1. Atomic Structure

Atomic Structures of Some Elements

The structure of atom of an element can be simply represented via the total number of protons, electrons, and neutrons present in it. The atomic structures of a few elements are illustrated below.


The most abundant isotope of hydrogen on the planet Earth is protium. The atomic number and the mass number of this isotope are 1 and 1, respectively.

Structure of Hydrogen atom: This implies that it contains one proton, one electron, and no neutrons ( total number of neutrons = mass number – atomic number)


Carbon has two stable isotopes – 12C and 13C. Of these isotopes, 12C has an abundance of 98.9%. It contains 6 protons, 6 electrons, and 6 neutrons.

Structure of Carbon atom: The electrons are distributed into two shells and the outermost shell (valence shell) has four electrons. The tetravalency of carbon enables it to form a variety of chemical bonds with various elements.


There exist three stable isotopes of oxygen – 18O, 17O, and 16O. However, oxygen-16 is the most abundant isotope.

Structure of Oxygen atom: Since the atomic number of this isotope is 8 and the mass number is 16, it consists of 8 protons and 8 neutrons. 6 out of the 8 electrons in an oxygen atom lie in the valence shell.

Bohr’s Atomic Theory

Neils Bohr put forth his model of the atom in the year 1915. This is the most widely used atomic model to describe the atomic structure of an element which is based on Planck’s theory of quantization.


  • The electrons inside atoms are placed in discrete orbits called “stationery orbits”.
  • The energy levels of these shells can be represented via quantum numbers.
  • Electrons can jump to higher levels by absorbing energy and move to lower energy levels by losing or emitting its energy.
  • As longs as, an electron stays in its own stationery, there will be no absorption or emission of energy.
  • Electrons revolve around the nucleus in these stationery orbits only.
  • The energy of the stationary orbits is quantized.

Limitations of Bohr’s Atomic Theory:

  • Bohr’s atomic structure works only for single electron species such as H, He+, Li2+, Be3+, ….
  • When the emission spectrum of hydrogen was observed under a more accurate spectrometer, each line spectrum was seen to be a combination of no of smaller discrete lines.
  • Both Stark and Zeeman effects couldn’t be explain using Bohr’s theory.

Heisenberg’s uncertainty principle: Heisenberg stated that no two conjugate physical quantities can be measured simultaneously with 100% accuracy. These will always be some error or uncertainty in the measurement.

Drawback: Position and momentum are two such conjugate quantities that were measured accurately by Bohr (theoretically).

Stark effect: Phenomenon of deflection of electrons in the presence of an electric field.

Zeeman effect: Phenomenon of deflection of electrons in the presence of a magnetic field.

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