BSc1Year Atomic Structure

Atomic Structure

Atomic Structure


          In Greek Atom means not divided. In 1803 john Dalton made a famous theory atomic theory according to this theory “matter is made up of tiny particles called atoms.” But in 1897 Sir J.J. Thomson proved experimentally that atoms are made of charged particles called electrons. And in the beginning of 20th century many scientists like Lord Rutherford, E. Goldstein proved that atom is not a smallest particle but is made up of electron, protons and neutrons.

Philosopher Scientists and their Ideas about Matter & Atom

1. Democritus

Ancient Greek Ideas About Matter



Democritus (460-370 B.C.)

Figure 1


·         The ancient philosopher Democritus (ca 460–370 BC) was one of the first to propose that matter is constructed of tiny indivisible particles known as atomos (or atoms).


·         Democritus also proposed that matter vary in shape and size depending on the substance they compose.


·         According to Democritus, atoms could not be created nor destroyed.


 2. Aristotle

Ancient Greek Ideas About Matter



Aristotle (384-322 B.C.)

Figure 2

·         Democritus’ ideas about atoms were opposed by more popular philosophers, Aristotle (384–322 B.C.).


·         He opposed the idea that atoms moved through empty space.


·         According to Aristole, matter was made of four elements namely:


1.      Earth

2.      Wind

3.      Water

4.      Fire



3. John Dalton


  • In the nineteenth century John Dalton (1766–1844), marks the beginning of the progress of modern atomic theory.


  • John Dalton challenges the Aristotelian theory. Dalton revived and revised Democritus’s ideas based on the results of scientific research he conducted.


  • The ideas of Democritus’s and Dalton’s were similar. Dalton carried out a number of experiments that allowed him to refine and support his hypotheses.


  • Dalton carried out numerous chemicals reactions where he was able to determine the mass ratios of different elements involves in the chemical reactions.


  • The outcomes of his research are known as Dalton’s atomic theory, which he proposed in 1803.


  • Based on Dalton’s atomic theory, John Dalton calculated the first relative weights of atoms.


  • He assessed the atomic weights of some elements according to the mass ratios in which they combined; with the hydrogen atom taken as unity.


Figure 3


Dalton’s Atomic Theory

Dalton’s Atomic Theory



Dalton (1766-1844)

Figure 4

1.      Elements consist of extremely small particles called atoms.


2.      All elements of a given element are identical, having the same size, mass and chemical properties.



3.      Compounds are composed of more than one element. In any compound the number of atoms of any two of the elements present is either an integer or a simple fraction.


4.      In a chemical reaction, atoms are separated, combined or rearranged.




Postulates of the Atomic Theory

Dalton expressed his theory in a series of postulates. He integrated the ideas of others into his own.

  1. Matter consists of atoms, which cannot be divided into simpler element. Therefore atoms can neither be created nor destroyed.
  • Atoms of one element cannot be transformed into other atoms of an element. In chemical reactions, the atoms of an element can combine with same or other atoms of an element to form new substances.
  • Atoms of an element have mass, physical and chemical properties and are different from atoms of any other element.

Compounds are formed from a chemical reaction of a specific ratio of atoms of different elements.

How the Theory Explains the Mass Laws

Mass conservation

According to postulate 1 and 2, atoms cannot be created, destroyed and converted into other types of atoms. This means that in a chemical reaction, in which different atoms of an element are combined together cannot result in a mass change.

Definite composition

According to postulate 3 and 4, a compound composed of a specific ratio of different atoms, each of which has a particular mass. Therefore, each element in a compound contributes to a fixed fraction of the total mass.

Multiple proportions

According to postulate 1 and 3, atoms of an element have the same mass and are invisible. For instance, the number of atoms of any two of the elements present is either an integer or a simple fraction.

Nuclear Atom Model

The electron (Discovery of the Electron and Its Properties)

  • In 1855, Sir William Crookes carried out a series on experiment to investigate the behaviour of heated metals in a vacuum. The experiments showed that a heated cathode produced radiation, which could make substances to emit light.
  • The radiation emitted from the cathode is called cathode rays.
  • According to some research, it was known that cathode rays could be deflected by a magnetic field and electric field, and they carried a negative charge (the cathode is the negatively charged electrode and because the beam originated from the cathode, it must therefore be negatively charged).

The cathode-ray tube

  • In 1897, Joseph J. Thomson demonstrated that both the beam and the charged particles could be bent by an electrical field is applied perpendicular to the path of the beam, as shown in Figure 5.
  • Thomson used the cathode-ray tube to show the deflection of electron by an applied electric field.
  • When the cathode is heated, cathode rays is produced which travel along the tube and hit the phosphor-coated end of the tube and emit a glowing spot of light (Figure 5).
  • The rays produced at the negative electrode (cathode) and moved to the positive electrode (anode).
  • It was concluded that cathode rays consist of negatively charged particles found in all matter.
  • By varying the electric field strength and measuring the angle of deflection, Thomson was able to determine the charge-to-mass (e/m) ratio of the particles, which are known as electrons.
  • Thomson measured the e/m ratio as −1.76 × 108 C/g.
  • According to Thomson, the e/m ratio is larger than the one expected compared to the atomic weights of the lightest of atoms. Therefore this indicates that the negatively charged electrons must be much smaller in size than a typical atom.
  • As a result of his experiment, Thomson proposed the plum pudding model of the atom, where the atom consisted of one or more of these tiny electrons distributed in a sea of positive charge.
The cathode-ray tube - Atomic Structure BSc 1st Year Chemistry Notes in English Hindi pdf
Figure 5 : The cathode-ray tube – Atomic Structure BSc 1st Year Chemistry Notes

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