Fundamental unit of matter is atom, which is capable of independent existence.Central body of atom is nucleus around which electrons revolves.
According to Neils Bohr, Atom consist of nucleus and nucleus contains neutrons and protons.Where neutrons are neutral charge particles and protons are positive charge particles.
Nucleus has a positive charge which attracts the electrons.Electrons would fall into nucleus if they don’t have required centrifugal force for their motion.So electrons travels in stable orbit with right velocity for centrifugal force to balance the nucleus attraction.
Now the electrons in each permitted orbit have a certain fixed amount of energy. Larger the radius of orbit ,the greater is the energy of electron.
If some additional energy is given to the electron it jumps to the larger orbit.Then the atom is said to be in the state of excitation.This state doesn’t last long since the electrons soon falls back to the original state or orbit and when it falls back to the original state , it radiates the acquired energy in the form of heat , light etc.
Atomic structure is shown below,
Take an example lets take an atom of silicon , Two dimensional structure is shown in figure,atomic number of silicon is 14.
Its electronics configuration is 2 8 4 .It has 14 electron ,14 protons (since number of electron is equal to number of protons in an atom).Now these electrons revolve around the nucleus in a particular orbit having radius r1 ,r2, r3 respectively.
In general, a shell can contain a maximum of 2 n^2 electrons. Where, n is the number of the shell. But to this rule there is an exception , the outermost orbit in an atom cannot accommodate more than 8 electrons .The electron present in the outermost orbit are called Valence Electrons.
So for silicon atom, two electrons revolve in the first Orbit, 8 in second, 4 in the last Orbit .The Silicon atom contains 4 electrons in the outermost orbit called Valence Electrons.Therefore Silicon atom is called tetravalent element.
Another example is , Germanium atomic number 32 , its electronic configuration is 2 8 18 4 , its outermost orbit contains four electrons so it is also a tetravalent element.
Another example is ,Boron atomic number is 5 ,its electronic configuration is 2 3 , its outermost orbit contains 3 electrons , its valency is 3, so it is called a trivalent element.
Other examples are aluminium ,gallium ,carbon etc. with atomic number numbers 13 ,31, 6 respectively.