Class 9 Charged particles in matter and The structure of an atom

### Topics to be covered

=> Charged particles in matter
=> The structure of an atom
=> Thomson's model of an atom
=> Rutherford's model of an atom
=> Bohr's model of atom

### 𝐂𝐡𝐚𝐫𝐠𝐞𝐝 𝐏𝐚𝐫𝐭𝐢𝐜𝐥𝐞𝐬 𝐢𝐧 𝐌𝐚𝐭𝐭𝐞𝐫

color{green}(•) J.J.Thomson identified that the atom was not a simple, indivisible particle but it contained at least one sub-atomic particle – the electron identified by J.J. Thomson.

color{green}(•) Before the discovery of electron, E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays.

color{green}(•) color{red}("𝐂𝐀𝐍𝐀𝐋 𝐑𝐀𝐘𝐒:") These rays were positively charged .It had a charge, equal in magnitude but opposite in sign to that of the electron. Its mass was approximately 2000 times as that of the electron. It was given the name of proton.

color{green}(•) In general, an electron is represented as color{red}(‘e^– ’) and a proton as color{red}(‘p^+ ’).

color{green}(•) The mass of a proton is taken as one unit and its charge as plus one.

color{green}(•) The mass of an electron is considered to be negligible and its charge is minus one.

color{green}(•) An atom was composed of protons and electrons, mutually balancing their charges.

color{green}(•) Protons were in the interior of the atom, for whereas electrons could easily be peeled off but not protons.

### 𝐓𝐡𝐞 𝐒𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐞 𝐨𝐟 𝐚𝐧 𝐀𝐭𝐨𝐦

According to Dalton’s atomic theory the atom was indivisible and indestructible. But the discovery of electrons and protons led to the failure of this aspect of Dalton’s atomic theory. So it became important to know the arrangement of electrons and protons in an atom. So various atomic models were proposed to study the structure of atom.

### 𝐓𝐇𝐎𝐌𝐒𝐎𝐍’𝐒 𝐌𝐎𝐃𝐄𝐋 𝐎𝐅 𝐀𝐍 𝐀𝐓𝐎𝐌

color{green}(•) Thomson proposed the model of an atom to be similar to that of a Christmas pudding.

color{green}(•) The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding.

color{green}(•) The structure of atom was also compared with watermelon, the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon.

color{green}("𝐓𝐡𝐨𝐦𝐬𝐨𝐧 𝐩𝐫𝐨𝐩𝐨𝐬𝐞𝐝 𝐭𝐡𝐚𝐭:")

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude.

So, the atom as a whole is electrically neutral.

color{green}("𝐃𝐫𝐚𝐰𝐛𝐚𝐜𝐤 𝐨𝐟 𝐭𝐡𝐞 𝐦𝐨𝐝𝐞𝐥.")

Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.

### 𝐑𝐔𝐓𝐇𝐄𝐑𝐅𝐎𝐑𝐃’𝐒 𝐌𝐎𝐃𝐄𝐋 𝐎𝐅 𝐀𝐍 𝐀𝐓𝐎𝐌

Rutherford designed an experiment to find out how electrons are arranged in an atom. In this experiment, fast moving alpha color{red}(α)-particles were made to fall on a thin gold foil.

color{green}(•) He selected a gold foil because he wanted as thin a layer as possible. This gold foil was about color{red}(1000) atoms thick.

color{green}(•) color{red}(α)-particles are doubly-charged helium ions. Since they have a mass of color{red}(4 u), the fast-moving color{red}(α)-particles have a considerable amount of energy.

color{green}(•) It was expected that color{red}(α)-particles would be deflected by the sub-atomic particles in the gold atoms. Since the color{red}(α)-particles were much heavier than the protons, he did not expect to see large deflections.

color{green}("𝐓𝐡𝐞 𝐟𝐨𝐥𝐥𝐨𝐰𝐢𝐧𝐠 𝐨𝐛𝐬𝐞𝐫𝐯𝐚𝐭𝐢𝐨𝐧𝐬 𝐰𝐞𝐫𝐞 𝐦𝐚𝐝𝐞:")

(i) Most of the fast moving color{red}(α)-particles passed straight through the gold foil.

(ii) Some of the color{red}(α)-particles were deflected by the foil by small angles.

(iii) Surprisingly one out of every color{red}(12000) particles appeared to rebound.

color{green}("𝐂𝐎𝐍𝐂𝐋𝐔𝐒𝐈𝐎𝐍 𝐎𝐅 𝐑𝐔𝐓𝐇𝐄𝐑𝐅𝐎𝐑𝐃’𝐒 𝐌𝐎𝐃𝐄𝐋 𝐎𝐅 𝐀𝐍 𝐀𝐓𝐎𝐌;")

(i) Most of the space inside the atom is empty because most of the color{red}(α)-particles passed through the gold foil without getting deflected.

(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

(iii) A very small fraction of color{red}(α)-particles were deflected by color{red}(180^0),indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

From the data he also calculated that the radius of the nucleus is about color{red}(10^5) times less than the radius of the atom..
Results of Rutherford model of an atom:

(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well-defined orbits.
(iii) The size of the nucleus is very small as compared to the size of the atom.

color{green}("𝐃𝐫𝐚𝐰𝐛𝐚𝐜𝐤𝐬 𝐨𝐟 𝐑𝐮𝐭𝐡𝐞𝐫𝐟𝐨𝐫𝐝’𝐬 𝐦𝐨𝐝𝐞𝐥 𝐨𝐟 𝐭𝐡𝐞 𝐚𝐭𝐨𝐦:")

Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable.

### 𝐁𝐎𝐇𝐑’𝐒 𝐌𝐎𝐃𝐄𝐋 𝐎𝐅 𝐀𝐓𝐎𝐌

Drawbacks of Rutherford’s model were overcome by Neils Bohr. He put forward the following postulates about the model of an atom:

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels.

(iii) These orbits or shells are represented by the letters color{red}("K,L,M,N,… ") or the numbers, color{red}("n=1,2,3,4,….")