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structure of atom 63 2 electrons in the orbitals belonging to the 1s orbital. Its configuration is, therefore, 1s . same subshell (p, d or f) does not take As mentioned above, the two electrons differ place until each orbital belonging to that from each other with opposite spin, as can be subshell has got one electron each i.e., it seen from the orbital diagram. is singly occupied. Since there are three p, five d and seven f orbitals, therefore, the pairing of electrons will The third electron of lithium (Li) is not start in the p, d and f orbitals with the entry allowed in the 1s orbital because of Pauli of 4th, 6th and 8th electron, respectively. It exclusion principle. It, therefore, takes the has been observed that half filled and fully next available choice, namely the 2s orbital. filled degenerate set of orbitals acquire extra The electronic configuration of Li is 1s 2s . 1 stability due to their symmetry (see Section, The 2s orbital can accommodate one more 2.6.7). electron. The configuration of beryllium (Be) atom is, therefore, 1s 2s (see Table 2.6, 2.6.5 Electronic Configuration of Atoms The distribution of electrons into orbitals of an page 66 for the electronic configurations of elements). atom is called its electronic configuration. If one keeps in mind the basic rules which In the next six elements—boron govern the filling of different atomic orbitals, (B, 1s 2s 2p ), carbon (C, 1s 2s 2p ), nitrogen (N, 1s 2s 2p ), oxygen (O, 1s 2s 2p ), fluorine the electronic configurations of different 2 2 5 2 2 6 atoms can be written very easily. (F, 1s 2s 2p ) and neon (Ne, 1s 2s 2p ), the 2p orbitals get progressively filled. This process The electronic configuration of different atoms can be represented in two ways. For is completed with the neon atom. The orbital picture of these elements can be represented example : as follows : (i) sap d ...... notation (ii) Orbital diagram s p d In the first notation, the subshell is represented by the respective letter symbol and the number of electrons present in the subshell is depicted, as the super script, like a, b, c, ... etc. The similar subshell represented for different shells is differentiated by writing the principal quantum number before the respective subshell. In the second notation each orbital of the subshell is represented by a box and the electron is represented by The electronic configuration of the an arrow (↑) a positive spin or an arrow (↓) 2 2 6 1 a negative spin. The advantage of second elements sodium (Na,1s 2s 2p 3s ) to argon notation over the first is that it represents all (Ar,1s 2s 2p 3s 3p ), follow exactly the same the four quantum numbers. pattern as the elements from lithium to neon The hydrogen atom has only one electron with the difference that the 3s and 3p orbitals are getting filled now. This process can be whichgoesintheorbitalwiththelowestenergy, namely 1s. The electronic configuration of the simplified if we represent the total number hydrogen atom is 1s meaning that it has of electrons in the first two shells by the name of element neon (Ne). The electronic one electron in the 1s orbital. The second electron in helium (He) can also occupy the configuration of the elements from sodium to Reprint 2026-27