The branch of engineering which deals with the flow of Electrons through vacuum, gas or semiconductor is called Electronics.

Electronics essentially deals with electronic devices and their utilization.

Atomic Structure

· Atom is the basic building block of all the elements. It consists of the central nucleus of positive charge around which small negatively charged particles called electrons revolve in different paths or orbits.

· An Electrostatic force of attraction between electrons and the nucleus holds up electrons in different orbits.

• Nucleus is the central part of an atom and contains protons and neutrons. A proton is positively charged particle, while the neutron has the same mass as the proton, but has no charge. Therefore ,nucleus of an atom is positively charged.

• atomic weight = no. of protons + no. of neutrons

• An electron is a negatively charged particle having negligible mass. The charge on an electron is equal but opposite to that on a proton. Also the number of electrons is equal to the number of protons in an atom under ordinary conditions. Therefore an atom is neutral as a whole.

• atomic number = no. of protons or electrons in an atom

• The number of electrons in any orbit is given by 2nsquare where n is the number of the orbit.

For example, I orbit contains 2x1square =2 electrons

II orbit contains 2x2square = 8 electrons

III orbit contains 2x3square = 18 electrons and so on

• The last orbit cannot have more than 8 electrons.

• The last but one orbit cannot have more than 18 electrons.

Positive and negative ions

• Protons and electrons are equal in number hence if an atom loses an electron it has lost negative charge therefore it becomes positively charged and is referred as positive ion.

• If an atom gains an electron it becomes negatively charged and is referred to as negative ion.

Valence electrons

The electrons in the outermost orbit of an atom are known as valence electrons.

• The outermost orbit can have a maximum of 8 electrons.

• The valence electrons determine the physical and chemical properties of a material.

• When the number of valence electrons of an atom is less than 4, the material is usually a metal and a conductor. Examples are sodium, magnesium and aluminium, which have 1,2 and 3 valence electrons respectively.

• When the number of valence electrons of an atom is more than 4, the material is usually a non-metal and an insulator. Examples are nitrogen, sulphur and neon, which have 5,6 and 8 valence electrons respectively.

• When the number of valence electrons of an atom is 4 the material has both metal and non-metal properties and is usually a semi-conductor. Examples are carbon, silicon and germanium.

Free electrons

• The valence electrons of different material possess different energies. The greater the energy of a valence electron, the lesser it is bound to the nucleus.

• In certain substances, particularly metals, the valence electrons possess so much energy that they are very loosely attached to the nucleus.

• The loosely attached valence electrons move at random within the material and are called free electrons.

The valence electrons, which are loosely attached to the nucleus, are known as free electrons.