1. Atomic Structure and Periodicity
CHAPTER-7
Atomic Structure and Periodicity BY
DR. GHULAM ABBAS

2. Electromagnetic Radiation
Visible light is just one form of electromagnetic radiation.
Light travels in space as a wave.
In vacuum, speed of light is constant and given the symbol “c”, c = 3.00 x 108 m/s.
Light waves have amplitude, frequency, and wavelength.
Wavelength () : distance between consecutive crests.
Frequency () : number of waves that pass a given point in one second (SI Unit is s-1 or Hz).
c = and  = c/

3. Which of the above waves has
are distances – S.I. unit is the meter
Which of the above waves has
the higher frequency?

4. The Electromagnetic Spectrum
The electromagnetic spectrum consists of all the different wavelengths of electromagnetic radiation, The only region in the entire electromagnetic spectrum that our eyes are sensitive to is the visible region.
Gamma rays have the shortest wavelengths, < 0.01 nanometers (about the size of an atomic nucleus). This is the highest frequency and most energetic region of the electromagnetic spectrum. Gamma rays can result from nuclear reactions.

5. The Electromagnetic Spectrum

6. The Electromagnetic Spectrum
X-rays range in wavelength from 0.01 – 10 nm (about the size of an atom). They are generated, when matter is irradiated by a beam of high-energy charged partciles such as electrons.
Ultraviolet radiation has wavelengths of 10 – 310 nm (about the size of a virus). Stars produce a lot of ultraviolet light.
Visible light covers the range of wavelengths from 400 – 700 nm (from the size of a molecule to a protozoan). Our sun emits the most of its radiation in the visible range, which our eyes perceive as the colors of the rainbow. Our eyes are sensitive only to this small portion of the electromagnetic spectrum.

7. The Electromagnetic Spectrum
Infrared wavelengths span from 710 nm – 1 millimeter (from the width of a pinpoint to the size of small plant seeds). At a temperature of 37 oC, our bodies give off infrared wavelengths with a peak intensity near 900 nm.
Radio waves are longer than 1 mm waves, have the lowest energy. Radio stations use radio wavelengths of electromagnetic radiation to send signals that our radios then translate into sound. Radio stations transmit electromagnetic radiation, not sound. Our radios receive the electromagnetic radiation, decode the pattern and translate the pattern into sound.

8. James Maxwell proposed in the mid-1800’s that light
is composed of perpendicular electric and magnetic waves

9. Sample Calculation – wavelength/frequency conversion
Calculate the frequency of visible light having a wavelength of 485 nm?
Remember to use S.I. units in your calculations!
 = c / 
= (3.00 x 108 m/s) ÷ (485 x 10-9 m)
= x 1014 s-1
What colour of visible light is this?

10. The Nature of Light Particles or Waves??
Light must be made of particles because it…
travels in a vacuum
reflects off of objects
exerts force (on the tails of comets)
Light must consist of waves because it…
reflects like waves
refracts and diffracts
exhibits interference
By the end of the 19th century,
scientists had concluded that
light is composed of WAVES!

11. History of Optics & Light Studies
Ibn Alhazen is considered the “Father of Optics.” He wrote the “Book of Optics”, which correctly explained and proved the modern theory of vision. His experiments included ones on lenses, mirrors, refraction, reflection, and the dispersion of light into its constituent colors. He studied the electromagnetic aspects of light, and argued that rays of light are streams of energy particles traveling in straight lines.
Ibn Alhazen
(965 – 1039)
Arab Muslim Scientist
“Father of Optics”

12. Historical Background of Spectroscopy
In 1608, Galileo Galilei is credited as the first to turn his telescope to the heavens. He soon discovered craters on our Moon, sun spots.
antwrp.gsfc.nasa.gov/ apod/ap html
Galileo Galilei

13. A Quantitative Study of Light
Sir Isaac Newton was one of the first man to study light scientifically.
In 1672, Newton directed a beam of white light through a triangular bar of glass, called a “prism”. He discovered that the light coming out of the prism was separated into bands of colors.
The arrangement of colors produced by a prism is called a “spectrum”.
Prior to this it was believed that “white light” was equal to purity.
lightoptics/spectrum.htm and web.mit.edu/.../research/ A-Vision/A10-1.html
Sir Isaac Newton

14. Original Studies Of Light Used Only One Prism.
When a narrow band of light from a “white” light source is sent through a prism, a continuous spectrum containing all wavelengths of visible light is formed.

15. Record Hydrogen line spectrum with a Scanning Spectrophotometer
The hydrogen line spectrum contains only a few discrete wavelengths.
In the visible region, there are only four wavelengths.

16. Proposed a model that the electron
In 1913, Bohr developed a quantum model for the hydrogen atom.
Proposed a model that the electron
in a hydrogen atom moves around the nucleus only in certain allowed circular orbits.
Niels Henrik David Bohr
Oct. 7, 1885 – Nov. 18, 1962
Danish Physicist
In atomic physics, the Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus — similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a quantum-physics based modification of the Rutherford model, many sources combine the two, referring to the Rutherford-Bohr model.
The Nobel Prize in Physics 1922 for the investigation of the structure of atoms and of the radiation emiting from them.
Solar System Model has electrons moving around the nucleus.

17. How are the electrons arranged in a Bohr Model of an atom?
Electrons orbit the nucleus.
Electrons are arranged in specific pathways called energy levels.
There’s a fixed number of energy levels.
Each energy level is capable of holding certain number of electrons.
An electron cannot be found between energy levels.

18. Bohr’s Quantum Model for Hydrogen
The electron in hydrogen occupies discrete energy levels.
The atom does not radiate energy when the electron is in an energy level.
When an electron falls to a lower energy level, a quantum of radiation is released with energy equal to the difference between energy levels.
An electron can jump to higher energy levels if the atom absorbs a quantum of radiation with sufficient energy.

19. Bohr’s Explanation of Line Spectra
Neils Bohr developed a mathematical model that could explain the observation of atomic line spectra.
He proposed that electrons orbit the nucleus in certain “allowed” orbits - or energy levels. That is, the electron’s energy is QUANTIZED (not continuous).
Working on a model of single-electron atoms, Bohr derived an equation to calculate the energy of an electron in the nth orbit of such an atom:
where Z = atomic number (nuclear charge) and n = electron energy level
- Ve sign means energy of the electron bound to nucleus is lower than it would be if electron were at infinite distance.

20. The Hydrogen Line Spectrum

21. Bohr’s Model: Calculations
Show that an electron transition from n = 4 to n = 2 results in the emission of visible light. Calculate the wavelength of the light emitted.
Step 1: Calculate E4 & E2
E4 = x J
E2 = x J
Step 2: Find DE42
DE = E2 – E4 = 4.09 x J
Step 3: Calculate l
l = 4.86 x 10-7 m = 486 nm

22. Bohr Model

23. What’s Wrong with Bohr’s Model??
Although it works great for single-electron atoms, Bohr’s model fails for atoms with 2 or more electrons!
It was a huge leap forward, but was fundamentally flawed.
Ultimately, the failure of Bohr’s model lay in the fact that he treated the electron as a charged particle orbiting the nucleus like a planet around the sun.
Electrons are more complicated …

24. Particle Theory of Light
In 1900, Max Planck turned the world of physics on its head by presenting the particle theory of light.
He proposed that light is composed of particles (quanta) each carrying a fixed amount of energy.
The amount of energy per quantum is directly proportional to the frequency of the light.
Max Karl Ernst Ludwig Planck
(April 23, 1858 – October 4, 1947)
German Physicist
This hypothesis was later extended by Albert Einstein. He presented light as small discrete particles of energy called photons.
The Nobel Prize in Physics 1918 for
The discovery of energy quanta.

25. EXAMPLE: