CHEM 1405 - Chapter 4
Models of the Atom
Dalton’s Atomic Theory
1.
2.
3.
4.
Matter is composed of very minute particles called atoms, which are indivisible.
Atoms of an element are identical in all respects.
Atoms of different elements have different mass and properties
Atoms of different elements combine in a fixed ratio by mass to form “compound
atoms”. (Molecules)
5. Atoms can neither be created nor destroyed
Thomson Model of the Atom
According to Thomson, electrons are present in homogeneous spheres of positive charge just like the
raisins in English plum pudding. This model is popularly known as the plum pudding model.
The Fundamental Constituents of atom are Electrons, Protons and Neutrons.
Electrons
J.J. Thomson discovered electrons by the cathode ray tube experiment.
R.A. Millikan calculated the magnitude of charge on electrons by his oil drop experiment.
Discovery of Nucleus of the atom
Rutherford discovered the nucleus of the atom through his gold leaf experiment. Nucleus is the
positively center of the atom.
Protons
Goldstein discovered protons by cathode ray tube experiment.
Neutrons
Chadwick discovered the neutrons while performing some experiments related to the artificial
transmutation of elements.
Rutherford Model of the Atom
Rutherford suggested that atom has a positively center called nucleus. The protons and neutrons are
present in the nucleus. Electrons revolve around the nucleus just like the planets around the sun.
He calculated the diameter of the atom as 1 x 10-8 cm and the diameter of nucleus as 1 x 10-13 cm.
Modern studies have shown that atoms are divisible.
Particle
charge Magnitud e
Mass (g)
Location
Electron
1-
1.602 x 10-19 coul.
9.109 x10 -28
outside nucleus
Proton
1+
1.602 x 10-19 coul .
1.672 x 10 -24
nucleus
Neutron
0
1.675 x 10 -24
nucleus
--
An atom is electrically neutral because it contains equal number of protons and electrons.
(i.e. same number of positive and negative charges, so that the net charge becomes zero).
Atomic number (Z)
Atomic number of an element is the total number of protons in its atom.
Atomic mass, Atomic weight or Mass number (A)
Atomic mass of an element is the total number of protons and neutrons present in its atom.
The unit of atomic mass is amu (atomic mass unit). 1 amu = 1.661 x 10-24 g
Atomic Notation
This is the shorthand method of representing an element using its symbol.
A
X
( X is the symbol of the element)
Z
Isotopes
Isotopes are the atoms of the same element having same atomic number but different atomic
mass.
Example
1. Hydrogen has 3isotopes.
1
2
3
H (hydrogen)
H (deuterium)
H (tritium)
1
1
1
*Other examples of Carbon, Chlorine and Nitrogen.
Q: Find out the sign and magnitude of charge of the following species and also the atomic number and
atomic mass
a.
12 p, 12 n and 10 e(b). 6 p, 8 n and 6 e(c). 8 p, 8 n and 10eQ: Find the number of protons, neutrons and electrons.
a). 40Ca
b) 31P
c) 238U
20
15
92
Atomic mass of elements (mass number)
It is defined as the average relative mass of one atom of the element, compared to the mass of a C12
isotope (C12 isotope with atomic mass of 12 amu is taken as the standard one). The unit of atomic mass is
amu. The instrument used to determine mass number is the mass spectrometer.
Q: Calculate the average atomic mass of Ne, which is composed of three different isotopes of atomic
masses 20.00 amu, 21.00 amu and 22.00 amu. The natural abundance of each isotope is 90.92%, 0.257%
and 8.82% respectively.
Q: Mg consists of three isotopes of masses 24.0 amu, 25.0 amu and 26.0 amu. With abundances of
78.70%, 10.13% and 11.17% respectively. Calculate the average atomic mass of Mg.
Electro-Magnetic Radiation
Radiations like visible light, X-rays, UV radiations, IR radiations, cosmic rays etc are called
electro-magnetic radiations. They are associated with electric and magnetic fields. They are
produced by the oscillation or vibration of charged particles. . All electromagnetic radiations
travel with the same velocity. They are charge less and do not contain any material particles.
Nature of Electro Magnetic radiations
1. Wave nature
According to Clerk Maxwell , Electromagnetic radiations travel continuously in the form of
waves. Each radiation has a definite wavelength and all other properties associated with the
waves.
C = 
Where, ‘C’ is the velocity of radiation, ‘’ is the frequency and ‘ ‘ is the wavelength
2.Particle nature. (Quantum Theory)
According to Max Planck, electromagnetic radiations travel discontinuously in the form of
minute packets of energy.
The energy of each packet is given as E=hv
‘ h ‘ is Planck’s constant (6.63 x 10 –34 J.s) and ‘v’ is the frequency of radiation
These energy packets are called ‘quanta’ and the energy of each packet is called a quantum
of energy. In visible light, these energy packets are known as photons.
Photoelectric Effect
The phenomenon of the ejection of electrons from the surface of metals when they are
exposed to radiations of certain frequency.
The minimum frequency needed for the ejection of electrons is called the ‘Threshold
Frequency’
Spectrum
A Spectrum is an arrangement of electromagnetic radiations in the increasing order of their
wavelengths.
Light from the sun, a lit candle or an incandescent bulb is called white light. This when
resolved through a prism, gives a continuous spectrum. Here the colors (wavelengths) are
arranged in the order VIBGYOR.
Atomic Spectra
When energy is given to an atom in the form of heat energy or electrical energy, the electrons
in the atom get excited to higher energy levels by absorbing energy. This is the excited state
of an atom, which is unstable. The electrons then start falling from higher levels to lower
levels, releasing energy. This energy when resolved through a spectroscope, we get different
lines of specific wavelengths. This is called Atomic Emission Spectrum or the Line Spectrum.
This is a discontinuous spectrum. Every element has its own characteristic emission spectrum,
which is used to identify the element. Hence, it is called the atomic finger print.
When radiations pass through some substances, certain wavelengths are absorbed,
giving an Absorption Spectrum.
Each element has its own Absorption spectrum. So, it is used as a tool for qualitative and
quantitative chemical analysis.
Bohr Model of the Atom
According to Bohr, atom has a positively center called nucleus. Protons and neutrons are
present in the nucleus. The electrons revolve round the nucleus in specified orbits known as
energy levels or energy shells.
Bohr’s Theory of the Hydrogen atom
The hydrogen atom has only one electron, which is present in the first shell. When energy is
supplied to the ‘H’ atoms the electrons get excited to higher energy levels by absorbing
energy. This state is unstable and the electrons start falling from higher levels to lower levels
by releasing energy. This emitted energy appears as discrete lines in the spectrum of
Hydrogen.
The energy released is given by
E=Ef–Ei
The different series of lines in H spectrum are Lyman (UV), Balmer (V), Paschen (IR),
Brackett (IR) and Pfund (IR).
Heisenberg’s Uncertainty principle
The principle states that it is impossible to find simultaneously both position and momentum
of a small moving particle like an electron.
Orbital
An orbital is a region of space around the nucleus where there is maximum possibility of
finding the electron.
Thus the electrons in the atom are distributed in different Shells, Sub shells and Orbital.
n
sub shell
# of orbital
max# of e-
1
s
1
2
2
s
1
2
p
3
6
s
1
2
p
3
6
d
5
10
s
1
2
p
3
6
d
5
10
f
7
14
3
4
Size and Shape of Orbitals
Electron configuration of atoms
The following rules govern the electron configuration in atoms
1.The Pauli exclusion principle
States that no two electrons in an atom can have the entire four quantum numbers same.
The principle follows that an orbital cannot accommodate more than two electrons.
2.Hund’s rule
The rule states that pairing of electrons any orbital of the atom is not possible until all the
available orbital of the given set contain one electron each.
3.Aufbau principle ( Aufbau is a German word meaning “ build up “
The principle states that electrons are filled in different orbital of the atom in the order of
increasing energy.
n =1
1s
2
2s
2p
3
3s
3p
3d
4
4s
4p
4d
4f
5
5s
5p
5d
5f
6
6s
6p
6d
7
7s
7p