d subshell.

13. Electronic Configuration using the AufbauPrinciple
Writing the Electron Configuration of Sulphur
The atomic number of sulphuris 16, implying that it holds a total of 16 electrons.
As per the Aufbauprinciple, two of these electrons are present in the 1s subshell, eight of them are present in the 2s
and 2p subshell, and the remaining are distributed into the 3s and 3p subshells.
Therefore, the electron configuration of sulphurcan be written as 1s22s22p63s23p4.
Writing the Electron Configuration of Nitrogen
The element nitrogen has 7 electrons (since its atomic number is 7).
The electrons are filled into the 1s, 2s, and 2p orbitals.
The electron configuration of nitrogen can be written as 1s22s22p3

14. The AufbauPrinciple
1. Electrons are placed in the lowest energetically
available subshell.
2. An orbital can hold at most 2 electrons.
3. If two or more energetically equivalent orbitals are
available (e.g., p, d etc.) then electrons should be spread out
before they are paired up (Hund'srule).