MUKESH SHARMA

Structure of atoms

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Atomic Structure 1

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Atomic Structure 2

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Atomic Structure 3

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Atomic Structure 4

Structure of atoms

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Atomic Structure 5

Dalton’s Atomic Theory (1803)

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Atomic Structure 6

• (1) Each element is composed of extremely small particles called atoms. �(2) Atoms of a particular element are like but differ from atoms of other element. �(3) Atom of each element is an ultimate particle and it has a characteristic mass but is structureless �(4) Atoms are indestructible i.e. they can neither be created nor be destroyed. �(5) Atoms of different elements take part in chemical reaction to form molecule

Matter

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MUKESH SHARMA

scientists rely upon models to help us to understand the atom.

Believe it or not this is a microscope. Even with the world’s best microscopes we cannot clearly see the structure or behavior of the atom.

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Atomic Structure 7

MUKESH SHARMA

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Atomic Structure 8

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Atomic Structure 9

Is this really an Atom?

The model above represents the most modern version of the atom.

(Artist drawing)

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Even though we do not know what an atom looks like, scientific models must be based on evidence.

The Chemophile

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10

Thomson’s Plum Pudding Model

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Atomic Structure 11

Thomson proposed that an atom consists of a uniform sphere of positive charge in which the electrons are distributed more or less uniformly.

• �This model of atom is known as "Plum-Pudding model" or "Raisin Pudding Model" or "Water Melon Model".

Drawbacks : • An important drawback of this model is that the mass of the atoms is considered to be evenly spread over that atom. �• It is a static model. It does not reflect the movement of electron. • �It couldn't explain the stability of an atom.

Discharge Tube Experiment

• An electron was discovered by cathode ray discharge tubes experiment.

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Atomic Structure 12

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Atomic Structure 13

Property of cathode rays

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1. They were produced by the negative electrode, or cathode
2. These are negatively charged
3. They travel in straight lines and cast sharp shadows.
4. They have energy and can do work.
5. They were deflected by magnetic fields
6. They have mass
7. The nature of cathode rays is independent of

(a) The nature of cathode and (b) The gas in discharge tube.�

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Atomic Structure 14

2.2

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Charge /mass =(e/m ) ratio of electron

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Atomic Structure 15

e/m = 1.75882 X 10¹¹ coulomb (C)/kilogram

Where, �E – Applied electric field�B – Applied magnetic field�R – Constant

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Atomic Structure 16

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(Charge and mass of on electron)�

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Atomic Structure 17

ANODE RAYS (CANAL RAYS) AND PROTONS�

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1. Like cathode rays, these positive rays are deflected by electric or magnetic fields, but in the opposite direction from cathode rays.
2. Canal ray particles have e/m ratios many times smaller than those of electrons, due to their much greater masses.
3. When different elements are in the tube, positive ions with different e/m ratios are observed.

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Atomic Structure 18

DISCOVERY OF NEUTRON :

• discovered by Chadwick in 1932 by bombarding a thin sheet of Beryllium with alpha particle

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Atomic Structure 19

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Atomic Structure 20

Ernest Rutherford’s�Gold Foil Experiment - 1911

• Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil
• Particles that hit on the detecting screen (film) are recorded

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Atomic Structure 21

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• particle velocity ~ 1.4 x 10⁷ m/s

(~5% speed of light)

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Rutherford’s Findings

1. The nucleus is small
2. The nucleus is dense
3. The nucleus is positively charged
4. Most of the space in the atom is empty
5. The electrons are distributed in the empty space of the atom around the nucleus in different concentric circular paths, called orbits

“The nucleus is heavily dense

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Conclusions:

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Atomic Structure 25

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Atomic Structure 26

1. This was not according to the classical theory of electromagnetism proposed by maxwell.According to this theory, every accelerated charged particle must emit radiations in the form of electromagnetic waves and loses it total energy which doesn’t happen .
2. Since energy of electrons keep on decreasing, so radius of the circular orbits should also decrease and ultimately the electron should fall in nucleus.
3. It could not explain the line spectrum of H-atom.
4. It says nathing about the electronic structure of atom i.e. how the e ñ are distributed around the nucleus and what are the energies of these e ñ.

Limitation OF RUTHERFORDíS MODEL :

Scales of Atoms

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MUKESH SHARMA

Slide 27 of 25

Useful units:

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• 1 amu (atomic mass unit) = 1.66054 x 10^-24 kg
• 1 pm (picometer) = 1 x 10^-12 m
• 1 Å (Angstrom) = 1 x 10^-10 m = 100 pm = 1 x 10^-8 cm
• e = 1.602X10^-19 Coulomb

• The heaviest atom has a mass of only 4.8 x 10^-22 g

and a diameter of only 5 x 10^-10 m.

atomic radius ~ 100 pm = 1 x 10^-10 m

nuclear radius ~ 5 x 10^-3 pm = 5 x 10^-15 m

Size of the nucleus :

2.2

MUKESH SHARMA

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Atomic Structure 28

Thus, diameter (size) of the atom is 100,000 times the diameter of the nucleus

The Atomic Scale

• Most of the mass of the atom is in the nucleus (protons and neutrons)
• Electrons are found outside of the nucleus (the electron cloud)
• Most of the volume of the atom is empty space

“q” is a particle called a “quark”

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Atomic Structure 29

About Quarks…

Protons and neutrons are NOT fundamental particles.

Protons are made of two “up” quarks and one “down” quark.

Neutrons are made of one “up” quark and two “down” quarks.

Quarks are held together

by “gluons”

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MUKESH SHARMA

Atomic Structure 30

Atomic Number

• Atoms are composed of identical protons, neutrons, and electrons
• How then are atoms of one element different from another element?
• Elements are different because they contain different numbers of PROTONS
• The “atomic number” of an element is the number of protons in the nucleus
• # protons in an atom = # electrons

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Atomic Structure 31

Atomic Number

Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

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Atomic Structure 32

Mass Number

Mass number is the number of protons and neutrons in the nucleus of an isotope.

Mass # = p⁺ + n⁰

8

8

18

18

Arsenic

75

33

75

Phosphorus

15

31

16

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Atomic Structure 33

Nuclear Symbols

Element symbol

Mass number

(p⁺ + n^o)

Atomic number

(number of p⁺)

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Atomic Structure 34

Isotopes

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• Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912
• Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
• Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

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Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

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Atomic Structure 36

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Atomic Structure 37

5 Isodiaphers: They are the atoms of different element which have the same difference of the number of Neutrons & protons.

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6 Isosters: They are the molecules which have the same number of atoms & electrons.

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Atomic Structure 38

For nitrogen atom.

No. of electron = 7 (given)

No. of neutrons = 7 (given)

 No. of protons = Z = 7

( atom is electrically neutral)

Atomic number = Z = 7

Light as wave

• The study of light led to the development of the quantum mechanical model.
• Light is a kind of electromagnetic radiation.
• Electromagnetic radiation includes many kinds of waves
• All move at 3.00 x 10⁸ m/s ( c)

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Atomic Structure 39

Parts of a wave

Origin

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Atomic Structure 40

Parts of Wave

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Atomic Structure 41

Refraction of Light

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we are almost blind

MUKESH SHARMA

I say we are almost blind because we are only seeing about 1.5 percent of the whole electromagnetic spectrum. Visible light is not different than any of these other forms of light except for the fact that our eyes evolved to see this narrow slice because that’s the light region that our sun mostly emitted. 

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Wavelength Table

Long

Wavelength

=

Low Frequency

=

Low ENERGY

Short

Wavelength

=

High Frequency

=

High ENERGY

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Atomic Structure 44

PLANCK'S QUANTUM THEORY

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Atomic Structure 45

1. The radiant energy emitted or absorbed by a body not continuously but discontinuously in the form of small discrete packets of energy and these packets are called quantum.
2. In case of light, the smallest packet of energy is called as 'photon' but in general case the smallest packet of energy called as quantum.
3. The energy of each quantum is directly proportional to frequency of the radiation i.e

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Atomic Structure 46

Order of wavelength in Electromagnetic spectrum ��Cosmic rays <  ñ rays < X-rays < Ultraviolet rays < Visible < Infrared < Micro waves < Radio wave

Higher Frequency and Energy

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Which one was a wrong version of android from above list ?