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Atoms are the building blocks of matter also known as the smallest unit of matter. The word atom comes from a Greek word meaning invisible. An atom is the smallest particle of the element which can take part in a chemical reaction. The parts of the periodic table are known as elements. It is made up of subatomic particles which are the electron, neutron & proton.

History of Atom

In the Past

  • Long back in India around 500bc an Indian philosopher Maharishi Kanad, explained that if we divide matter, we will get smaller and smaller particles.
  • Ultimately, a time will come will when we shall come to the smallest particle and further division would not be possible. He named these particles as ‘parmanu’.
  • TheIndian philosopher Pakundha Katyayama went a step further and proposed that the particles of matter i.e. parmanu normally exist in a combined form.
  • He also said that the combinations of particles give us various kind of matter.
  • Ancient Greek philosophers Democritus and Leucippus also explained the same theory and called these particles ‘Atom’ meaning invisible.

Dalton's Atomic Theory

John Dalton was born in England 1766 to a poor weaver’s family. He got his early education from his father and village school. He started teaching in the village at the age of 12. Dalton left for Manchester to teach physics, chemistry, and maths in a college in 1793. On 1808, Dalton presented his atomic theory to explain the properties of matter and atoms. 
His theory stated the following:
  • All the matter is made up of very small particles called ‘atoms’.
  • Atoms can neither be created nor destroyed.
  • Atoms cannot be divided.
  • All atoms of a particular element are same in every respect. They have the same mass, size, and chemical properties.
  • Atoms of different elements have different mass, size, and chemical properties.
  • Chemical combinations of atoms form molecules of compounds.
  • The number and kind of atoms in a compound are fixed.
  • Atoms of the same element can combine in more than one ration to form more than one compound.
  • The Dalton theory is based on-
  1. Law of conservation of mass as it states ‘atoms can neither be created nor destroyed’.
  2. Law of constant proportion as it states ‘the number and kind of atoms in a compound is fixed’.

Drawbacks of Dalton's Atomic Theory
  • Dalton’s theory included that atoms couldn’t be divided but under special, atoms can be further divided into smaller particles called electrons, protons, and neutrons.
  • His theory says that all atoms of an element have exactly same mass but atoms of the same element have slightly different masses.
  • His theory also explained that atoms of different elements can have the same mass.

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