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Structure of the Atom : Chapter Notes
Matter is made up of tiny particles called atoms. Atoms are further made of three fundamental particles or sub – atomic particles called electron, proton and neutron.
Earlier Dalton postulated that atom is indivisible i.e. cannot be further divided which proved to be wrong by discovery of sub – atomic particles.
Types of Sub - atomic Particles
Note -
Protons and neutrons are almost of same mass. The mass of proton is approximately 2000 times as that of the electron.
Discovery of Sub- atomic Particles
(a) Electron (e ):
These negatively charged particles were discovered by J. J. Thomson in 1897. In the experiment, a gas at low pressure was taken in a discharge tube made up of glass. At the ends of the discharge tube two electrodes were placed, connected to a battery for high
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voltage supply. The electrode connected to the negative end was known as cathode and that to the positive as anode. He then observed a stream of negatively charged particles coming out of cathode towards the anode. These particles were called electrons.
The collection of negatively charged particles emitted from cathode of discharge tube is called cathode rays.
(b) Proton (p ):
In 1886, Goldstein observed in the same experiment, with different situations that the anode emitted positive particles which were called
Canal rays: The positively charged radiations produced in the discharge tube from anode
are called Canal rays.
(c) Neutron ( n ):
These neutrally charged particles were discovered by James Chadwick in 1932. Neutrons are present in atoms of all elements except Hydrogen.
Activities related to charged particles
Activity 1 - Comb rubbed with dry hair attracts paper pieces.
Procedure:
Rub a comb on your dry hair.
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The Magnitude of positive and negative charge is same inside atom, so net charge inside an atom is zero.
2. Rutherford’s “α- particle scattering experiment”
Rutherford designed an experiment, He used:
α - Particles are doubly charged helium ion. Since they have a mass of 4 u, the fast- moving α - particle have a considerable amount of energy. A thin gold foil with thickness of about 1000 atoms. Procedure :
A lot of fast moving α - particles were bombarded on thin gold foil. After passing the foil the α – particles, hit the screen.
Following are the observations and inferences made:
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Rutherford on the basis of this experiment concluded The Nuclear model of an atom. According to Nuclear model of an atom:
- Most of the space inside the atom is empty.
- At the Centre of an atom, a small, heavy, positively charged nucleus is present.
- Electrons revolve around the nucleus.
- Total positive charge in nucleus is same as total negative charge on all electrons of atom as atom has net zero charge.
Drawbacks:
According to Rutherford model, electrons revolve around positive charged nucleus. According to the, Theory of Maxwell, if any charged particle does accelerated motion it must radiate energy. Therefore, if a charged body (e ) rotates around another charged body (nucleus), it will radiate energy. Due to energy loss through the radiations, speed of electrons will decrease and eventually it will fall into the nucleus. But such collapse does not occur and atoms were found to be quite stable.
Note -
Rutherford after this experiment gets the credit of discovery of nucleus. The protons and neutron, collectively known as ‘Nucleons’ were present in the nucleus. It was found that almost all mass of an atom resides in nucleus.
3. Bohr’s Model of Atom -
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For atoms:
Atomic number = number of proton = number of electron
For ions:
Atomic number = number of proton ≠ number of electron
For example, in aluminium atom number of electrons is equal to atomic number but in aluminium ion it is not so.
Mass number
Mass number is equal to the number of nucleons present inside the nucleus of an atom. It means it is the sum of number of protons and neutrons present in the nucleus of an atom. It is denoted by letter A.
Mass number of element = Atomic mass of element = number of protons + number of neutrons
For example -
Carbon : Mass number = 12 (no. of p = 6, no. of n = 6 ) Nitrogen : Mass number = 14 (no. of p = 7, no. of n = 7 ) Fluorine : Mass number = 19 (no. of p = 9, no. of n = 10 )
The symbol of the element with atomic number and mass number -
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For example - Nitrogen is written as , where 14 is the mass number and 7 is the atomic number.
Isotopes
Isotopes are atoms of same element having same number of protons but different number of neutrons. Isotopes have similar chemical properties but different physical properties.
For example -
1. Hydrogen - It exists in three different isotopic forms: 2. Carbon - It exists in three different isotopic forms:
Uses of Isotopes:
- An Isotope of uranium is used in nuclear power plants to generate electricity.
- Used for medical purposes:
An Isotope of cobalt is used in the treatment of cancer. An isotope of iodine is used in the treatment of goiter.
Fractional Mass number of elements: If an element is a mixture of isotopes, it may
have fractional mass number.
(^147) N
( 23592 Ur )
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- Orbit are filled from inside to outside. First, n = 1 shell is filled, then n = 2, and so on.
- The outermost occupied shell of an atom can have a maximum of 8 electrons even if it can accommodate more electrons.
For example -
1. Sulphur - It has 16 electrons. Therefore the electronic configuration is 2, 8, 6.
n = 1 or K shell : 2 electrons n = 2 or L shell : 8 electrons n = 3 or M shell : 6 electrons
2. Sodium - It has 11 electrons. Therefore the electronic configuration is 2, 8, 1.
n = 1 or K shell : 2 electrons n = 2 or L shell : 8 electrons n = 3 or M shell : 1 electron
Valency
Valency of an element is the number of electrons that its atoms should give away or take to attain stable electronic configuration i.e. the atom should accommodate 8 electrons in the outermost shells or valence shells except the K shell which can accommodate 2 electrons to the maximum.
The electrons present in the outermost orbit of an atom are known as valence electrons.
Three ways to obtain stable electronic configuration: (a) By losing electrons (b) By Gaining electrons Download (c) By sharing electrons APP
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January 12, 2016 at 10:28 pm (http://www.dronstudy.com/book/structure-of-the-atom- chapter-notes/#comment-37140) Reply (http://www.dronstudy.com/book/structure-of-the-atom-chapter-notes/? replytocom=37140#respond)
For example -
Hydrogen molecule: Hydrogen has only one electron in its outermost orbit thus requires one more electron to complete its outermost orbit (K shell). For this, hydrogen atom shares one electron with another Hydrogen atom and forms H (Hydrogen molecule).
Valency of atoms of first ten elements with Electronic configuration
Note - Atoms do chemical reactions to attain stable configuration. But noble gases are inert as they already have stable electronic configuration.
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