Basic Elements of Atomic Structure in the Structure of Matter:

An atom is the fundamental unit of matter, consisting of a nucleus composed of protons and neutrons, surrounded by electrons in orbit.The study of atoms is fundamental to chemistry and physics, providing insights into the structure, behavior, and interactions that govern the properties of matter, particularly the intricate details of atomic structure.

Atoms: Fundamental Building Blocks of our world:

• Atoms are the fundamental building blocks of all matter. 
• The existence of different kinds of matter is due to different atoms constituting them. 
• Our entire world is made up of atoms. 

What Does Atomic structure mean in Nanometer dimensions?

• Atoms are very small, they are smaller than anything that we can imagine or compare with in the realm of atomic structure. 
• Radius: Atomic radius is measured in nanometers.

1/109m = 1 nm 

1m = 109  nm

How did the Discovery of Electrons and Protons shape Atomic Structure?

• The discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of Dalton’s atomic theory. 
• Various Models: Many scientists proposed various atomic models to explain how electrons and protons are arranged within an atom, emphasizing the intricate nature of atomic structure.

Thomson Pudding Model and the Foundations of Atomic Structure

• Pudding Model and the Atomic Structure Analogy: Thomson proposed the model of an atom to be similar to that of a Christmas pudding. 
  • The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding,  illustrating the distribution within the atomic structure. 
• Thomson’s Atomic Proposal: Balanced charges and electric neutrality of the Atom: An atom consists of a positively charged sphere and the electrons are embedded in it. 
• The negative and positive charges are equal in magnitude. 
• So, the atom as a whole is electrically neutral. 

Rutherford’s Model of an Atom: Nucleus and Electron Paths in Atomic Structure:

• Rutherford’s Electron Experiment: Atomic Arrangement: Ernest Rutherford designed an experiment to know the arrangement of electrons within the atom. 

• Use of α- Particle: Rutherford’s Gold Foil Experiment: In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil. 
  • Selection of Gold Foil: He selected a gold foil because he wanted as thin a layer as possible and this gold foil was about 1000 atoms thick. 
  • About α-particles: These particles are doubly-charged helium ions. 
    • Since they have a mass of 4u, the fast-moving α-particles have a considerable amount of energy. 
  • Deflection: It was expected that α-particles would be deflected by the subatomic particles in the gold atoms. 
• Deviated Result in Rutherford’s Alpha-Particle scattering experiment: The α-particle scattering experiment gave totally unexpected results. 
  • Most of the fast moving α-particles passed straight through the gold foil. 
  • Some of the α-particles were deflected by the foil by small angles. 
  • One out of every 12000 particles appeared to rebound. 
• Observations: Atom’s Empty Spaces and Concentrated Positive Charge: Rutherford concluded from the α-particle scattering experiment that– 
  • Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected. 
  • Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space. 
  • A very small fraction of α-particles were deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom. 

• Calculating Nucleus Radius:: He also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.
• Nuclear Model of an Atom and the Central role of the nucleus: On the basis of his experiment, he put forward the Nuclear Model of an Atom.
  • There is a positively charged centre in an atom called the nucleus. 
  • Nearly all the mass of an atom resides in the nucleus. 
  • The electrons revolve around the nucleus in circular paths. 
  • The size of the nucleus is very small as compared to the size of the atom. 
• Drawbacks of Rutherford’s model: Stable Reality of Atoms: The revolution of the electron in a circular orbit is not expected to be stable. 
  • Any particle in a circular orbit would undergo acceleration and would radiate energy. 
  • Thus, the revolving electron would lose energy and finally fall into the nucleus. 
  • If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. 
    • Atoms are quite stable.

How did Bohr reshape Atomic Structure with Discrete Electron Orbits and Energy Levels?

• Overcoming Objections with Key Postulates: In order to overcome the objections raised against Rutherford’s model of the atom, Neils Bohr put forward the following postulates: 
  • Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom. 
  • While revolving in discrete orbits the electrons do not radiate energy, a crucial aspect of atomic structure stability.. 
• These orbits or shells are called energy levels . 
• These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,…., illustrating the hierarchical arrangement in the atomic structure.

J. Chadwick Neutron: Bridging the Mass Balance in Atomic Structure

• Chadwick’s Neutron Discovery: Neutral Force in Atomic Structure: In 1932, J. Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton named as neutron (represented as ‘n’).
• Elemental Presence, Excluding Hydrogen: Neutrons are present in the nucleus of all atoms, except hydrogen. 
• Mass Calculation in Atoms: The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.