Short Notes of Atomic Structure

 Short Notes of Atomic Structure:

 *Dalton's Theory:* 

Smallest indivisible particle of matter is ATOM.

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This theory failed after discovery of Cathode rays during experiments in discharge tube by J J Thomson.

J.J. Thomson discovered the sub-atomic particle namely ‘electron.’

Then he proposed an atomic model. It is known as PLUM PUDDING model 

Key points of 

Thomson’s Model of Atom

(1)Atom is spherical in shape

(2)Atom consists of a positively charged spread throughput sphere and the electrons (negative charge) are embedded in it 

(2) The negative and positive charges are equal in magnitude. Thus, the atom as a whole is electrically neutral.

ANODE RAYS:

In 1886, E. Goldstein discovered the presence of new radiations in discharge tube when perforated cathode taken. And named them canal rays.

These canal rays assumed to consisting another positively charged sub-atomic particle  named it as proton.

Rutherford’s Model of Atom

➖E. Rutherford is popular as the ‘Father’ of nuclear physics.

➖Rutherford is largely known for his work on radioactivity and the discovery of the nucleus of an atom with the gold foil experiment (as shown in the image given below.

➖Rutherford said that in an atom, there is a positively charged center known as the nucleus.

➖Rutherford said that nearly all the mass of an atom exists in in the nucleus.

➖According to Rutherford, the electrons revolve around the nucleus in well-defined orbits.

Bohr’s Model of Atom

Neils Bohr further extended Rutherford’s model and improved his drawbacks.

According to Bohr, only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

Bohr said that electrons do not radiate energy while revolving in discrete orbits.

Bohr named orbits or shells as energy levels.

Bohr represented these orbits or shells are by the letters K, L, M, N,… or the numbers, n = 1,2,3,4,….

Discovery of Neutron

In 1932, J. Chadwick discovered a new sub-atomic particle i.e. neutron.

Neutron has no charge and a mass nearly equal to that of a proton.

Neutrons are present in the nucleus of all atoms, except hydrogen.

Electrons Distributed in Different Orbits (Shells)

➖The maximum number of electrons that can be present in a shell is given by the formula 2n2.

➖‘n’ is the orbit number or energy level index, i.e. 1, 2, 3,….

➖According to the given formula −

First orbit i.e. K-shell will be = 2 × 12 = 2

Second orbit i.e. L-shell will be = 2 × 22 = 8

Third orbit i.e. M-shell will be = 2 × 32 = 18

Fourth orbit i.e. N-shell will be = 2 × 42 = 32

➖Likewise, the maximum number of electrons that can be accommodated in the outermost orbit is 8.

➖Electrons are not filled in a given shell, unless the inner shells are filled. It means, the shells are filled in a step-wise manner; starting from inner shell to outer shell.

 

*Valence electrons:* 

The electrons, those are present in the outermost shell of an atom, are known as the valence electrons.

 *Bohr-Bury model:* 

the outermost shell of an atom can have a maximum of 8 electrons.

 *Atomic Number:* 

Denoted by Z

Defined as the total number of protons  present in the nucleus of an atom or total number of electrons in a neutral atom

 *Nucleons:* 

Protons and neutrons collectively are known as nucleons.

Mass Number:

Denoted by A

It is the sum of the total number of protons and neutrons, present in the nucleus of an atom.

 *Isotopes* 

The atoms of the same element, having the same atomic number but different mass numbers, is known as isotopes. 

Examples:

Hydrogen  has 3 isotopes

(a) protium(H)

(b) deuterium(D)

(c)  tritium(T)

Important points of istopes

(1)The chemical properties of isotopes of an atom are similar because number of electrons are same have same electronic configuration

(2) But their physical properties are different, because physical properties depends on Mass and they have different mass number.

(3) They have same number of protons

(4) Their mass number different due to different number of neutrons

(5) Rate of reaction of isotopes will be different due which is called as isotopic effect

 *Isobars* 

Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars. E.g. calcium’s atomic number is 20and argon’s atomic number is 18; further, the number of electrons in these atoms is different, but the mass number of both these elements is 40.

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