Structure of Atom


An atom is the smallest particle that cannot be redivided further. Atom is the foundation of every object that we use today and no wonder, the atom is the most invisible but powerful unit every substance. An atom in an inanimate object is compared to a cell of a living entity hence it is the lifeblood of science. Furthermore, atomic force is  one of the strongest and  destructive force because of the release of nuclear energy during its fission. But, we wonder how can such a smaller unit result in the most devastating nuclear energy? What makes an atom to release such a lethal force? What is an atom and how atoms are arranged and what is it made up of? These questions are beyond the scope of  this topic now. Nevertheless, let us understand the basic concepts of the atom such as atomic number, and different models explaining the atom. 


Atomic number

“Atomic Number represents the total number of protons in an atom of an element”. For example, aluminium has atomic number 13 as it has 13 protons. The number of protons in an atom of an element is very crucial as it is used to distinguish one atom from another. In any given neutral atom, the number of protons is equivalent to the number of electrons because the two have opposite charges that keep the atom in a neutral state. It is fair to say, Atomic number= number of protons= number of electrons.



Isotopes refer to atoms of the same element having the same atomic number but different mass numbers. For example, neon exists naturally as Ne-11, Ne -21 and Ne-22. Isotopes can either exist naturally or artificially. The results obtained from the spectrometer study showed that  RAM of the elements are varied. The results contradicted with one of Dalton’s proposals on the atomic theory. Furthermore, the results that some atoms of elements had the same atomic number. These atoms, however, had different mass numbers due to the difference in the number of neutrons in the nucleus of an element. 



The atom of different elements may contain the same mass number but different atomic number. Isobars are the atoms or nuclides of 2 or more different chemical elements that have the same number of nucleons (protons+ neutrons). Alfred Walter Stewart has found the isotopes for the first time in the year 1918. In the Greek language- isos means equal and bar means weight. Isobars vary each other. This is because of the difference in atomic numbers but the number of neutrons makes up the difference in the number of nucleons. As they vary in their chemical elements, they exhibit different chemical properties. Some of the examples are

  1. 32 Ce7634Se76

  2.  40S40Cl40Ar40K, and 40Ca

  3. 18Ar40 19K40 20Ca40



Introduction to Atomic models and theories

The atomic theory states that every matter composed of discrete, unique components called atoms. Atoms are the reason behind the existence of matter presenting a  unique structure, functions, and behaviour of an underlying matter.  The concept of atoms began as a philosophical concept in Ancient Greece and revolutionized into a science in the early 19th century. Some of the most common theories of atomic models are as follows.

  1. Thomson model (Plum pudding model)

  2. Dalton model (Billiard ball model)

  3. Lewis model (Cubical atom model)

  4. Nagaoka model (Saturnian model)

  5. Rutherford model (Planetary model)

  6. Bohr model (Rutherford–Bohr model)

  7. Bohr–Sommerfeld model (Refined Bohr model)

  8. Gryziński model (Free-fall model)


Thomson’s model and its limitations

An English physicist J.J. Thomson suggested that an atom of an element contains protons which have a positive charge in a spherical model and negatively charged electrons that embed inside the orbital.



Thomson’s Atomic Model- Postulates

According to the postulates of Thomson’s atomic model, an atom resembles a sphere of positive charge with electrons (negatively charged particles) present inside the sphere. Thomson’s model is also known as the plum-pudding model and it is called so because, the plum pudding model has electrons surrounded by a volume of positive charge, like negatively-charged "plums" embedded in a positively-charged "pudding". The model has also been compared to a watermelon as the red-coloured edible part of a watermelon looked like a sphere of an atom having a positive charge and the black seeds around the area looked similar to the electrons inside the sphere. His theory was opposed because it failed to explain how alpha particles scattered using a thin metal foil.


Rutherford’s model and its limitations

Rutherford’s conducted an experiment by allowing the with α-particles to bombard against a thin sheet of gold. He observed the trajectory of these particles after their interaction with the gold foil. According to Rutherford, an atom of an element is made up of the nucleus which is concentrated in a small volume. The composition of the nucleus includes the positively charged protons and neutrons which don't have the charge. Rutherford assumed that the remaining part of the atom is empty. He further suggested that the electrons in the atom always revolve around the nucleus and both are supported by the electrostatic forces of attraction.

Rutherford has observed that:

  1.  A major fraction of the α-particles sheet passed through the gold sheet without any deflection, and hence most of the space in an atom is empty.

  2. With a small angle, some of the α-particles were deflected by the gold so the positive charge in an atom is not uniformly distributed. The positive charge in an atom is concentrated in a very small volume.

  3. Very few of the α-particles were deflected back, because these  α-particles had nearly 180o angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.


Limitations of the Rutherford model

Rutherford had proposed that the negatively charged electrons in an atom of an element accelerate as it orbits around the nucleus emits radiations. However, this is not the case always. According to his postulations, no information is given concerning the distribution of negatively charged electrons around the nucleus and also their corresponding energies.


Bohr’s Model and its limitations

The theory was put forth by Niels Bohr in the year 1913. Bohr was Rutherford's product and came up with an advanced model of the structure of the atom. According to Bohr, the single electron in a hydrogen atom is capable of orbiting around the nucleus with uniform energy and from a fixed point which he referred as the radius. Bohr called the path is moved by the electron's energy levels. He further stated that the amount of energy of an electron orbiting around the nucleus is always constant and the atom neither loses nor gains energy during orbiting. From the above suggestions, it is now possible to compute the frequency of a given radiation as follows:

where h is the plank’s constant.  V is the frequency of the radiation and   Eand Eare higher and lower energy states respectively. It should be noted that an electron can only move to energy levels that have angular momentum which is a multiple of   This can mathematically be expressed as:

Mvr = n. h/2l.

Where n= 1, 2, 3……


Limitations of Bohr’s Model

  1. The theory didn't account for the splitting of the line in the spectrum in the hydrogen atom.

  2. The theory was biased since it didn't explain the spectrum of atoms with two or more atoms.

  3. It was unable to account for Zeeman effect (splitting of the spectral line in the presence of electronic field)

  4. The theory also failed to account for stark effect (splitting of the spectral line in the presence of an electric effect)

  5.  Bohr didn't explain what happens during the formation of molecules through chemical bonding.



Review  question

  1. Atomic number= number of protons is this statement true.
  2. Who proposed atomic theory?
  3. What is Plum pudding model?
  4. What is an isobar? Give an example?
  5. Explain Rutherford’s model
  6. Distinguish between a neutron and a proton
  7. Which are the 2 elements in the periodic table has the most number of isotopes?


Solved questions 

1. Naturally occurring boron exists as two isotopes, B-10 with a relative abundance of 20% and B-11 with a relative abundance of 80%. How many electrons does each of the boron atoms contain?

Ans; Boron -10        Atomic number=4

Electrons =10-4  = 6 electrons

2. An atom is identified as platinum-195. What does the number represent?

Ans: The mass number of platinum.





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