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Atomic Theory


Dalton’s Atomic Theory

Every substance consists of small particles called atoms. Atom is an indivisible particle. All atoms of the same element have the same weight, size, and chemical properties. The atomic weights of different elements vary in size and chemical properties. In a chemical reaction, atoms are rearranged only. They can neither be created nor destroyed by chemical reactions.

Thomson’s Atomic Model 

The model of the first atomic structure was introduced in 1998 by Sir JJ Thomson. According to them, the atom is a positively charged ball of the size of 10 – 10 meters. In which an equal amount of negatively charged electrons are distributed. It is also called the plum pudding model. From this model, Thomson had clarified that the atom has the same amount of positive charge and negative charge, and the atoms are electrically neutral.

Rutherford’s Experiment

  1. Ernest Rutherford did an experiment with the bombardment of alpha particles on a very thin gold membrane in 1911.
  2. In this experiment, the nucleus of high-energy alpha particles helium was bombarded with a gold thin membrane thickness of 10 – 7 m or 100 nanometers.
  3. A circular curtain coated with zinc sulfide was placed around the membrane so that after bombardment the alpha particle collided with this screen to produce a flash.

Observations of this experiment 

  1. Most of the alpha particles escaped directly from the gold membrane without being deflected.
  2. Very few alpha particles were deflected by some angle.
  3. One of the approximately 20000 alpha particles was deflected with an angle of 180°.

Rutherford Experiment Findings

  1. The majority of the atom is hollow and chargeless, so most of the alpha particles are ejected directly. In this empty region, electrons move in a circular path. These circular paths are called orbits.
  2. Some alpha particles are deflected, meaning there must have a strong repulsion force. Therefore, all investments are concentrated in one place inside the atom.
  3. The volume of positive charge in an atom is much less than its total volume. This positively charged volume is called the nucleus. The diameter of an atom is about 10 to the power of 10 meters and the diameter of the nucleus is about 10 to the power of 15 meters.

Niels Bore’s Hypothesis

The following hypothesis is given by Niels Bore

  1. There is a nucleus in the center of the atom, in which there are positively charged protons.
  2. Electrons move around the nucleus in a path with a fixed radius and energy. These fixed energy paths are called the orbit, shell, or energy level.
  3. These orbits are arranged around the nucleus and centrally. These are denoted by n. Their values ​​are always integers such as 1,2,3,4, …. and are also represented by K, L, M, N, …. respectively. The maximum number of electrons in a shell is 2n2.
  4. As the value of n increases, the orbit moves away from the nucleus, and their energy increases. The energy of the n = 1 or K orbit is the lowest.
  5. According to Bore, there is no change in the energy of the electron when it rotates in a certain orbit.
  6. When the electron absorbs any kind of energy from outside the atom, it gets excited and moves to a higher energy level. If the electron emits energy, it moves from a higher energy level to lower energy level orbit.




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