Until World War II, the world was familiar with only mechanical, chemical, acoustic, thermal, optic, magnetic and electrical forms of energy. But on a fateful day in 1945 when the U.S. exploded atom bomb over Hiroshima, the world got aware of nuclear energy.
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When towards the end of the Second World War the first atomic bomb blasted out of existence the peaceful and prosperous city of Hiroshima, and (6th August, 1945) without warning, the conscience of the world was stirred to its depth.
The most important use of nuclear energy is to generate electricity. Three per cent of the energy produced in India is obtained from nuclear reactors. Bhabha Atomic Research Centre (BARC) is the centre for research and development work in field of atomic energy of India.
The number of protons present in the nucleus of an atom of the element is called atomic number of the element. Since all atoms of a particular element have same number of protons in their nuclei, the atomic number of an element is characteristic of the element.
In 1913, Mosley introduced an atomic parameter called atomic number. Atomic number of an element is denoted by with, and can have only an integral value.
Various postulates of Bohr’s atomic model are: 1. In an atom, the electrons revolve around the nucleus in certain definite circular paths called orbits, or shells. 2. Each shell or orbit corresponds to a definite energy. Therefore, these circular orbits are also known as energy levels or energy shells.
Variations in the atomic properties of elements in any period are discussed below. Atomic number: In a period, the elements are arranged in the order of increasing atomic number. The atomic number increases by one unit atomic each element from left to right in a period.
All elements in a group generally show the same valence. From the values given in the above table, the valence of the main group elements is either equal to the number of valence electrons or 8-number of valence electrons.
In 1803, John Dalton, an English school teacher, proposed the Atomic theory of matter based on his experimental observations. The main postulates of Dalton’s atomic theory are as follows.
According to Dalton’s atomic theory, an atom is the ultimate, discrete and indivisible particle of matter. Later researches proved that Dalton’s atomic theory was not wholly correct.