The structure of an atom
Today we are going to learn about The structure of an atom. under this chapter we will know about some important points like
Existence of charged particles in atom, Thomson's model of an atom, Rutherford's model of an atom, Bohr's model of atom, electrons, proton and Neutrons
We will also learn that, How are electrons distributed in different orbits
We will also cover many other important points that is related to this chapter like
valency, atomic number and atomic mass, isotopes, isobars
so lets begin the chapter
Existence of charged particles in atom
In dalton's theory Dalton said that atoms are smallest particles of a matter that can not be splited.
But in 1895, scientist observed different types of things that one matter attracted other matter. They observed that pieces of paper were attracted by plastic comb that was rubbed on dry hair.
They also observed that a wall was attracted by a balloon that was rubbed on dry hair.
Before 1895 scientists knew two theories of attraction or repulsion.
1- attraction or repulsion between poles of magnet ( magnetism )
2- attraction or repulsion between different charge particles
After observation they found that this happened because of charged particles that is present inside the atoms of a matter.
This observation proved the Dalton's theory wrong because the atoms were now divisible.
1-Existence of charged particle in atom
Before 1895 scientists knew that electrically charged objects can attract the uncharged objects by inducing opposite charge in them.Existence of charged particles in atom was discovered with the help of some events like comb acquires electric charge and attracts the pieces of paper , glass rod acquires electric charge and attracts the pieces of paper. This is because, combs or glass rod constitute matter and matter is made up of atoms, there must be some charged particles present in the matter and hence in the atom. In other word we can say that, atom is divisible. More and more experiments were performed which showed that atom is not the smallest particles but is made up of still smaller particles called subatomic particles. The three subatomic particles were electrons, protons and neutrons.
2-Discovery of electron
Electron is discovered by J.J. Thomson in 1897. He discovered electrons by cathode ray experiment. J.J Thomson performed this experiment with the help of discharge tube. He took a discharge tube sealed at both ends and is fitted with two metal electrodes. both electrodes are connected to source of high voltage ( approx 10000 v). The electrode that is connected to negative terminal is called cathode and the electrode that is connected to positive terminal is called anode. The discharge tube is connected to a vacuum pump for increase or decrease the pressure. The discharge tube is filled with air or a gas like hydrogen.
Experiment
In case 1 he passed electricity at high voltage approx 10000v and at very high pressure which is 1atm.
In case 2 he passed electricity at high voltage approx 10000v but at low pressure which is 1mm of Hg.
In case 3 he passed electricity at high voltage approx 10000v and at very low pressure which is 0.001 mm of Hg.
Note:- The atmospheric pressure is 760 mm of Hg.
Observation
In case 1, when the voltage was high and pressure was 1 atp than no electricity could not flow through air.
In case 2, when the voltage is high and pressure is reduced to 1 mm of Hg than electricity started flowing through the tube and the light was emitted inside the tube but the light is invisible.
In case 3, when the voltage is high and pressure is reduced to 0.001 mm of Hg than the wall of discharge tube opposite to the cathode and coated with fluorescent coating of zinc sulfide started glowing with greenish light called fluorescence.
The fluorescence is generated due to bombardment of rays emitted from cathode on the opposite wall of tube.
since the cathode rays consist of negative charged particles, hence they are called electrons.
Characteristics of electron
1- The charge on an electron is 1.6 × 10-19 . It is the smallest negative charge carried by any particles hence it is taken as unit negative charge.
2- The absolute mass of an electron is 9.1 × 10-31 kg .
Discovery of Proton
Proton is discovered by a german scientist E. Goldstein. He discovered protons by anode ray experiment. This experiment is also called canal ray experiment. He discovered protons by using modified discharge tube. He fitted perforated cathode in discharge tube. He sealed at both ends of discharge tube and coated the part of discharge tube by zinc sulfide that is opposite to anode. both electrodes are connected to source of high voltage ( approx 10000 v). The discharge tube is connected to a vacuum pump for increase or decrease the pressure. The discharge tube is filled with air or a gas like hydrogen.
Experiment
In case 1 he passed electricity at high voltage approx 10000v and at very high pressure which is 1atm.
In case 2 he passed electricity at high voltage approx 10000v but at low pressure which is 1mm of Hg.
In case 3 he passed electricity at high voltage approx 10000v and at very low pressure which is 0.001 mm of Hg.
Observation
In case 1, when the voltage was high and pressure was 1 atp than no electricity could not flow through air.
In case 2, when the voltage is high and pressure is reduced to 1 mm of Hg than electricity started flowing through the tube and the light was emitted inside the tube but the light is invisible.
In case 3, when the voltage is high and pressure is reduced to 0.001 mm of Hg than the wall of discharge tube opposite to the anode and coated with fluorescent coating of zinc sulfide started glowing with greenish light called fluorescence.
The fluorescence is generated due to bombardment of rays emitted from anode on the opposite wall of tube passing through perforated cathode.
since the anode rays consist of positive charged particles, hence they are called protons.
These positively charged particles were named as 'protons' by a British scientist, Earnest Rutherford.
Describe Thomson's model of an atom
After the discovery of subatomic particles it became necessary to find out how these particles are arranged inside these atoms. J.J. thomson was the first scientist who propose a model for structure of an atom. J.J. thomson proposed his model of the atom in 1904. At this time only two subatomic particles electron and proton are discovered.
In Thomson's model of an atom, J.J. Thomson proposed that
1- An atom consists of a positively charged sphere and the negatively charged electrons embedded in it.
2- The positive and negative charges in an atom are equal in magnitude. So that an atom is electrically neutral.
J.J. Thomson compared his atomic model with Christmas pudding, hence it is also called plum pudding model.
Describe Rutherford's model of an atom
Earnest Rutherford was interested in knowing how the electrons are arranged within an atom. For this he performed an experiment called gold foil experiment. For this experiment he used 𝝰-particles, thin gold foil and zinc sulfide screen.
𝝰-particle- It is a positive charged particles which have mass. Helium becomes 𝝰-particle when it looses its two electron. 𝝰-particles are the double charged helium ions which have a mass of 4u. The fast moving 𝝰-particles have a considerable amount of energy.
He selected a gold foil because he wanted as thin a layer as possible.The gold foil was about 1000 atoms thick.
apparatus that is used in this experiment
He used a lead box to produce 𝝰-particles. He also made a small hole in lead box to fire 𝝰-particles on gold foil. A screen of zinc sulfide was used to surround the gold foil. A slit was also made in screen of zinc sulfide so that the 𝝰-particles easily reached to the gold foil.
Expectation
He was expected that 𝝰-particles would be deflected by the sub-atomic particles in the gold atoms. since the 𝝰-particles were much heavier than the protons, he did not expected to see large deflection.
Observation
But the 𝝰-particles scattering experiment gave totally unexpected result. The following observation were made.
1- Most of the fast moving 𝝰-particles passed straight through the gold foil.
2-Some of the 𝝰-particles were deflected by the foil by small angles
3- surprisingly one out of every 12000 particles appeared to rebound.
After this experiment Rutherford said that 'This result was almost as incredible as if you fire a 15-inch shell at a piece of tissue paper and it comes back and hits you.
conclusion
The conclusions that Rutherford gave from the 𝝰-particles scattering experiment were
1-Most of the space inside the atom is empty because most of the 𝝰-particles passed through the gold foil without getting deflected.
2-Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
3-A very small fraction of 𝝰-particles were deflected by 1800 , indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
4- He also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.
On the basis of his experiment Rutherford gave the nuclear model of an atom. The features of the nuclear model of an atom are
1- There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom present in the nucleus.
2- The electrons revolve around the nucleus in circular path.
3- The size of the nucleus is very small as compared to the size of the atom.
Drawbacks of Rutherford's model of the atom
The revolution of the electron in a circular orbit is not expected to be stable. Any particles 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.
What are discrete orbits of an atom?
Ans:- The certain special orbits of an atom which have different radius and different energy level . as we go far away from the nucleus the energy level increases. as we go far away from the nucleus the size of radius increases
Bohr's model of the atom
Niels bohr was Danish physicist. He proposed a new model of atom in 1913. Niels Bohr put forward the following postulates about the model of atom.
1- electrons revolve only in certain fixed orbits around the nucleus without losing energy in the form of radiations.
2- only certain special orbits known as discrete( अलग ) orbits of electrons are allowed inside the atom.
3-While electrons revolve on discrete orbits the electrons do not lose energy in the form of radiation. These orbits or shells are called energy levels or energy cells. They are represented by the letters K, L, M, N ........... or the numbers 1, 2, 3, 4.............
The main points or the postulates of Bohr's model of atom
1- An atom consists of a small heavy positively charged nucleus in the centre and the electrons revolve around it in circular path called orbits.
2- in a particular atom, the orbits in which the electrons revolve are the discrete orbits. They are also called energy level or energy cells. The energy of every orbits are fixed therefor it is also called stationary state. These are numbered as 1, 2, 3, 4........etc as we go out side of the nucleus. They are also represented by the letters K, L, M, N........etc. The energy of these cells increases as we go outwards from the nucleus. Thus the 1st shell is represented by the energy E1 . The 2nd shell is represented by the energy E2 . the 3rd shell is represented by the energy E3 . the 4th shell is represented by the energy E4 etc. where E1<E2<E3<E4
3- As an electron is revolving in a particular orbit, it can neither lose energy nor gain energy, thus the atom is stable and does not collapse. This state of atom with lowest energy is called ground state of atom.
4- Energy is lost or gained by ab electron only when it jumps from one orbit to the another. The energy gained or loss is equal to the difference of energy of the two energy levels involved. If an electron gains outer energy, it will jump to some outer shell.
when an electron gains or loses energy, than the atom is said to be excited state. In excited state, the atom is not stable.
Discovery of Neutrons
In 1932, J Chadwick discovered another sub-atomic particles which had no charge. It had mass nearly equal to that of proton. It was eventually named as neutron. J. Chadwick discovered the neutron with the help of light element like Beryllium, Boron . He bombard the ∝ particles on these elements one by one. After bombarding the alpha particles on these elements, he saw that some other types of particles were arises. For these particles he said that, these particles had no charge but had mass.Now the scientist discovered the third types of sub atomic particles whose properties were completely different from electron and proton. Neutrons are present in the nucleus of all atoms except hydrogen. The neutron is represented by 'n'.
What is Neutron?
Ans:- Neutron is a sub-atomic particles which carries no charge but has mass nearly equal to proton.
Characteristics of electron, proton and neutron
|
Particles |
Charge on the particle |
Mass of the Particle |
Symbol |
Location in the atom |
|
Electron |
-1 unit (-1.602 × 10-19)coulomb |
9.11 × 10-31kg |
e- |
Outside of the nucleus |
|
Proton |
+1 unit 1.602 × 10-19 coulomb |
1.602 × 10-27kg |
P+ |
In the nucleus |
|
Neutron |
No charge |
1.602 × 10-27kg |
n |
In the nucleus |
Atomic number
What is atomic number?
Atomic number of an electron is equal to the number of protons present in the nucleus of the atom of that element
Ex:- The atomic number of hydrogen is 1 because the nucleus of hydrogen has 1 proton
The atomic number of carbon is 6 because the nucleus of carbon has 6 proton
The atomic number of chlorine is 17 because the nucleus of chlorine has 17 protons
Why proton is used to assign atomic number of an element
Ans:- Because the number of protons in a nucleus of an element is always fixed
Write some important characteristics of atomic number
Ans:-👉 Number of protons present in a nucleus must be equal to the number of electrons present on discreet orbit. In neutral atom, the atomic number is equal to number of electron and also to the number of proton
👉Atomic number of an element = number of proton in the nucleus = Number of electron on the discreet orbit.
👉Atomic number should always be calculated from number of protons present in the nucleus.
👉 Atomic number is always a whole number because atom always contains whole number of protons
👉 All atoms of the same element have same number of protons in the nucleus so they have the same atomic number.
👉No two elements have same atomic number
👉Atomic number is always a whole number, this is because an atomic number of an element does not change during chemical reaction. This happens because in chemical reaction, electrons rearranges but protons remain same.
👉 Atomic number of an element is usually denoted by 'Z'
Mass number
What is mass number?
Ans:- The sum of the number of protons and neutrons present in an atom of the element is called mass number.
Mass number of an element = Number of protons + number of neutrons
Protons and neutrons are collectively called nucleons because they are present in the nucleus of an atom. Mass number of an element is equal to the number of nucleons present in the atom of that element
Mass number of an element is represented by 'A'
In the case of neutral atom
Z = p+ = e-
A = p + n = Z + n
n = A- Z
Numerical-1
Calculate the number of electrons , protons and neutrons in sodium atom. If the atomic number of sodium is 11 and the mass number is 23
Arrangement of electrons in an atom or distribution of electrons in an atom
What is electronic configuration?
Ans:- The distribution or arrangement of the electrons in the different shells of the atom is called the electronic configuration of the element.
The arrangement of electrons in the different shells should be such that it should have maximum stability. Thus , to write the electronic configuration
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