ATOMIC STRUCTURE
Discovery of sub-atomic particles.
Shortcomings of Dalton’s atomic theory.
1. The theory did not account for the existence of subatomic particles (it suggested that atoms are
indivisible).
2. By suggesting that all atoms of an element must have identical masses and sizes, Dalton’s atomic
theory did not account for the existence of isotopes. Furthermore, this theory also did not account
for the existence of isobars (nuclides of different chemical elements with the same mass number).
3. Dalton’s atomic theory failed to explain the dissimilarities in the properties of different
allotropes of an element.
4. This theory states that elements must combine in simple, whole-number ratios to form
compounds. However, this is not necessarily true. Several complex organic compounds do not
feature simple ratios of their constituent elements.
Various experiments that led to the discovery of neutrons, protons, electrons and
nucleus [Cathode ray, Millikans cathode ray, Rutherford and Chadwick experiment].
Discovery of Neutron (Chadwick Experiment)
In 1932, Chadwick bombarded alpha radiation at beryllium sheet from a polonium source. This
led to the production of an uncharged, penetrating radiation. This radiation was made incident on
paraffin wax, a hydrocarbon having a relatively high hydrogen content. The protons ejected from
the paraffin wax (when struck by the uncharged radiation) were observed with the help of an
ionization chamber. The range of the liberated protons was measured and the interaction between
the uncharged radiation and the atoms of several gases was studied. He concluded that the
unusually penetrating radiation consisted of uncharged particles having (approximately) the same
mass as a proton. These particles were later termed ‘neutrons’. The diagram of Chadwick
experiment is shown in Fig 1.
Figure 1: Diagrammatic representation of the Chadwick Experiment
,Discovery of Electron (Cathode Ray Experiment)
In 1897, J. J. Thomson showed that when a potential difference of 5 000V was applied across a
glass tube containing a gas at a very low pressure of about 0.0001 atm, the tube began to glow.
When the potential difference was increased to 15 000V, a bright green glow appeared on the
glass. Thomson was able to prove that the glow was due to some kind of rays which travelled in
straight lines from the cathode. He called these rays cathode rays.
Further experiments showed that cathode rays
travels in straight lines and cast shadows of opaque objects placed in their paths;
are composed of only negatively charged particles;
are capable of producing mechanical motion;
are identical in nature and in the ratio of charge to mass, irrespective of the nature of the
residual gas in the discharge tube or the metals used in the electrodes.
Thomson argued that these particles were fundamental particles, i.e. they were present in all matter.
He concluded that these particles must be the electrons proposed earlier by other scientists to
explain the conduction of an electric current in wires and in solutions of electrolytes. Thomson
determined the mass to charge ratio of the cathode ray particles, which led to a fascinating
discovery, the mass of each particle was much, much smaller than that of any known atom.
Figure 2: Diagrammatic representation of the cathode ray experiment
In 1910, Millikan successfully measured the charge of the electron in his oil drop experiment.
Millikan used an atomizer to spray droplets of oil into a chamber in which the oil droplets
eventually would fall into a strong electric field. Within this field (between charged metal plates),
X-ray was used to ionize gas molecules which produced free electrons that attached themselves to
the oil droplets. Thus, the oil droplets acquired negative charge.
,These experiments are important because they proved the existence of the electron which carries
the fundamental unit of electric charge, the negative charge, and provided a vital clue to the
structure of the atom.
Figure 3: Diagrammatic representation of the Millikan oil drop experiment.
Discovery of Protons
Since the atom as a whole is electrically neutral, there must exist inside the atom enough positively
charged components to balance the negative charge of the electrons. Thomson repeated his earlier
experiments, but used a discharge tube with a central cathode which had a hole in it. He noticed a
reddish glow in the opposite direction to the green glow and proved that this reddish glow was due
to positively charged rays.
Further experiments showed that
unlike the cathode rays, the positive rays required much larger magnetic fields to cause
their deflections (indicating that they are much heavier than the electrons);
the mass of the positively charged particles depended on the nature of the gas in the tube.
Figure 4: Diagrammatic representation of the Thomson experiment.
, Discovery of the Nucleus (The Gold Foil experiment).
Geiger and Marsden, at the suggestion of Rutherford, used positively charged particles called alpha
particles to bombard the atom. In their classical experiment, they used a narrow beam of alpha
particles emitted from a radioactive source to bombard a thin gold foil. The scattering of the
particles from the gold foil was detected by a movable zinc sulphide screen which could be rotated
to various positions around the foil. Each time an alpha particle hit the screen, a visible flash light
or scintillation was produced. This was observed by a microscope attached to the screen.
Geiger and Marsden found that most of the alpha particles followed a straight path through the
gold foil. But some of them were scattered through large angles and a few were even scattered in
the backward direction. In 1911, Rutherford deduced that the atom consists largely of empty space,
since the majority of the alpha particles passed through the gold foil without being deflected. He
explained that the alpha particles which were deflected through large angles or rebounded must
have collided with that part of the atom in which the positive charge and mass of the atom were
concentrated. To account for the observation in the gold-foil experiment, Rutherford proposed a
nuclear theory of the atom. According to this theory, the atom consists of a positive core called the
nucleus, where most of the mass of the atom is contained, and electrons which move around the
nucleus.
Figure 5: Diagrammatic representation of the Gold foil experiment.
Planck’s theory
Blackbody radiation
A blackbody is an idealized object that absorbs all the radiation incident on it. When a solid object
is heated to about 1000 K, it begins to emit visible light, as you can see in the soft red glow of
smoldering coal (Figure 6, left). At about 1500 K, the light is brighter and more orange, like that
Discovery of sub-atomic particles.
Shortcomings of Dalton’s atomic theory.
1. The theory did not account for the existence of subatomic particles (it suggested that atoms are
indivisible).
2. By suggesting that all atoms of an element must have identical masses and sizes, Dalton’s atomic
theory did not account for the existence of isotopes. Furthermore, this theory also did not account
for the existence of isobars (nuclides of different chemical elements with the same mass number).
3. Dalton’s atomic theory failed to explain the dissimilarities in the properties of different
allotropes of an element.
4. This theory states that elements must combine in simple, whole-number ratios to form
compounds. However, this is not necessarily true. Several complex organic compounds do not
feature simple ratios of their constituent elements.
Various experiments that led to the discovery of neutrons, protons, electrons and
nucleus [Cathode ray, Millikans cathode ray, Rutherford and Chadwick experiment].
Discovery of Neutron (Chadwick Experiment)
In 1932, Chadwick bombarded alpha radiation at beryllium sheet from a polonium source. This
led to the production of an uncharged, penetrating radiation. This radiation was made incident on
paraffin wax, a hydrocarbon having a relatively high hydrogen content. The protons ejected from
the paraffin wax (when struck by the uncharged radiation) were observed with the help of an
ionization chamber. The range of the liberated protons was measured and the interaction between
the uncharged radiation and the atoms of several gases was studied. He concluded that the
unusually penetrating radiation consisted of uncharged particles having (approximately) the same
mass as a proton. These particles were later termed ‘neutrons’. The diagram of Chadwick
experiment is shown in Fig 1.
Figure 1: Diagrammatic representation of the Chadwick Experiment
,Discovery of Electron (Cathode Ray Experiment)
In 1897, J. J. Thomson showed that when a potential difference of 5 000V was applied across a
glass tube containing a gas at a very low pressure of about 0.0001 atm, the tube began to glow.
When the potential difference was increased to 15 000V, a bright green glow appeared on the
glass. Thomson was able to prove that the glow was due to some kind of rays which travelled in
straight lines from the cathode. He called these rays cathode rays.
Further experiments showed that cathode rays
travels in straight lines and cast shadows of opaque objects placed in their paths;
are composed of only negatively charged particles;
are capable of producing mechanical motion;
are identical in nature and in the ratio of charge to mass, irrespective of the nature of the
residual gas in the discharge tube or the metals used in the electrodes.
Thomson argued that these particles were fundamental particles, i.e. they were present in all matter.
He concluded that these particles must be the electrons proposed earlier by other scientists to
explain the conduction of an electric current in wires and in solutions of electrolytes. Thomson
determined the mass to charge ratio of the cathode ray particles, which led to a fascinating
discovery, the mass of each particle was much, much smaller than that of any known atom.
Figure 2: Diagrammatic representation of the cathode ray experiment
In 1910, Millikan successfully measured the charge of the electron in his oil drop experiment.
Millikan used an atomizer to spray droplets of oil into a chamber in which the oil droplets
eventually would fall into a strong electric field. Within this field (between charged metal plates),
X-ray was used to ionize gas molecules which produced free electrons that attached themselves to
the oil droplets. Thus, the oil droplets acquired negative charge.
,These experiments are important because they proved the existence of the electron which carries
the fundamental unit of electric charge, the negative charge, and provided a vital clue to the
structure of the atom.
Figure 3: Diagrammatic representation of the Millikan oil drop experiment.
Discovery of Protons
Since the atom as a whole is electrically neutral, there must exist inside the atom enough positively
charged components to balance the negative charge of the electrons. Thomson repeated his earlier
experiments, but used a discharge tube with a central cathode which had a hole in it. He noticed a
reddish glow in the opposite direction to the green glow and proved that this reddish glow was due
to positively charged rays.
Further experiments showed that
unlike the cathode rays, the positive rays required much larger magnetic fields to cause
their deflections (indicating that they are much heavier than the electrons);
the mass of the positively charged particles depended on the nature of the gas in the tube.
Figure 4: Diagrammatic representation of the Thomson experiment.
, Discovery of the Nucleus (The Gold Foil experiment).
Geiger and Marsden, at the suggestion of Rutherford, used positively charged particles called alpha
particles to bombard the atom. In their classical experiment, they used a narrow beam of alpha
particles emitted from a radioactive source to bombard a thin gold foil. The scattering of the
particles from the gold foil was detected by a movable zinc sulphide screen which could be rotated
to various positions around the foil. Each time an alpha particle hit the screen, a visible flash light
or scintillation was produced. This was observed by a microscope attached to the screen.
Geiger and Marsden found that most of the alpha particles followed a straight path through the
gold foil. But some of them were scattered through large angles and a few were even scattered in
the backward direction. In 1911, Rutherford deduced that the atom consists largely of empty space,
since the majority of the alpha particles passed through the gold foil without being deflected. He
explained that the alpha particles which were deflected through large angles or rebounded must
have collided with that part of the atom in which the positive charge and mass of the atom were
concentrated. To account for the observation in the gold-foil experiment, Rutherford proposed a
nuclear theory of the atom. According to this theory, the atom consists of a positive core called the
nucleus, where most of the mass of the atom is contained, and electrons which move around the
nucleus.
Figure 5: Diagrammatic representation of the Gold foil experiment.
Planck’s theory
Blackbody radiation
A blackbody is an idealized object that absorbs all the radiation incident on it. When a solid object
is heated to about 1000 K, it begins to emit visible light, as you can see in the soft red glow of
smoldering coal (Figure 6, left). At about 1500 K, the light is brighter and more orange, like that
