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Atomic Structure



From Early times man has speculated, on the nature of matter. 
That is, they used to wonder, what matter comprised of.
Initially, they used to believe, that all matter was comprised, of water, 
- fire, earth, air and the sky. 
In the fifth century B C, the Greek philosopher Democritus,  
- proposed the concept of an atom. 
He thought of dividing matter, till a point, where we reached a particle, 
- which could not be divided further. 
He gave the name 'atom' to this particle. 
He said all matter, are made up of atoms. 
We must remember that all this was, only speculated theory. 
There was no experimental, or scientific evidence, for these theories. 
However, man continued to think about, and explore,
- the basic nature of matter, in history.


Is all matter the same? 
By observation, we notice that different matter, has different properties. 
For example, the properties of iron, are different from the properties, of gold. 
We now know that there are, different elements in nature. 
For example, hydrogen, oxygen, carbon, copper, iron, gold, aluminium, are Elements.
Each element is unique.
Each element, has its own unique properties. 
We have succeeded in discovering, all the elements available today. 
One hundred and eighteen elements, are known to us. 
Out of this, 90 are found in nature, rest are made in the laboratory. 
If there are 118 elements, does it mean, they all have the same atoms? 
Does hydrogen, oxygen, Iron, and gold have the same atoms? 
We subsequently discovered, that each unique element, has an unique atom. 
So hydrogen is comprised, of hydrogen atoms. 
Oxygen is comprised, of oxygen atoms. 
Iron is comprised, of iron atoms. 
All the 118 known elements, have 118 unique atoms. 
We can also say that, any substance made up of the same kind of atom, is an element. 
Is all matter, only elements? 
Is water an element? 
No, water is not an element. 
Water is a compound, of the elements, hydrogen, and oxygen. 
Matter in nature, comprise of both, elements and compounds. 
As of now, we will discuss only elements. 

Early Atomic theory

In 1803, English chemist Dalton, proposed an atomic theory. 
The main postulates of the theory, are. 
Elements, are made up of very small, indivisible particles, called atoms. 
All atoms of a particular element, are identical in shape, mass and other properties. 
Atoms of one element, are different from Atoms, of other elements. 
Atom of one element, cannot be converted into an atom, of another element. 
Atoms of an element, combine with atoms of other elements, to form a molecule. 
An atom can neither be created, nor destroyed. 
Daltons atomic theory, was a good starting point, of understanding of matter. 
However, these postulates were purely theoretical,  
- and not backed by experimental, or scientific proof. 
We now know, that some of these postulates are not correct, or not fully correct. 
Later, many scientists improved, our knowledge of the atom. 
It will be interesting to note that, research on the atom, is still going on. 
Scientists are still discovering, more new and exciting things, about the nature of matter. 

Nature of the atom

What would an atom be like? 
Is it possible to divide, the atom further? 
Scientists, started doing experiments, to understand, the nature of the atom. 
Many scientists, have contributed to this process. 
Due to this, we have a fairly good idea now, on the nature of the atom. 
 Fundamental particles


Thomson, and others conducted some experiments, on the nature of matter. 
Thomson, used a discharge tube apparatus, for his experiments. 
The discharge tube is a glass tube, with two metal plates attached to the ends. 
This is somewhat similar to a tube light. 
The discharge tube, was connected to a vacuum pump, which could evacuate the air inside. 
He evacuated the air, inside the glass tube, by using the vacuum pump. 
This created very low pressure, inside the discharge tube. 
He then applied, a high voltage to the metal plates. 
These metal plates, acted as electrodes. 
The metal plate, with the positive charge, is called an anode. 
The metal plate, with the negative charge, is called a cathode. 
When the current was passed, he observed a greenish glow, near the anode. 
The particles, which were emitted from the cathode to the anode, was the cause for the greenish glow. 
The streams of particles emitted, from the cathode, are called cathode rays. 
He placed a light paddle wheel, in the path of the cathode rays. 
The paddle wheel started to rotate. 
He concluded that cathode rays, are a stream of particles. 
He conducted another experiment. 
He placed a positively charged plate, near the cathode ray. 
The cathode rays deflected, towards the positively charged plate. 
The positively charged plate, was attracting the cathode rays. 
He concluded that cathode rays, are negatively charged. 
He named these particles, in the cathode rays, as electrons. 
He conducted the above experiment, with different gases, in the discharge tube. 
Regardless of the gas used, the deflection of the cathode rays, was the same. 
The electrons of all the gases were the same. 
The electrons are negatively charged particles. 
Electrons, are lighter than any atom. 
The mass of an electron, is very small, when compared to an atom. 
He showed that an atom is divisible, and it has very small particles, namely the electron. 
Dalton, had proposed that atoms, were in divisible. 
After the discovery of the electron, it was proved, that atoms are divisible,
- and have smaller particles. 

Anode rays

After the discovery of electrons, scientists started to rethink, about the nature of the atom.
If an atom, is composed of only electrons, then atoms and matter, should be negatively charged. 
The fact is that, all matter and atoms, are electrically neutral. 
That is, a whole atom is, neither negatively charged or positively charged. 
Could this mean, that atoms, have positively charged particles also? 
May be, the positive charged particles, cancels out the negative charge, of  the electron. 
To explore this theory,  Goldstein, conducted an experiment. 
In 1886, Goldstein used a discharge tube, with a perforated cathode.. 
He observed, that some rays, were travelling from the anode, to the cathode. 
This was unlike, the cathode rays which travelled, from the cathode to the anode. 
The rays travelling, from anode to cathode, are called anode rays, or positive rays. 
Anode rays contain, positively charged particles. 
He conducted another experiment. 
He exposed, the anode rays to a negative, electric charge. 
He observed that, the path of the rays deflected, towards the negative  charge. 
The deflection of anode rays, was very little, when compared to the deflection, of cathode rays.  
This was because, the anode rays were heavier, than the cathode rays. 
He conducted further experiments, using different gases, in the discharge tube. 
He noticed that, the deflection was different, for each gas. 
In the case of cathode rays, comprising of electrons, there was no difference,
- when different gases were used. 
In the case of anode rays, he noticed the deflection was different, for different gases. 
This led to the conclusion, that anode rays depend, on the type of gas, inside the tube. 
These findings paved the way for discovering the proton. 

Characteristics of anode rays

They consist, of positively charged particles. 
They are deflected by electrical, and magnetic fields. 
The deflection of anode rays, is less compared, to cathode rays. 
Anode rays, comprise of heavier particles, than cathode rays, which are electrons. 
The deflection of the anode rays, depends on the nature of the gas, in the discharge tube. 
If the gas is heavier, the deflection is lower. 
If the gas is lighter, the deflection was more. 
When hydrogen gas, is taken inside the discharge tube, the deflection was maximum. 
This led to the conclusion, that the positive particles, in hydrogen atom are the lightest.
The concept of the elementary positively charged particle, was born.

The e/m(charge/mass) ratio depends on the gas ratio.


The positive particles, in atoms,  are called protons. 
Protons are much heavier, than electrons. 
The hydrogen atom has, only one proton, and one electron. 
The mass of one proton, is about the mass of 1,840 electrons. 
The charge of a proton, is positive. 
The charge of an electron, is negative. 
The charge of an proton is, opposite of the charge of an electron, but of the same magnitude. 
That is why, the hydrogen atom, as a whole is neutral. 
That is, the positive charge of the proton, is balanced by the negative charge, of the electron. 
The charge of a proton, is + 1.
The charge of an electron is minus one.
The mass of a proton, is defined as, 1 atomic mass unit, or 1 A M U. 

  • Structure of an atom

Rutherford's experiment

It was initially believed that, electrons and protons are,  
- uniformly distributed in an atom. 
Rutherford, conducted an experiment, to test this concept. 
He used positively charged particles, or alpha particles for this experiment. 
He directed the high speed, positively charged particles, towards a thin gold foil. 
He observed, that most of the alpha particles, passed through the foil, without deflection. 
A few of them were heavily deflected, after they passed the foil. 
A few of them, bounced back and returned. 
This experiment raised, 3 major questions, and possible explanations. 
Why did most of the particles, pass through the foil? 
Possibly, there is a lot of empty space, in the atom. 
This is why, many particles were able to pass, through the gold foil. 
Why did a few of them heavily deflect, from their original path? 
Some of the particles, came into contact with particles, in the atom. 
This is why they got deflected. 
This is like the coins, in the carom board, which get deflected, when they hit another coin. 
Similarly, the particles in the ray, hit the particles in the gold foil, and got deflected. 
Why did a few of them rebound? 
Some of the particles, in the ray, collided head on, with the particles in the gold foil. 
These particles bounced back. 

Based on this experiment, Rutherford proposed a prototype model, of an atom. 
Positively charged particles of an atom, are held together in the central region, of the atom. 
It is called, the nucleus. 
It is surrounded, by electrons. 
The volume of the nucleus is small, when compared to the volume of an atom. 
The mass of an atom, is concentrated in the nucleus. 
The nucleus is positively charged. 
The size of the nucleus is different, for different elements.  
The charge of the nucleus is different, for different elements. 
Electrons orbit around, the nucleus. 
There are a number of positively charged particles, in the nucleus. 
These positively charged particles, are the protons. 
The number of electrons that orbit the nucleus, is equal to the number of protons, in the nucleus. 
The atom as a whole, is neutral in charge. 
That is, the total number of protons in the nucleus, 
- is equal to the total number of electrons, that orbit it. 
Heavier elements, have more number of protons, in the nucleus. 
For example, oxygen has more number of protons, than hydrogen. 
Carbon has more number of protons, than oxygen. 
Iron has more number of protons, than carbon. 
We can say, the heavier the element, the more, the number of protons, that it will have. 
Scientists were able to correlate, the weight of an element, with the number of protons. 
But, they were not able to explain fully, the mass of an element, with protons. 
There seemed to be something else, which contributed to the mass of a atom. 
That is, the mass of the atom, seem to be always much more, 
- than the mass of all the protons, in the nucleus. 


Rutherford, predicted the existence, of neutrally charged particles, in the nucleus. 
Neutrally charged particles have no charge, either positive or negative. 
The neutrally charged particles, are called protons. 
The mass of a neutron, is the same as that, of a proton. 
So a nucleus of an atom contains, both protons and neutrons. 
Now scientists were able to explain, the mass of the atom. 
In 1932, James Chadwick, experimentally proved, the presence of such particles, called neutrons. 

Bohr's Atomic Model

In 1913, Neils Bohr, presented a model of an atom, which is now called the Bohr model.
It built upon, and improved the Rutherford model, of the atom.
The atom,  comprises of other smaller particles. 
They are Protons, Neutrons, and Electrons. 
At the core of the atom, is the nucleus. 
The nucleus comprises, of the protons and neutrons. 
The electrons orbit around, the nucleus. 
To take an analogy, Planets orbit the sun.
Different planets have different fixed orbits.
Mercury has an orbit, close to the sun. 
Pluto has an orbit, very far from the sun. 
There is a lot of empty space between the sun and the planets.
Electrons also orbit around the sun, in fixed orbits called shells.
There is a lot of empty space, between the electrons, and the nucleus. 


Electrons, revolve around the nucleus, only in certain permissible circular orbits. 
Each orbit is called, a shell. 
In an atom, electrons orbit in different circular shells, around the nucleus. 
Each shell is at a different distance, from the nucleus. 
The first shell, closest to the nucleus is called the K, shell. 
The subsequent shells are, L, shell, M, shell, N, shell, O, shell and so on. 
Electrons in each shell, are associated with a definite, amount of energy. 
Electrons in a higher shell, have more energy, than those nearer to the nucleus. 
The energy of an electron remains constant, so long as it revolves, in its own shell. 
The shells are also called energy levels, or stationary energy levels. 
The concept of shells, was able to explain, characteristics of the atom.
If we give a number to each shell, we can call that shell number, as lower case n. 
- So, n=1, for the K, shell. 
- n=2, for the L shell. 
- n=3, for the M shell. 
- n=4, for the N shell and so on. 
The number of electrons, that can be accommodated in a shell, is fixed. 
It is equal to 2, multiplied by, n to the power of 2. 
When n = 1, the number of electrons, will be 2 multiplied by, 1 to the power of 2, which is two. 
So the K shell, or n, = 1 shell, can accommodate two electrons. 
When n = 2 , it can accommodate, 
- 2 multiplied by, 2 to the power of 2, which is 2 into 4, equal to 8. 
So the capacity of the L shell, is 8 electrons. 
When n = 3, it can accommodate, 
- 2 multiplied by, 3 to the power of 2, which  is 2 into 9, equal to 18. 
So the capacity of the M shell, is 18 electrons. 
It is like saying, that each shell, has a fixed number of rooms. 
- K shell, has 2 rooms. 
- L shell, has 8 rooms. 
- M shell, has 18 rooms. 
The lightest element is hydrogen. 
It is a very light gas. 
You might have noticed, that a hydrogen balloon tends to fly off, if we do not hold on to it. 
This is because, hydrogen is lighter, than even the air around us. 
A hydrogen atom has, one proton and one electron. 
This electron is in the first K shell. 
Carbon has 6 protons, and 6 neutrons, in the nucleus. 
It has 12 electrons, orbiting this nucleus. 
Two electrons, are accommodated in the K shell. 
Eight electrons, are accommodated in the L shell. 
Two electrons, are accommodated in the M shell. 
Oxygen has 8 protons, and 8 neutrons, in the nucleus. 
It has 16 electrons, orbiting this nucleus. 
Two electrons, are accommodated in the K shell. 
Eight electrons, are accommodated in the L shell. 
Six electrons, are accommodated in the M shell. 


A nucleon is a particle, in the nucleus of an atom. 
A nucleon can be a proton, or a neutron. 

Atomic number

The number of protons, present in the nucleus of an atom, is known as  atomic number. 
The number of electrons, always equals the number of protons, in an atom. 
Atomic number also represents, the number of electrons, in the atom. 
Atomic number is denoted, by the symbol Z. 

Mass Number

The total number of protons and neutrons, present in the nucleus of an atom, is known as mass number. 
The total number of protons and neutrons, is also the total number of nucleons. 
Mass number is denoted, by the symbol A. 
It is important to understand, that atomic mass number is only a number,  and not the mass. 
Any element can be uniquely identified, if we know the number of protons and neutrons, in the nucleus. 
That is, if we know the atomic number, and the mass number, we know the element. 
So, if X is an element, we represent the atomic number Z, as a subscript. 
We represent the mass number A, as the superscript. 
Element X which has an atomic number of Z, and a mass number of A, is represented as Z, X, A. 
  • Hydrogen. Symbol H
- Number of protons equals one. 
- Number of neutrons equals zero. 
- Atomic number equals one. 
- Atomic mass number equals one. 
  • Carbon. Symbol C.
- Number of protons equals 6. 
- Number of neutrons equals 6. 
- Number of nucleons equals 12. 
- Atomic number equals 6. 
- Atomic mass number equals 12. 
  • Nitrogen. Symbol N.
- Number of protons equals 7. 
- Number of neutrons equals 7. 
- Number of nucleons equals 14. 
- Atomic number equals 7. 
- Atomic mass number equals 14. 
  • Oxygen. Symbol O
- Number of protons equals 8. 
- Number of neutrons equals 8. 
- Number of nucleons equals 16. 
- Atomic number equals 8. 
- Atomic mass number equals 16. 
  • Sodium. Symbol N a.
- Atomic number equals 11. 
- Atomic Mass number equals 22. 
  • Aluminium. Symbol A l.
- Atomic number equals 13. 
- Atomic Mass number equals 26. 
  • Chlorine. Symbol C l.
- Atomic number equals 17. 
- Atomic Mass number equals 35. 
  • Iron. Symbol F e.
- Atomic number equals 26. 
- Atomic Mass number equals 55. 
  • Copper. Symbol C u.
- Atomic number equals 29. 
- Atomic Mass number equals 63. 


In many elements, the configuration of protons, neutrons, and electrons are the same. 
In some elements, the same element can have in the nucleus, a different  number of neutrons. 
In this case, the atomic number, is the same. 
But the atomic mass number, will be different. 
The element is the same, but it has variants. 
It is like having different models, of the same element.
These variants of elements, are called isotopes. 
For example, neutral carbon has six protons, and six neutrons. 
It has a atomic number of 6, and a mass number of 12. 
This neutral or pure carbon, is designated as 6, C 12. 
This is the most common, form of carbon. 
But carbon has minute quantities of isotopes, which has 8 neutrons. 
The atomic number of this isotope, is the same 6. 
The mass number of this isotope is 14, because it has 6 protons and 8 neutrons. 
This isotope of carbon is designated, as C 14. 
When we refer to isotopes in future, we will refer it only with the mass number. 
When we normally mention carbon, we mean neutral or pure carbon, which is C 12 carbon. 
Atomic mass, is discussed, in another module. 
Atomic mass also takes into account, the proportion of the isotopes present, in the element.

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