Page 105 - Chemistry ICSE Class IX

Atomic Structure and Chemical Bonding 93
4. The D-particles have appreciable mass. These are de ected by the
nucleus. It means that almost the entire mass of the atom lies at its /Ĩ ǁĞ ŵĂŐŶŝĨLJĂŶĂƚŽŵƚŽƚŚĞ ƐŝnjĞ ŽĨ Ă
centre, i.e., the entire mass of an atom is concentrated inside its nucleus. ĨŽŽƚďĂůů ƐƚĂĚŝƵŵ ;ĂďŽƵƚ ϭϬϬ ŵͿ͕ ƚŚĞŶ
ƚŚĞ ŶƵĐůĞƵƐ ǁŽƵůĚ ďĞ ƌĞƉƌĞƐĞŶƚĞĚ
ďLJ Ă ƉĞĂ ƉůĂĐĞĚ Ăƚ ƚŚĞ ĐĞŶƚƌĞ ŽĨ ƚŚĞ
Rutherford’s Nuclear Model of Atom ƉŝƚĐŚ͘

On the basis of the famous D-particle scattering experiment, Rutherford
proposed his nuclear model of the atom in 1911. The main points of
Rutherford’s nuclear model are described below and shown in Fig. 4.9.
ƒ An atom consists of a positively charged nucleus, which is surrounded
by electrons moving around it.
ƒ Electrons and the nucleus are held together by coulombic force of
attraction.
ƒ The size of the nucleus is very small as compared to the size of the
atom. Experimentally, it was found that the, Sir Ernest Rutherford
Radius of the nucleus of an atom = 10 –14 to 10 –15 m ;ϭϴϳϭ ͲϭϵϯϳͿ
Radius of an atom = 10 –10 m ZƵƚŚĞƌĨŽƌĚ ǁĂƐ ďŽƌŶ Ăƚ EĞůƐŽŶ͕
Thus, the size of the nucleus is about ten-thousandth part of the size EĞǁ ĞĂůĂŶĚ͘ ,Ğ ǁĂƐ ĞĚƵĐĂƚĞĚ Ăƚ
of an atom. ƚŚĞ hŶŝǀĞƌƐŝƚLJ ŽĨ EĞǁ ĞĂůĂŶĚ ĂŶĚ
ĐŽŶĚƵĐƚĞĚ ƌĞƐĞĂƌĐŚ ǁŽƌŬ ŝŶ ϭϴϵϱ
ƒ Almost the entire mass of an atom is concentrated in its nucleus. ǁŝƚŚ :͘ :͘ dŚŽŵƐŽŶ Ăƚ ƚŚĞ ĂǀĞŶĚŝƐŚ
ƒ Atom, as a whole, is electrically neutral. So, number of protons inside >ĂďŽƌĂƚŽƌLJ͘ /Ŷ ϭϵϬϳ͕ ŚĞ ĂĐĐĞƉƚĞĚ
the nucleus of an atom and the number of electrons surrounding the >ĂŶŐǁŽƌƚŚLJ WƌŽĨĞƐƐŽƌƐŚŝƉ Ăƚ
nucleus are equal. DĂŶĐŚĞƐƚĞƌ hŶŝǀĞƌƐŝƚLJ ŝŶ ŶŐůĂŶĚ
ǁŚĞƌĞ ŚĂĚǁŝĐŬ ǁĂƐ ŽŶĞ ŽĨ ŚŝƐ
What are the drawbacks in the Rutherford’s nuclear model ƐƚƵĚĞŶƚƐ͘ ZƵƚŚĞƌĨŽƌĚ ǁŽŶ ƚŚĞ
EŽďĞů WƌŝnjĞ ŝŶ ϭϵϬϴ ĨŽƌ ŚŝƐ ĐůĂƐƐŝĐ
of an atom ŝŶǀĞƐƟŐĂƟŽŶƐ ŝŶ ƚŚĞ ĮĞůĚ ŽĨ ĂƚŽŵŝĐ
ƉŚLJƐŝĐƐ͘ ZƵƚŚĞƌĨŽƌĚ ĞdžƉůĂŝŶĞĚ ƚŚĂƚ
The Rutherford model explains the structure of an atom in a very simple ƚŚĞ ŵĂũŽƌ ƉŽƌƟŽŶ ŽĨ ƚŚĞ ĂƚŽŵŝĐ
way. But, it suffers from the following drawbacks: ŵĂƐƐ ŝƐ ĐŽŶĐĞŶƚƌĂƚĞĚ ŝŶ Ă ŵŝŶƵƚĞ͕
ƒ An electron revolving around the nucleus gets accelerated towards ĐĞŶƚƌĂůůLJ ůŽĐĂƚĞĚ͕ ƉŽƐŝƟǀĞůLJ ĐŚĂƌŐĞĚ
the nucleus. An accelerating charged particle must emit radiation and ŶƵĐůĞƵƐ͘ Ɛ ĞĂƌůLJ ĂƐ ŝŶ ϭϵϮϬ͕ ŚĞ
lose energy. Thus, the electrons in an atom must continuously emit ƉƌĞĚŝĐƚĞĚ ƚŚĞ ƉƌĞƐĞŶĐĞ ŽĨ ƉĂƌƟĐůĞƐ
radiation and lose energy. Because of this loss of energy, the electron ǁŝƚŚ ƵŶŝƚ ŵĂƐƐ ďƵƚ ŚĂǀŝŶŐ ŶŽ
ĐŚĂƌŐĞ͘ ,Ğ ŶĂŵĞĚ ƐƵĐŚ ƉĂƌƟĐůĞƐ
would slow down and will not be able to withstand the attraction of ŶĞƵƚƌŽŶƐ͘
the nucleus. As a result, the electron would follow a spiral path and
ultimately fall into the nucleus (Fig. 4.10). If it happens, then the atom
–8
should collapse in about 10 second. But, this does not happen—
atoms are stable. This indicates that there is something wrong in
Rutherford’s nuclear model of atom.
ƒ Rutherford’s model of atom does not say anything about the
arrangement of electrons in an atom.

Böhr’s Model of Atom Fig. 4.10 An electron in an

atom. According to the classical
Rutherford’s nuclear model of atom, although a great contribution to the electromagnetic theory, it should
science, could not explain certain observations. A brilliant description of continuously lose energy to ultimately
atomic structure was proposed by Neils Böhr in 1913. fall into the nucleus.

What are the postulates of Böhr’s atomic model
The main postulates of Böhr’s atomic model are:
ƒ In an atom, the electrons revolve around the nucleus in certain de nite
circular paths called orbits or shells.