Atomic Theory

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In Science, matter refers to a substance that has mass and occupies space. Matter is everything around us. All physical objects are composed of matter. The atomic theory explains the nature of matter, which states that matter is composed to tiny units known as atoms. The atomic theory has undergone several variations and developments by various scientists over time, thereby upgrading and shaping our current understanding of the concept. Let us study some of the atomic theories.

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  1. Democritus’ Theory – Around 400 BCE, the Greek philosopher introduced the idea of atom as the basic building block of matter. He named them atomos meaning ‘not to be cut’. As per him, atoms are tiny, hard, and indivisible particles moving around randomly in matter surrounded by empty spaces.
  1. Dalton’s Theory – In 1808, English chemist John Dalton proposed a theory of atoms was based on actual observation. He incorporated the concept of Elements into the Atomic theory. (An element is a pure substance that is made from a single type of atom. Examples of elements include iron, oxygen, hydrogen, gold, iron, etc.)

Dalton’s atomic theory states that –

  1. Elements are made of small particles called atoms.
  2. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.
  3. Atoms cannot be subdivided, created, or destroyed.
  4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
  5. In chemical reactions, atoms are combined, separated or rearranged.

 

For e.g., as per his theory, water is formed when hydrogen and oxygen combine as below-

  1. J.J. Thompson’s theory – English physicist proposed the plum pudding model of the atom in 1904, after the discovery of electrons in 1897. His model was based on his findings that stated that an atom has a positively charged spherical structure in which negatively charged electrons are studded like fruits in a plum pudding. He also said that the amount of positive charge of the sphere is equal to the amount of negative charges of the electrons.
  1. Rutherford’s hypothesis – In 1911, New Zealand-born physicist Ernest Rutherford did an experiment to test the plum pudding model along with his students Hans Geiger and Ernest Marsden. They directed a beam of alpha particles (a type of positively charged radiation) at a thin gold foil. Most of the radiations passed through the foil, a very few particles were deflected by large angles, and fewer came straight back off the foil at 180 degrees.

They concluded that –

  • Most of the space inside an atom is empty.
  • There is a concentration of positive charge in the atom (since like charges repel).
  • The mass of the atom and its positive charge is concentrated in a tiny volume (that is the nucleus).
  1. Bohr’s Theory – In 1913, Neils Bohr proposed that an atom is a system consisting of a small dense positively charged nucleus surrounded by negatively charged electrons orbiting around it. This structure is similar to the solar system. It says that an electron revolves in a definite circular orbit around the nucleus that has a set size and energy. Radiation is absorbed or emitted when an electron moves from one orbit to another.

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DISCOVERY OF PROTONS AND NEUTRONS

In 1917, Ernest Rutherford discovered the positively charged particles known as protons and also suggested the presence of neutral particles in the nuclei of atoms in 1920. But neutrons were discovered eventually by Sir James Chadwick in the year 1932.

6. MODERN ATOMIC THEORY

The modern atomic theory states that atoms of one element are the same, while atoms of different elements are different. The fundamental characteristic that all the atoms of the same element share are their number of protons. Some of the key concepts of the Modern Atomic Theory are –

  • ATOMIC NUMBER – The number of protons in an atom of an element is its atomic number. For e.g. the Atomic number of hydrogen element is 1, the Atomic Number of carbon is 6 and the Atomic number of oxygen is 8.
  • MASS NUMBER – The sum total of the number of protons and neutrons in the nucleus is called the mass number of the atom. Usually, the number of electrons in an atom is the same as the number of protons.
  • ATOMIC SYMBOLS – Every element has been given a symbol for the sake of simplifying their representation. It consists of one or two letters abbreviated from their names, the first letter is in the capital and the second in lower case. For example:

PERIODIC TABLE –

 It is a special chart in which elements are grouped and placed together based on the ascending order of their atomic numbers. Try to spot Carbon (C), Oxygen (O), and Iron (Fe).

The Modern Atomic Theory can be summarized as –

  1. All matter is composed of atoms.
  2. All atoms are composed of protons, neutrons and electrons.
  3. 3. Each element has its own atomic number which is equal to the number of protons in its nucleus.
  4. 4. Elements are represented by an atomic symbol.
  5. 5. The periodic table is a chart that organizes all the known elements.

Check Point

  1. An ___________is a pure substance that is made from a single type of atom.
  2. The positively charges particles in an atom are found in the nucleus and are known as _____.
  3. The electrons revolve around the nucleus in an atom in fixed paths known as _______.
  4. J.J. Thompson’s model of an atom that stated that electrons are scattered in a positively charged sphere was known as the __________ model.
  5. The number of protons in an atom of an element is its _____________.

Answer Key

  1. Element
  2. Protons
  3. Orbits
  4. Plum Pudding
  5. Atomic number

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