Thus, the two electrons in the carbon 2 p orbitals have identical n, l, and m s quantum numbers and differ in their m l quantum number (in accord with the Pauli exclusion principle).

As per the principle electrons are filled in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s. You can calculate the orbital energy by the sum of azimuthal and the principal quantum number. The orbitals are filled as described by Hund’s rule: the lowest-energy configuration for an atom with electrons within a set of degenerate orbitals is that having the maximum number of unpaired electrons. According to the Aufbau Principle in NCERT Book In the ground state of the atoms, the orbitals are filled in order of their increasing energies. We now have a choice of filling one of the 2 p orbitals and pairing the electrons or of leaving the electrons unpaired in two different, but degenerate, p orbitals. The remaining two electrons occupy the 2 p subshell. Four of them fill the 1 s and 2 s orbitals. When drawing orbital diagrams, we include empty boxes to depict any empty orbitals in the same subshell that we are filling.Ĭarbon (atomic number 6) has six electrons. There are three degenerate 2 p orbitals ( m l = −1, 0, +1) and the electron can occupy any one of these p orbitals. Because any s subshell can contain only two electrons, the fifth electron must occupy the next energy level, which will be a 2 p orbital.

The n = 1 shell is filled with two electrons and three electrons will occupy the n = 2 shell. The fourth electron fills the remaining space in the 2 s orbital.Īn atom of boron (atomic number 5) contains five electrons. Thus, the electron configuration and orbital diagram of lithium are:Īn atom of the alkaline earth metal beryllium, with an atomic number of 4, contains four protons in the nucleus and four electrons surrounding the nucleus.

Aufbau principle order