NCERT Gist: Structure of The Atom | Additional Study Material for UPSC PDF Download

- An atom is divisible and consists of charged particles.

Canal rays

- Were positively charged radiations which ultimately led to the discovery of another sub-atomic particle.
- The mass of a proton is taken as one unit and its charge as plus one.
- The mass of an electron is considered to be negligible and its charge is minus one.
- An atom was composed of protons and electrons, mutually balancing their charges.
- Also appeared that the protons were in the interior of the atom, for whereas electrons could easily be peeled off but not protons.
- Neutrons are present in the nucleus of all atoms, except hydrogen. In general, a neutron is represented as - n.
- The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.
- The maximum number of electrons that can be accommodated in the outermost orbit is 8.
- The electrons present in the outermost shell of an atom are known as the valence electrons.
- An outermost-shell, which had eight electrons was said to possess an octet.
- Atoms would thus react, so as to achieve an octet in the outermost shell.
- Done by sharing, gaining or losing electrons.
- The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element, that is, the valency.
- The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.
- Isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers.
- There are three isotopes of hydrogen atom, namely protium, deuterium and tritium.
- Many elements consist of a mixture of isotopes.
- Each isotope of an element is a pure substance. The chemical properties of isotopes are similar but their physical properties are different, (i) An isotope of uranium is used as a fuel in nuclear reactors, (ii) An isotope of cobalt is used in the treatment of cancer, (iii) An isotope of iodine is used in the treatment of goitre.
- Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.