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26 CHEMISTRY
UNIT 2
STRUCTURE OF ATOM
The rich diversity of chemical behaviour of different elements
can be traced to the differ ences in the internal structure of
atoms of these elements.
After studying this unit you will be
able to
• know about the discovery of
The existence of atoms has been proposed since the time
electron, proton and neutron and
of early Indian and Greek philosophers (400 B.C.) who
their characteristics;
were of the view that atoms are the fundamental building
• describe Thomson, Rutherford blocks of matter. According to them, the continued
and Bohr atomic models; subdivisions of matter would ultimately yield atoms which
• understand the important would not be further divisible. The word ‘atom’ has been
features of the quantum derived from the Greek word ‘a-tomio’ which means ‘uncut-
mechanical model of atom; able’ or ‘non-divisible’. These earlier ideas were mere
speculations and there was no way to test them
• understand nature of
experimentally. These ideas remained dormant for a very
electromagnetic radiation and
Planck’s quantum theory;
long time and were revived again by scientists in the
nineteenth century.
• explain the photoelectric effect
The atomic theory of matter was first proposed on a
and describe features of atomic
firm scientific basis by John Dalton, a British school
spectra;
teacher in 1808. His theory, called Dalton’s atomic
• state the de Broglie relation and theory, regarded the atom as the ultimate particle of
Heisenberg uncertainty principle; matter (Unit 1).
• define an atomic orbital in terms In this unit we start with the experimental
of quantum numbers; observations made by scientists towards the end of
• state aufbau principle, Pauli
nineteenth and beginning of twentieth century. These
exclusion principle and Hund’s established that atoms can be further divided into sub-
rule of maximum multiplicity; atomic particles, i.e., electrons, protons and neutrons—
a concept very different from that of Dalton. The major
• write the electronic configurations problems before the scientists at that time were:
of atoms.
• to account for the stability of atom after the discovery
of sub-atomic particles,
• to compare the behaviour of one element from other
in terms of both physical and chemical properties,
2015-16
,STRUCTURE OF ATOM 27
• to explain the formation of different kinds
of molecules by the combination of
different atoms and,
• to understand the origin and nature of the
characteristics of electromagnetic
radiation absorbed or emitted by atoms.
2.1 SUB-ATOMIC PARTICLES
Dalton’s atomic theory was able to explain
the law of conservation of mass, law of Fig. 2.1(a) A cathode ray discharge tube
constant composition and law of multiple
proportion very successfully. However, it failed stream of particles moving in the tube from
to explain the results of many experiments, the negative electrode (cathode) to the positive
for example, it was known that substances electrode (anode). These were called cathode
like glass or ebonite when rubbed with silk or rays or cathode ray particles. The flow of
fur generate electricity. Many different kinds current from cathode to anode was further
of sub-atomic particles were discovered in the checked by making a hole in the anode and
twentieth century. However, in this section coating the tube behind anode with
we will talk about only two particles, namely phosphorescent material zinc sulphide. When
electron and proton. these rays, after passing through anode, strike
the zinc sulphide coating, a bright spot on
2.1.1 Discovery of Electron the coating is developed(same thing happens
In 1830, Michael Faraday showed that if in a television set) [Fig. 2.1(b)].
electricity is passed through a solution of an
electrolyte, chemical reactions occurred at the
electrodes, which resulted in the liberation
and deposition of matter at the electrodes. He
formulated certain laws which you will study
in class XII. These results suggested the
particulate nature of electricity.
An insight into the structure of atom was
obtained from the experiments on electrical
discharge through gases. Before we discuss
these results we need to keep in mind a basic Fig. 2.1(b) A cathode ray discharge tube with
rule regarding the behaviour of charged perforated anode
particles : “Like charges repel each other and The results of these experiments are
unlike charges attract each other”.
summarised below.
In mid 1850s many scientists mainly (i) The cathode rays start from cathode and
Faraday began to study electrical discharge
move towards the anode.
in partially evacuated tubes, known as
cathode ray discharge tubes. It is depicted (ii) These rays themselves are not visible but
in Fig. 2.1. A cathode ray tube is made of glass their behaviour can be observed with the
containing two thin pieces of metal, called help of certain kind of materials
electrodes, sealed in it. The electrical (fluorescent or phosphorescent) which
discharge through the gases could be glow when hit by them. Television
observed only at very low pressures and at picture tubes are cathode ray tubes and
very high voltages. The pressure of different television pictures result due to
gases could be adjusted by evacuation. When fluorescence on the television screen
sufficiently high voltage is applied across the coated with certain fluorescent or
electrodes, current starts flowing through a phosphorescent materials.
2015-16
,28 CHEMISTRY
(iii) In the absence of electrical or magnetic (ii) the mass of the particle — lighter the
field, these rays travel in straight lines particle, greater the deflection.
(Fig. 2.2). (iii) the strength of the electrical or magnetic
(iv) In the presence of electrical or magnetic field — the deflection of electrons from
field, the behaviour of cathode rays are its original path increases with the
similar to that expected from negatively increase in the voltage across the
charged particles, suggesting that the electrodes, or the strength of the
cathode rays consist of negatively magnetic field.
charged particles, called electrons. When only electric field is applied, the
(v) The characteristics of cathode rays electrons deviate from their path and hit the
(electrons) do not depend upon the cathode ray tube at point A. Similarly when
material of electrodes and the nature of only magnetic field is applied, electron strikes
the gas present in the cathode ray tube. the cathode ray tube at point C. By carefully
Thus, we can conclude that electrons are balancing the electrical and magnetic field
basic constituent of all the atoms. strength, it is possible to bring back the
electron to the path followed as in the absence
2.1.2 Charge to Mass Ratio of Electron
of electric or magnetic field and they hit the
In 1897, British physicist J.J. Thomson screen at point B. By carrying out accurate
measured the ratio of electrical charge (e) to measurements on the amount of deflections
the mass of electron (me ) by using cathode observed by the electrons on the electric field
ray tube and applying electrical and magnetic strength or magnetic field strength, Thomson
field perpendicular to each other as well as to
was able to determine the value of e/me as:
the path of electrons (Fig. 2.2). Thomson
argued that the amount of deviation of the e
me = 1.758820 × 10 C kg (2.1)
11 –1
particles from their path in the presence of
electrical or magnetic field depends upon:
Where me is the mass of the electron in kg
(i) the magnitude of the negative charge on
and e is the magnitude of the charge on the
the particle, greater the magnitude of the
charge on the particle, greater is the electron in coulomb (C). Since electrons
interaction with the electric or magnetic are negatively charged, the charge on electron
field and thus greater is the deflection. is –e.
Fig. 2.2 The apparatus to deter mine the charge to the mass ratio of electron
2.1.3 Charge on the Electron
Millikan’s Oil Drop Method
R.A. Millikan (1868-1953) devised a method
In this method, oil droplets in the form of
known as oil drop experiment (1906-14), to mist, pr oduced by the atomiser, were allowed
determine the charge on the electrons. He to enter thr ough a tiny hole in the upper plate
found that the charge on the electron to be of electrical condenser. The downward motion
– 1.6 × 10–19 C. The present accepted value of of these dr oplets was viewed through the
electrical charge is – 1.6022 × 10–19 C. The telescope, equipped with a micrometer eye
mass of the electron (me) was determined by piece. By measuring the rate of fall of these
combining these results with Thomson’s value droplets, Millikan was able to measure the
of e/me ratio. mass of oil dr oplets.The air inside the
chamber was ionized by passing a beam of
e 1.6022 × 10–19 C X-rays through it. The electrical charge on
me = = these oil dr oplets was acquired by collisions
e/ m e 1.758820 × 1011C kg –1
with gaseous ions. The fall of these charged
= 9.1094×10–31 kg (2.2) oil droplets can be retar ded, accelerated or
made stationary depending upon the charge
2.1.4 Discovery of Protons and Neutrons
on the droplets and the polarity and strength
Electrical discharge carried out in the of the voltage applied to the plate. By carefully
modified cathode ray tube led to the discovery measuring the ef fects of electrical field
of particles carrying positive charge, also strength on the motion of oil dr oplets,
known as canal rays. The characteristics of Millikan concluded that the magnitude of
these positively charged particles are listed electrical charge, q, on the dr oplets is always
an integral multiple of the electrical charge,
below.
e, that is, q = n e, where n = 1, 2, 3... .
(i) unlike cathode rays, the positively
charged particles depend upon the
nature of gas present in the cathode ray
tube. These are simply the positively
charged gaseous ions.
(ii) The charge to mass ratio of the particles
is found to depend on the gas from which
these originate.
(iii) Some of the positively charged particles
carry a multiple of the fundamental unit
of electrical charge.
(iv) The behaviour of these particles in the
magnetic or electrical field is opposite to
that observed for electron or cathode Fig. 2.3 The Millikan oil dr op apparatus for
measuring charge ‘e’. In chamber, the
rays.
forces acting on oil drop ar e :
The smallest and lightest positive ion was gravitational, electrostatic due to
obtained from hydrogen and was called electrical field and a viscous drag force
proton. This positively charged particle was when the oil drop is moving.
characterised in 1919. Later, a need was felt
properties of these fundamental particles are
for the presence of electrically neutral particle given in Table 2.1.
as one of the constituent of atom. These
particles were discovered by Chadwick (1932) 2.2 ATOMIC MODELS
by bombarding a thin sheet of beryllium by Observations obtained from the experiments
α-particles. When electrically neutral particles mentioned in the previous sections have
having a mass slightly greater than that of suggested that Dalton’s indivisible atom is
the protons was emitted. He named these composed of sub-atomic particles carrying
particles as neutr ons. The important positive and negative charges. Different
2015-16
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