Structure of Atom

• According to Dalton’s theory, matter is made up of an atom, and atoms are indivisible.
• However, many scientists proved that an atom is divisible and it consists of charged sub-atomic particles.
• So, numerous models of atoms were suggested.
• The modern model of the atom is based on Rutherford-Bohr model.
• It was suggested by Neil Bohr in 1913.
• He gave a planetary model of the atom and suggested that electrons are distributed in concentric circular orbits while nucleus is the center.
• Bohr and Bury gave the following rules for the distribution of electrons around the nucleus.
  • Electrons are revolving around the nucleus in different orbits or shells. These energy shells are represented by number 1, 2, 3, 4 or K, L, M, N.
  • The maximum number of electrons in any shell cannot exceed 2n², where n is the number of that energy level. Thus for
    • K – shell, n = 1, no. of electrons =2 x 1² = 2
    • L – shell, n = 2, no. of electrons =2 x 2² = 8
    • M – shell, n = 3, no. of electrons =2 x 3² = 18
    • N – shell, n = 4, no. of electrons =2 x 4² = 32
  • The maximum number of electrons that can be accommodated in the outermost orbit is 8. The next to the outermost shell (penultimate shell) can have at the most 18 electrons.
  • It is not absolutely necessary that an orbit has its full quota of electrons before starting to fill the next higher orbit but the shells are filled in a stepwise manner.

Consider the following examples.

Electronic configuration of Hydrogen:

Atomic number = 1

K – shell / 1^st Orbit: n = 1

Electronic configuration of Carbon:

Atomic number = 6

K – shell / 1^st Orbit = 2

L – shell / 2^nd Orbit = 4

Electronic configuration of Neon:

Atomic number = 10

K – shell / 1^st Orbit = 2

L – shell / 2^nd Orbit = 8

• Depending on the way in which electrons are distributed among orbitals the stability of the orbitals also varies.
• Elements are arranged in the periodic table based on their electronic structure.
• With such an arrangement on the basis of Electronic Configuration in Periods and Groups, elements with the same behavior can be identified easily.