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Dalton’s Atomic Theory

Dalton’s atomic theory states that:
  1. All matter is made up of small indivisible particles called atoms.
  2. Atoms can neither be created nor be destroyed.
  3. Atoms of an element are identical, but they differ from the atoms of other elements.
  4. A chemical combination takes place between small and whole numbers of atoms.
Dalton’s atomic theory gave an elementary understanding of the differences between elements and compounds and also about the formation of chemical compounds.
Although Dalton had postulated that atoms were indivisible particles, experiments conducted around the beginning of last century showed that atoms themselves consist of particles. These experiments showed that an atom consists of three kinds of subatomic particles, namely positively charged proton, neutron with no charge and negatively charged electron. Proton and neutron constitute the nucleus of an atom. The nucleus is a dense core at the centre of an atom which is responsible for the entire mass of the atom. An electron has negligible mass with equal and opposite charge to that of a proton. It is present in the region around the nucleus. The nucleus is positively charged and is known as nuclear part.
Table gives a brief description of the subatomic particles.
Table: Characteristics of the Fundamental Particles of an Atom
  Electron Proton Neutron
Symbol 0–1 e 11 p 10 n
Year of discovery 1897 1920 1932
Scientist J. J. Thomson Rutherford James Chadwick
Experiment Cathode ray tube experiment Canal ray tube experiment Bombarded beryllium atom with α-particles
Nature Negatively charged Positively charged Neutral
Relative charge –1 1 0
Absolute charge 1.602 × 10–19 C 1.602 × 10–19 C 0
Relative mass Description: 23205.png 1 1
Absolute mass 9.109 × 10–28 g 1.67 × 10–24 g 1.67 × 10–24 g
  • An atom is electrically neutral. Hence, the number of protons is equal to the number of electrons in an atom.
  • Protons and neutrons that constitute the nucleus carry the entire mass of an atom.
  • Negatively charged electrons are present in the extra-nuclear part of an atom and are distributed in different shells, subshells and orbitals.

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