Atom and molecules are the building blocks of matter.The existence of different kinds of matter around us is due to different types of atoms and molecules present in them.For a long time it was thought that the atoms are invisible ,so they don't have a inner structure. We now know that the atoms are divisible and do have inner structure. Atoms have smaller particle in them which particles in them which are called subatomic particles.

Atoms are made up of three subatomic particles:electron, proton ,neutrons. Electron has negative charge, proton has positive charge , whereas neutron has no charge, it is neutral.

Protons and neutrons are present in a small nucelus at the center of the atom. Almost the entire mass of the atom is in the nucleus , because a electron, which are outside the nucleus,have very, very small mass . Due to the presence of proton,nucleus has positive charge.

Electrons are outside the nucleus. The electrons in an atom revolve rapidly round the nucleus in fixed circular paths are called energy level or shells. Since the atom on the whole is electrically neutral therefore the numbers of electron outside the nucleus is equal to the number of protons inside the nucleus.

Atoms of all the elements ( except hydrogen) are made up of three subatomic particles: electron, proton and neutrons. Hydrogen atom is made up of only one electron and one proton . It doesn't contain any neutron. The atoms of different elements differ in the number of electrons, protons and neutrons. We will now describe how the electrons, protons and neutrons where discovered and put together to give the structure of atom.

•Charged particles in matter

If we rub comb on dry hair , then this comb attract small pieces of paper. And if we rub a glass rod with a piece of silk cloth and bring it near an inflated balloon , then the glass rod attract the balloon. We know that an electrically charged objects can attract an uncharged object. This means on rubbing with dry hair, a comb gets an electric charge and by rubbing with silk cloth , a glass rod also gets an electric charge. Now, the question arises: When does the electric charge came from? The obvious answer is : from within the atoms present in the comb and glass rod. These simple experiments tells us that some charged particles are present in the atoms of matter . So, the atom is divisible.

Discovery Of Electron

The existence of electrons in an atom was shown by J.J.Thomson in 1897. Thomson passed electricity at a high voltage through a gas at a very low pressure taken in a discharge tube. Streams of the minute particles were given out by the cathode. These streams of particles are called cathode rays.The mass and charge of cathode ray particles doesn't depend on the nature of gas taken in the discharge tube. Cathode Ray's consist of small , negatively charged particles called electrons. Since all the gases form cathode rays, it was concluded that all the atoms contain negatively charged particles called electrons.

Thomson explained the formation of Cathode rays as follows. The gas taken in the discharge tube, consists of the atoms contain electrons.When high electrical voltage is applied the electrical energy pushes out some of the electrons from the atoms of the gas. These fast moving electrons form cathode rays.Thus the formation of Cathode rays shows that one of the subatomic particles present in all the atoms is the negatively charged ' electron'. We can know define an electrons as follows.

The electrons is a negatively charged particles found in the atoms all the elements. The electrons are located outside the nucleus in the atom. Only hydrogen atom contains one electron, all other atoms contain more than one electron. An electron is usually represented by the symbol e-(e for electron and minus sign for negative charge).

Characteristics of an electron

The two important chacteristics of an electron are in its mass and charge. These are describe below.

1. Mass of the electrons: The mass of an electron is about 1/1840 of the mass of hydrogen atom. Since the mass of a hydrogen atom is 1u, we can say that the relative mass of an electron is 1/1840u. The absolute mass of electron is however , 9×10^-28 gram. The mass of the electron is so small that it is considered to be negligible. This is why the mass of electron is ignored while calculating the atomic mass of an element.

2. Charge of an Electron: The absolute charge on an electron is 1.6×10^-19 coulomb of the negatively charge. Now 1.6×10^-19 coulomb has been found to be the smallest negative charge carried by an particles. So this is taken as the unit of negative charge. This means that the electron has 1 unit of negative charge. In the other words , the relative charge of an electron is ,-1 ( minus one).

Discovery of Proton

The formation of Cathode rays that all the atoms contain negatively Charged particles to balance the negative charge of the electrons. It has actually been found by experiments that all the atoms contain positively charged particles are called protons.

The existence of protons in the atoms was shown by E. Goldstein. When Goldstein passed electricity at high voltage through a gas at a very low pressure taken in a discharge tube , streams of heavy particles where given out by the anode . These streams of particles vare called anode rays. Anode rays consist of positively charged particles having different masses and different charges.

Hydrogen gas is the lightest gas and hydrogen atom is the lightest atom . So the positive particles obtained from hydrogen gas are the lightest and have the smallest charge. The anode rays obtained from hydrogen gas are made up of the same type of positive particles. These particles are called protons. Thus ,the anode rays obtained from hydrogen gas consist of protons. A proton is formed by the removal of an electron from a hydrogen atom.

Goldstein explained the formation of protons as follows. Hydrogen gas consist of hydrogen atoms . When high electrical voltage is applied to hydrogen gas, the electrical voltage is applied to hydrogen gas, the electrical energy removes the electrons from the hydrogen atoms. After the removal of negatively Charged electron from the hydrogen atom, a positively charged particles are called protons is formed. These fast moving proton from the anode .Please note that hydrogen atoms are the lightest of all the atoms, so hydrogen atoms form the lightest positively charged particles called protons. The heavier atoms of other gases form heavier positively charged particles which are made up of a number of protons held together. We can now define a proton as follows.

The proton is a positively charged particle found in the atoms of all the elements. The protons are located in the nucleus of an atom. Only hydrogen atom has one proton in its nucleus, atoms of all other elements contain more than one proton. A proton is usually represented by the symbol p^+ ( p for proton and plus for positive charge).

Characteristic of a proton

There are two important chacteristics of proton are its mass and charge. These are described below.

1. Mass of a proton : The proton is actually hydrogen atom which has lost its electrons. Since the mass of an electron is very small,we can say that the mass of a proton is equal to the mass of hydrogen atom. But the mass of a hydrogen atom is 1u , therefore the relative mass of the proton is 1u . If however we compare the mass of proton with that of an electron , then the mass of the proton is 1840 times that of the electron. The absolute mass of proton is 1.6 ×10 ^-24gram.

2. The charge of a proton is equal and opposite to the charge of an electron. So, the absolute charge of a proton is 1.6× 10^-19 coulomb has been found to be the smallest positive charge carried by any particle. So, this is taken as the unit of positive charge. This means that the proton carries 1 unit positive charge. In other words , the relative charge of a proton is+1(plus one).

The formation of Cathode rays and anode rays on passing electricity through gases at very low pressure tells us that atom is not indivisible , it is made up of smaller particles. Actually , the formation of Cathode rays tells us that atoms contain negatively Charged particles inside them whereas the formation of anode rays tells us that atom contains positively charged particles in them.

Discovery of Neutron

After the discovery of proton and electron, it was noticed that all the mass of an atom can't be accounted for on the basis of only protons and electrons present in it. For example , a carbon atom contains 6 protons and 6 electrons . Now , the mass of ekectrons is so small that it can be ignored. So, the atomic mass of carbon should be only 6u, which is the mass of 6 protons. This, however, is wrong because the actual atomic mass of carbon is 12u. Then, how do we explain wrong because the actual atomic mass of carbon is 12u.Then, how do we explain this extra mass of 6 units?This problem was solved by the discovery of another subatomic particles by James Chadwick in 1932. This particles is called neutron. The neutron is a neutral particle found in the nucleus of an atom. Atoms of all the elements contains neutrons except ordinary hydrogen atom which doesn't contain any neutron . Thus, the subatomic particles not present in hydrogen atom is neutron. A hydrogen atom contains only one proton and one electron. A neutron is represented by the symbol 'n'.

•Chacteristics of a Neutron

The two important chacteristics of a Neutron are its mass and charge . These are described below.

1. Mass of a neutron : The mass of a neutron is equal to the mass of a proton. In other words , the relative mass of neutron is 1u. The absolute mass of a Neutron is 1.6×10^-24 gram.

2. Charge of a Neutron: Neutron has no charge. It is electrically neutral.

We are now in the position to explain why the atomic mass of carbon is 12u. It is now know that a carbon atom contains 6 protons and 6 neutrons, each having a mass of 1u. Now.

Atomic mass=Mass of 6protons+

Massof 6 neutrons

=6×1 +6×1

=12u

Thus , the atomic mass is given by the sum of the masses of protons and neutrons present in the nucleus of an atom. The mass of electrons present in an atomis very, very small, so it is ignored.

Structure of Atom

Dalton's atomic theory suggested that atom was indivisible-which couldn't be broken down into smaller particles. But the discovery of subatomic particles such as electrons and protons inside the atom discovered this postulate of Dalton's atomic theory.The discovery of electrons and protons suggested that atoms are divisible and they do have an inner structure.After the discovery that atom contains electron and protons ,it became necessary to find out how these electrons and protons were arranged inside the atom.J.J Thomson was the first scientist to propose a model for the structure for the structure of atom.This is described below.

•Thomson's model of the atom

When J.J Thomas proposed his model of the atom in 1903, then only electrons and protons were known to be present in the atom. According to Thomson's model of the atom:

1.An atom consists of a sphere of a positive charge with negatively Charged electrons embedded in it.

2.The positive and negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral.It has no overall positive or negative charge

•Rutherford's Experiment-Discovery of

nucleus

After the discovery of electrons, protons and neutrons, it became clear that an atom is made up of these three subatomicparticles.

Experiments were then carried out to find out how electrons, protons and neutrons were arranged in an atom. It was Rutherford's alpha particle scattering experiment which led to the discovery of a small positively charged nucleus in the atom containing all the protons and neutrons.

When fast moving alpha particles are allowed to strike a very thin gold foil in vaccum,it is found that:

1.Most of the alpha particles pass straight through the gold foil without any deflection from the original path.

2.A few alpha particles are deflected through small angles and a few are deflected through large angles.

3.A very few alpha particles completely rebound on hitting the gold foil and turn back on their path.

Rutherford explained these observation in the following way:

Gold foil is made up of atoms. If the atoms were solid throughout the volume ,then every alpha particles striking them should have changed its path and got deflated. Since most of the alpha particles pass straight through the gold foil without any deflection,it shows that there is a lot of empty space in the atom.

We know that similar charges repel each other. So, a positively charged boby will repel another positively charged body. The observation that some of the alpha particles are deflected through small and large angles shows that there is a center of positive charge in the atom which repels the positively charged alpha particles and deflects them from their original path.This center of positive charge in the atom which repels the positively charged alpha particles and deflects them from their original path. This center of positive charge in the atom is known as nucleus.Thus , the scattering of alpha particles by a thin gold foil shows the existence of a positively charged nucleus in the atom.

A very few of the alpha particles are turned back on their path. This fact can't be explained only on the basis of repulsion due to positive charge of the nucleus is very dense and hard. So, the observation that a very few alpha particles completely rebound on hitting the gold foil shows that the nucleus is very dense and hard which doesn't allow the alpha particles to pass through it. Since the nucleus is very dense, practically the whole mass of an atom is centered at its nucleus.As the number for alpha particles which are deflected is very small,we conclude that the size of the nucleus must be very small,we conclude that the size of the nucleus . As the number of alpha particles which are deflected is very small, we conclude that the size of the nucleus must be very small as compared to the size of the atom as a whole. In fact , the radius of the nucleus has been found to be about 10^5 times smaller than the radius of the atom.

From the above discussion we conclude that Rutherford's alpha particle scattering experiment shows the presence of a nucleus in the atom. It also gives the following important information about the nucleus of an atom:

1.Nucleus of an atom is positively charged.

2.Nucleus of an atom is very dense and hard.

3.Nucleus of an atom is very small as compared to the size of the atom as a whole.

•Nuclues

The nucleus is a small positively charged part of the center of an atom.The nucleus contains all the protons and neutrons, therefore almost the entire mass of an atom is concentrated in the nucleus.The positive charge on the nucleus is due to the presence of protons in it.The number of protons in the nucleus determines the number of positive charges in the nucleus.The neutrons which are also present in the nucleus has no charge,it is neutral.Protons and neutrons are taken together are known as nucleons.The volume of the nucleus of an atom is very small as compared to the volume of the extranuclear part of the atom.The existence of positively charged nucleus in an atom was shown by the alpha particle scattering experiment of Rutherford.

Please note that when Rutherford put forward his nuclear model of atom in 1911, even then only electrons and protons were known as to be present in the atom.This is because Neutron was discovered much later in 1932. So, the original model of an atom given by Rutherford contained only protons in the nucleus. It contained no neutrons. Rutherford's model of the atom was improved later on by including neutrons in the nucleus. We will study this improved model of atom now.

• Rutherford's model of the atom

On the basis of alpha particle scattering experiment, Rutherford gave a nuclear model of the atom. Rutherford's model of atom can be described as follows:

1.An atom consists of a positively charged,s

dense and very small nucelus containing all the protons and neutrons. Almost the entire mass of an atom is concentrated in the nucleus.

2.The nucleus is surrounded by negatively charged electrons.The electrons are revolving round the nucleus in circular paths at very high speeds. The circular paths of the electrons are called orbits.

3.The electrostatic attraction between the positively charged nucleus and negatively charged electrons and negatively charged electrons hold the atom together.

4.An atom is electrically neutral. This is because the number of protons and electrons in an atom is equal.

5.Most of the atom is empty space.

We will now describe the structures of the atom on the basis of Rutherford's model of atom.

The simplest atom is that of hydrogen. It contains one proton and one electron. According to Rutherford's theory, hydrogen atom consists of a small nucelus containing one proton, and one electron revolving around it.The nucleus is almost at the center of the atom. Since the hydrogen atom contains an equal number of protons and electrons,it is electrically neutral.Please note that the nucleus of an ordinary hydrogen atom doesn't contain any neutrons in it.

The next simplest atom is that of helium.A helium atom consists of a small central nucleus containg 2 protons and 2 neutrons, and there are 2 electrons revolving around this nucleus.Since the helium atom contains an equal number of protons and electrons, therefore, it is electrically neutral.

•Drawbacks of Rutherford's model of the atom

A major drawback of Rutherford's model of the atom is that it doesn't explain the stability of the atom. This point will become more clear from the following discussion.

In the Rutherford's model of an atom, the negatively Charged nucleus in circular paths. Now,we known that if an object moves in a circular paths.Now, if we apply this electromagnetic theory to the Rutherford's model of an atom,it will mean that the negatively charged electrons revolving around the nucleus with accelerated motion, will lose their energy continuously by radiation. Thus, the energy of revolving electrons will decrease gradually and their speed will also go on decreasing.The Electron will then be attracted more strongly by the oppositely charged nucleus due to which they will come more and more close to the nucleus.And ultimately the electrons should fall into the nucleus by taking special path.This should make the atom very unstable and hence the atom should collapse.

But this doesn't happen at all.We know that the electrons don't fall into the nucleus of an atom.Rather,atoms are very stable and don't collapse on their own.