The Atom consists of a nucleus as we saw in Rutherford’s experiment. In the nucleus, there is this densely populated pocket of positive charges but in this nucleus there exists not only positive charges, there are also some neutrally charged subatomic particles. And the negatively charged particles exist in a cloud surrounding the nucleus of the atom. As per the modern view of the atomic structure, an atom has no net electrical charge or net positive charge. Scientists have been able to work out the structure of an atom.

Structure of an atom

Atomic Structure centers primarily around three subatomic particles – electron, proton, and neutron. Each of the following subatomic particles together form the atom.

  1. Electrons are the negatively charged particles that exist in a cloud around the nucleus. 
  2. Protons are the positively charged particles found in the nucleus, and 
  3. Neutrons are the neutrally charged particles in the nucleus

Mass of Proton and neutron are about 1.67 * 10-27 kg but mass of electron is actually (orders of magnitude -> smaller and lighter than proton & neutron) about 9.11 * 10-31 kg. 

Particles

Charge

Mass (in Kg)

Electron

-1

9.11 * 10-31

Proton

+1

1.67 * 10-27

Neutron

0

1.67 * 10-27

 

Electrons do not contribute much in terms of mass but it takes up the majority of space. Hence, a lot of volume of an atom is taken up by these electrons existing in this cloud of charge surrounding a nucleus and a nucleus takes up very little space.

From Rutherford’s experiment, we saw that the nucleus of an atom is really dense and tightly packed so it does not take up much volume but it gives primarily the mass for the atom.

These three subatomic particles are very crucial to your understanding of the atoms, molecules and chemistry in general. 

Some important Definitions

  • Isotopes: These are the atoms of an element which have the same atomic number but they are different in their mass numbers.

                           Avg. atomic weight of an element=W1X1+W2X2+W3X3X1+X2+X3 

              Where,           W1X1+W2X2+W3X3 = mass number of isotopes

                                      X1+X2+X3 = possibility of occurrence in nature

  • Isobars: Given by Aston, Isobars are the atoms of different elements which have the same mass number but different atomic number.
  • Isodiaphers: They are the atoms of different elements which have the same difference of the number of neutrons & protons (isotopic numbers).
  • Isotones / Iso-neutronic Species: They are the atoms of different elements which have the same number of neutrons.

Bohr’s Atomic Model

In 1912, Danish physicist Niels Bohr published a groundbreaking atomic model. According to Bohr’s atomic model, an atom is neutral in terms of its electronic configuration. The number of protons are equal to the number of electrons present in an atom. 

  • He explained that the nucleus of an atom is surrounded by a number of shells and energy levels and named them as K, L, M, N,…..so on. 
  • As per this model, angular momentum of orbiting electrons is quantized and the possible orbits have :                              L= nh2π     

                        Where n = 1,2,3,4,5,….  &   h = Planck constant

  • He proposed that an electron in a higher energy level moves into a lower one by emitting a single photon.
  • It explained the atomic line spectra and predicted the wavelengths of hydrogen light.
  • In 1920s, it was established that a moving particle also has the properties of  a wave and its De Broglie wavelength ( ) is related to its momentum.

                      λ= hp                      or                 λ= hmu

Modern Periodic Table

The modern periodic table (also known as the long form of periodic table) is based on the modern periodic laws. The elements in the modern periodic table have been arranged in the increasing order of their atomic numbers in tabular form. It is the present form of the periodic table i.e. it is used in the current times in the field of science.  It comprises of :- 

  1. Groups: These are the vertical columns. There are a total of  18 groups. Each group comprises elements having the same outer shell electronic configuration.
  2. Periods: Periods are the horizontal rows. There are a total of 7 periods in the periodic table. Each period reflects the number of shells / energy levels present in the atom of an element.
  3. The Modern periodic table also has a separate panel at the bottom of it which comprises of 14 elements of the 6th period referred to as lanthanoids and 14 elements in the 7th period referred to as actinoids.

Conclusion

This article has summed up the fundamentals of an atom and its structure of modern view. Apart from Rutherford’s atomic model, Bohr’s atomic model explained important aspects of atomic structure and tried to resolve a few shortcomings of the Preceding atomic models and brought about a significant mark in the field of science. This article also covered the modern periodic table in which elements have been arranged on the basis of atomic numbers in increasing order.