Structure of Atom

An atom cosists of a small positively charged heavy nucleus, with negatively charged plight electrons revolving around the nucleus in circular paths called orbits. Atom is made up of three fundamental particles:

  1. Electron
  2. Proton
  3. Neutron


Nutrones and protons are inside the nucleus whereas electrons are found outside the nucleus.


Proton is a positively charged particle having a charge of +1.6 × 10-19 coulombs. The mass of a proton is about 1836 electrons, the relative mass of a proton is 1 a.m.u(1 atomic mass unit), which is the same as that of a hydrogen atom. The hydrogen atom have the lowest mass.


Electron is a negatively charged particle having an absolute charge of -1.6 × 10-19 coulombs. A normal atom has an equal number of protons and electrons and since a proton and an electrons and since a proton and an electon have equal and opposite charge, an atom on the whole is electrically neutral.


Neutron is an uncharged particle or neutral particle which is present in the nucleus of an atom. The mass of a neutron is the same as that of proton i.e. 1amu. The relative charge of a neutron is zero

Atomic Number

The number of protons present in the nucleus of an atom of an element is called its atomic number. For example, if an element has 2 protons in the nucleus of its atom, then its atomic number is 2.

Mass Number

The total number of protons and neutrons present in the nucleus of one atom of an element is called its mass number.
Mass Number = No. of Protons + No. of Neutrons.


All the atoms of an elements contain the same number of protons but they may have different number of neutrons. The atoms of an element having the same atomic number but different mass numbers are called isotopes.


The heavy elements like uranium, thorium and radium disintigrate themselves in an uncontrollable manner and emit alpha, beta particles and gamma rays. This phenomenon of disintigration is known as radioactivity.

Chemical Reactions

In a chemical reactions, only the outermost electrons of the atoms takes part, the nuclei of the atoms remain unaffected.

Nuclear Reactions

In nuclear reactions, the nucleus of an atom undergoes a chang forming new atoms and releasing a tremendous amount of energy. New atoms can be produced in a nuclear reaction, which is not possible in the case of a chemical raction. The first ever nuclear reaction was carried out by Tutherford in 1919.

Transuranic Element

The elements above atomic number 92(i.e of uranium) are called as transuranic elements.

Forces insde the Nucleus

It is found that the nuclei of small atoms are quite stable but the nuclei of big atoms are unstable. There are two type of forces operating insde the nucleus of an atom. These are:

  1. Electrostatic Force: The force which a charged particle exerts on another charged particle is called electrostatic force. The electrostatic force may cause attraction or repulsion depending upon the nature of charges of particle. If two particle have opposite charges then the electrostatic force cause attraction between them but they have similar charges then the electrostatic force will be repulsive.
  2. Nuclear Force: TThe force with which two protons, two neutrons or a proton and a neutron attract one another in the nucleus is called nuclear force(Strong force)


Nuclear Fission

An atom contains protons and neutrons in its central nucleus. In fission, the nucleus splits, either through radioactive decay or because it has been bombarded by other subatomic particles known as neutrinos. The resulting pieces have less combined mass than the original nucleus, with the missing mass converted into nuclear energy. This process is known as nuclear fission

Nuclear Fusion

In nuclear reaction two or more atomic nuclei come very close and then collide at a very high speed and join to form a new nucleus. During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy). This process is known as nuclear Fusion

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