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Dalton's theory can be summarized as follows: - All matter consists of atoms.
- Atoms are indivisible indivisible & cannot be created or destroyed.
- Atoms of a particular element are identical in mass.mass.
- Atoms rearrange in chem. reactions.
- Compounds are formed from the combination of the atoms of two or more elements.Atoms combine in simple numerical ratios.
He studied the deflection of cathode rays towards (+) charged plates of his specially designed cathode-ray tube.
This shows that the ray is composed of (-) charged particles.
J. J. Thomson
He proposed the plum pudding model of the atom.
Thomson proposed that an atom was composed of a spherical ball of (+) charge with "corpuscles" of (-) charge imbedded in it.
The corpuscles would later become known as electrons.
Ernest Rutherford
He bombarded thin gold foil with α particles.
He observed that most of the α particles passed through the foil un-deflected.
Very few particles were deflected when passed through the foil.
One particle out of 8000 particles was deflected at 90o
Few particles were deflected at different angles.
Major portion of the atom is empty.
The whole mass of the atom is concentrated in the centre of atom called nucleus.
The (+) charged particles are present in the nucleus of atom.
•
The charge on the nucleus of an atom is equal to charge of proton.
The e- revolve around the nucleus in different circular orbits.circular orbits.
Size of nucleus is very small as compare to the size of atom.
The Bohr model depicts the atom containing a (+) charged nucleus surrounded by e- that travel in successively successively larger orbits larger orbits around the nucleus.
This is similar in structure to the solar system.
Bohr model of the atom
- The eThe e-- can only travel in special orbits. can only travel in special orbits.
- The e- do not continuously lose energy as they travel.
-They can only gain and lose E by jumping from 1 orbit to another,
-They absorb or emit electromagnetic radiation with a frequency determined by the energy diff. of the levels.
He studied radiation from bombardment of He with α particles,
He noted a particle with approx. the same mass as a proton being released.
He determined that, as the particle was not bent by electrical fields it was electrically neutral.
Discoverer DiscoveryJohn Dalton 1st Modern Atomic Theory
J.J Thomson Electrons => studied CRTs.
Ernest Rutherford Nucleus => bombarded Au with α particles.
Niels Bohr Structure of atom => e- travel in successively larger orbits.
James Chadwick Neutrons => bombarded He with α particles.
Atomic Number-The no. of protons in the nucleus of an atom.
-The atomic no. = no. of e- in a neutral atom.
Mass Number
-The no. of protons & neutrons in the nucleus of an atom.
ISOTOPES
- They are elements having atoms that have same no. of protons but diff. no of neutrons.neutrons.
- Hence, the atomic no. is same, but mass no. is different.
- Isotopes have different physical prop. but similar chem. prop.
- The mass no for each element is rarely a whole no. because elements have few isotopes that are present in diff. compositions.
MODERN ATOMIC THEORY
• Niels Bohr unlocked secret of Line Emission Spectrum.
• Electrons orbiting in shells around the nucleus.
• Won the 1922 Nobel Prize for Physics.
Radiowaves
Microwaves
Infrared .
Ultra-violet
X-Rays
GammaRays
Low energy
High energy
Low Frequency
High Frequency
Long Wavelength
Short WavelengthVisible Light
Hydrogen Emission Spectrum
Hydrogen Emission Spectrum
When H2 (g) is placed in a Crookes Discharge tube & high voltage is applied, the tube glows with a pale violet light.
If this light is passed through a triangular prism, the screen beyond is illuminated with a series of red, blue-green, blue & violet lines.
How are the lines of the line spectrum formed?
The e- in the atom became excited by energy supplied from high voltage & move to a higher energy level.
The e- then returned from an excited level to a more stable energy level closer to the nucleus.
In making this transition betw the levels, the e- released a FIXED quantity of energy in the form of electromagnetic radiation (light rays) of a fixed wavelength.
In other words, transitions/movement from higher energy levels lead to DISCRETE emissions of energy.
n=1
n=2
n=3
n=4
Spectrum
UV
IR
Vi s ible
Ground State
Excited State
Excited StateExcited State unstable and drops back down
•Energy released as a photon•Frequency proportional to energy drop
Excited State
But only as far as n = 2 this time
Line spectrum of various elements
The e- moves from the lower (ground) level to a
higher excited level
(in this case from n=1 to n=4 level)
In this eg, the excited e- falls from the n=4 to the n=2 level.
The light photons in this case produce a spectral line
that is blue-green
FLAME TEST - APPLICATION OF LES
Erwin Schrodinger – proposed that e- do not travel in orbits but move in space called orbitals.
-This theory is known as quantum mechanics.
The Quantum Mechanical Model of the atom presents a more accurate model of the atom.
The Quantum Mechanical Model introduces the concept of:
- Electrons – they behave like a cloud of (-) charge
- Electrons move in regions of space surrounding the nucleus called orbitals.
(1) The element chlorine has two isotopes. Which of the following statements concerning these isotopes is incorrect?
(a) They have the same nuclear charge
(b) They have the same number of electrons
(c) Their atoms have the same mass
(d) They are in the same position in the periodic table
QUESTIONS
(3). Two isotopes of Chlorine 35Cl and 37Cl are present in chlorine gas. How many types of chlorine molecules, which differ by mass, can exist?
(a) 3 (b) 2 (c) 1 (d) 4
2 An atom of Ceasium, 55Cs, ionizes to form the Cs+ ion. This ion has
(a) 53 electrons (b) 54 electrons (c) 55 electrons (d) 56 electrons
4 Which one of the following has the following has the least number of electrons?
(a) Cl
(b) Cl-
(c) Ar
(d) S2-
Thank You Very Much!
