Phy 000                TOPIC: ATOM                                                          lesson No: 01

For science students and for Engineering students this is a most important topic to understand. Most Science and Engineering flows out of Atom only. Though there is much detail, we are concerned with such more important features and concepts that are essential for our understanding. We, as Engineering students, are so much dependent on an electron that without it we are lost. There can be no electricity and no electronics! No mobiles and nothing!

Let us get on with it.

If there was placed one atom inside a room and if we could look into this room what would we see?
We shall see a guthli (kernel) and 2-3 (cheenti) electrons revolving around (in a circle) the guthli at some distance, say one foot. Outside this circle there is a bigger circle, say, of two-foot diameter and some cheentis revolving around the guthli. There could be several such circles.



This is what we shall be telling our friends like watching on a bio-scope. Let us say in this room there were 4 circles to see as above.

black dots: electrons
black arrow: distance of orbit from center
blue: energy increasing with increasing radius of the orbit

In another similar room there could be 2 or 3 circular paths around the guthli. Because atom in the second room was from a different material.

One important thing we must remember: Irrespective of the fact how big or small the radius of the circle is, ALL cheetis complete one rotation together ie in SAME time from start to finish. Remember this!

One thing that immediately becomes clear to us: Since bigger diameter (or radius) means longer circumference, cheenti on this bigger circle must be running faster than the cheenti running on a smaller radius circle as time of running is same for both.

Let’s call the guthli a “nucleus” and cheenti an “electron”.

Now we can say in more technical terms:

In an atom there is a nucleus around which electrons revolve in circular paths. There can be several such circular paths each having some number of electrons, depending upon what material the atom is from.

We see here only one circle for each circular path. This is because we have drawn our fig in 2-dimensional plane.

But it is not so. It is like the Oon-ka-gola (wool ball) that our mother makes before starting knitting a sweater. Such is a diagram that we often see shown for an atom. Try picturing it in your mind, in three dimensions.

Remember: each electron is uniquely identifiable like our mobile number. A mobile has an IMEI number, it belongs to an area circle like MP, Punjab or UP etc; It is operated by Jio or Idea, and it has a number. Let us say, this mobile number is our ID.

Similarly, every atom has many such characteristics. Let’s say (for simplicity of learning) an atom is recognizable to us by the circle in which it is revolving. Each circle has a different radius. We consider only one electron. Don’t worry! Things are much more difficult but then we are only trying to understand Fundamentals. Things become easy.

We know that every material is made up of atoms, and each atom is made up of negatively charged electrons and positively charged protons.

And that an atom is electrically NEUTRAL meaning that there is no resultant charge. This is why we do not feel anything when we touch materials.

In simple words: number of negative charges e = number of + charges
Net charge: e + p = 0 charge in Coulombs

All positive charges called protons are inside the nucleus only and remain there as they are heavy in mass. Thus nucleus forms a positive centre.

Around this nucleus the electrons being lighter in mass are rotating. Thus actually an atom spreads from nucleus to the electrons in the outermost orbit like an oon-ka-gola.

Now, if we remove an electron from an atom then there result less number of electrons than protons in the atom, meaning thereby that atom is no longer electrically neutral; it is more positive or less negative.

Similarly if we added one electron…….

In both cases an atom becomes an ION. This is the difference.

** We know circumference of a circle = 2x π x radius meters

**+ve and negative charges attract each other ie a force of attraction exists between them. Copy Coulomb’s Law here and write what each symbol means to you.

** A rotating electron is rotating at some speed in the circular path. Say V m/sec

** A rotating thing has centrifugal force. Write it here.

**There is an equal and opposite force called centripetal force in inward direction.

** Electron has some mass m.

** An electron running at a speed (velocity) will have some momentum. Write formula here and write what each symbol means to you.

** It will have kinetic energy. Write formula here and write what each symbol means to you.

When we talk of properties of a material (electrical) we speak of electrons present ONLY in OUTERMOST circle. There is nothing we can do about electrons in inner circles. They are all FULL to their capacity.

A material is known to be different from other depending on how many electrons it has in all; how they are arranged in each circle; how many circles there are and how many electrons there are in outermost circle. Look at any table in your book.

Outermost orbit will have 1,2,3,4 … electrons.

ENERGY that an electron possesses is written as : Coulomb’s Lawwrite formula here.

** Energy is negative in our case. Since electron is negatively charged the energy is negative.

** See the radius coming in denominator. Larger the radius lesser becomes RHS; but then we know that
-6 is smaller than -2; -1 is bigger than -2; -0.008 is bigger than -0.07 and so on.

We could show energy levels in different circles at various distances from nucleus as follows:

We know that there cannot be more than 8 electrons in the outermost circle.

**Now come back to energy;
Let’s us say there are only two circular paths, one inside, smaller radius and one bigger with bigger radius. Accordingly, electrons on the smaller circle will have lesser velocity because of smaller circumference, more attraction force, less momentum, lesser kinetic energy, less centrifugal force than electron on bigger circle.

Let us say that total energy in outer circle is divided into 2 electrons that are present in this circle. Then full energy is spread into two.

Now since outermost circle can have 1,2,3,4,5,6,7,8 electrons (WHY not 0 also) then depending on the actual number of electrons present total energy will be distributed among them. If there happens to be only one then all energy will be carried by this electron only.

** As we will learn more later, more number of electron in a not-fully-filled-orbit, more difficult it becomes to make an electron free. But you know it from above. If there are 8 electrons in the outermost orbit it is IMPOSSIBLE to remove an electron.

** We know that in a circle of radius R, velocity V an electron will be having KE = ……… . Write formula here.

**If we could add some energy into the outermost circle it would mean either V will increase or the radius increases. (Write down all formula and understand meaning of each symbol).

That is, this electron will go into circle of higher radius. But we said we are talking of electrons in the outermost shell only. And there is no more circle outside the outermost circle.

So, this electron LEAVES this outermost circle and becomes FREE.

Where does it go? Don’t worry we’ll tell you in later lessons.  

Free means it is available for doing our work. and the work is carrying charge from one place to the other. Once we have a free electron, we can make it work for us.

All our effort to understand atom is to understand how we can dislodge one electron from outermost circle.

Why not two?

*** Additional energy can be supplied in type of mechanical, electrical, thermal or chemical.

Unless we can FREE an electron all these atoms are of no use to us.

*** Now you might ask, why are the electrons rotating at all? Unless they have energy, they cannot rotate.

My atom was lying on my table and I can see no source of additional energy. Where the energy came from?  You can say maybe, temperature inside my room is high. Okay, if the climate is hot in Mumbai what about Siberia where it is all ice and snow. At least there the electrons should be lying at rest. Or in ice all electrons should be sleeping.

** Answer to this is:

Have you heard of temperature in degree Kelvin?

On this scale 0-degree K is -273 degree Celsius.

So, even if we go to Shimla, the temperature inside the ice is much greater than 0 deg K.

It is here the electrons are taking energy to rotate!

We will never attain 0 deg K and never will the electron motion shall stop.

** In actual fact an electron needs a very small energy to break its bonds with the outermost circle and become homeless.

We are interested only in a FREE electron.

This is all electrical and electronics are all about. One FREE electron from an atom.

This is only beginning. Much more will come in continuation.

NEXT: What happens when an electron becomes free?

NOTE: If you feel that there is something that can be added to this lesson please  tell us at