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Atomic structure

– An element is defined as a substance which cannot be decomposed into other substances. The smallest particle of an element which takes part in chemical reaction is known as atom.

– All matter is composed of atoms which are infinitesimally small.

– All atoms are made of electrons, protons and neutrons. Most solid materials are classed,

from the stand point of electrical conductivity, as conductors, semiconductors or Insulators .To be conductor, the substance must contain some mobile electrons that can move freely between atoms. These free electrons come only from the valence (outer) orbit of the atom Conductivity depends on the number of electrons in the valence orbit.


“The energy level of an electron increases as its distance from the nucleus increases. Thus an electron in the second orbit possesses more energy than electron in the first orbit, electrons in the third orbit have higher energy than in the second orbit and so on. It follows, therefore, that electrons in the last orbit will possess very high energy. These high energy electrons are less bound to the nucleus and hence they are more mobile. It is the mobility of last orbit electrons that they acquire the property of combining with other atoms. Further due to this combining power of last orbit electrons of an atom they are called valence electrons”.


  • Atoms with fewer than four valence electrons are good conductors.
  • Atoms with more than four valence electrons are poor conductors.
  • Atoms with four valence electrons are semiconductors.

 Important data of an atom :

(i) Electron

Mass of an electron     = 9.11 x 10 -31 kg

= 1 / 1840 mass of proton

Charge of electron         = – 1.602 x 10 -19 coulomb

Diameter of an electron = 10 -15 m

(ii) Proton                  

Mass of proton            = 1.67 x 10 -27 kg

Charge on proton       = + 1.602 x 10 19 coulomb


(iii) Neutron

Mass of neutron          = mass of proton(= 1.67 x 10-27 kg)

Charge of neutron       = Nil

Diameter of nucleus … … of the order of 10-14 m

Diameter of orbits      = 104 times the dia. of the molecule


Normally, the atoms are electrically neutral, that is, the number of electrons and protons are the same, cancelling each other’s electrical force. Atoms “stay together’ because unlike charges attract each other. The electrical force of the protons hold the electrons in their orbits. Like electrical charges repel each other so negatively charged electrons will not collide with each other.


Positive and negative ions :

When an electron is removed from a neutral atom, this atom becomes positively charged and is called positive ion. However if an electron is added to a neutral atom, it becomes negatively charged and is called a negative ion. Thus an atom becomes an ion by the gain or loss of an electron.