notes on chemistry Chapter atomic structure
Answers
Answer:
Sub-atomic particles of atom includes
(a) Electrons – negatively charged with charge of –1 and negligible mass.
(b) Protons – Positively charged with charge of +1 and mass is 1 u.
(c) Neutrons – Particles are with no charge and mass of 1 u.
Thomson’s model of an atom
• Atom is electrically neutral consisting of positively charged sphere with electrons embedded in it.
• If failed to explain the results of experiments carried out by other scientists.
Bohr’s model of an atom
• An atom has a positively charged nucleus and electrons revolve in permitted circular orbits with fixed radii and energy.
• Bohr explained the drawback of Rutherford’s model of an atom.
Rutherford’s model of an atom
• He performed the α-particle scattering experiment and proposed that an atom is electrically neutral, with a positively charged nucleus having protons and neutrons and negatively charged electrons revolving around the nucleus.
Bohr-Bury scheme
• The number of electrons that can be present in a given shell is 2n2, when ‘n’ is the number of shell.
• Maximum number of electrons in the outermost orbit should not exceed 8.
Valency
• The combining capacity of the atoms of an element is valency.
• Elements having 1, 2, 3, and 4 electrons in the valence shell, has valency equal to number of electrons.
• Elements having more than 4 electrons in the valence shell has valency equal to 8 minus the number of valence electrons.
Atomic number and mass number
• Atomic number (z) is number of protons in one atom of an element.
• Mass number (A) is the sum of protons and neutrons present in the atom of the element.
• Notation for an atom

Isotopes
• Isotopes are the atoms of the same element, having the some atomic number but different mass number.
• Isotopes are useful as nuclear fuel, in medical field, in carbon dating, etc.
Answer:
Aoms:
Atom is the smallest indivisible particle of the matter. Atom is made of electron, proton and�neutrons.
PARTICLE ELECTRON PROTON NEUTRON
Discovery Sir. J. J. Thomson
(1869) Goldstein (1886) Chadwick (1932)
Nature of charge Negative Positive Neutral
Amount of charge 1.6 x 10 -19 Coloumb 1.6 x 10 -19 Coloumb 0
Mass 9.11 x 10 -31 kg 1.672614 x 10 -27 kg 1.67492 x10 -27 kg
Electrons were discovered using cathode ray discharge tube experiment.
Nucleus was discovered by Rutherford in 1911.
Cathode ray discharge tube experiment: A cathode ray discharge tube madeof glass is taken with two electrodes. At very low pressure and high voltage,current starts flowing through a stream of particles moving in the tube fromcathode to anode. These rays were called cathode rays. When a perforatedanode was taken, the cathode rays struck the other end of the glass tube atthe fluorescent coating and a bright spot on the coating was developed
Results of Rutherford experiments:
a. Cathode rays consist of negatively charged electrons.
b. Cathode rays themselves are not visible but their behavior can be observed with help of fluorescent or phosphorescent materials.
c. In absence of electrical or magnetic field cathode rays travel in straight linesd. In presence of electrical or magnetic field, behaviour of cathode rays is similar to �that shown by electrons
� Neutrons were discovered by James Chadwick by bombarding a thin sheet of beryllium by ?- particles. They are electrically neutral particles having a mass slightly greater than that of the protons.
� Atomic number (Z) : the number of protons present in the nucleus(Moseley1913).�������������
�Mass Number (A) : Sum of the number of protons and neutrons present in thenucleus.
Thomson model of an atom:
This model proposed that atom is considered asa uniform positively charged sphere and electrons are embedded in it.An important feature of Thomson model of an atom was that mass of atom isconsidered to be evenly spread over the atom.Thomson model of atom is also called as Plum pudding, raisin pudding orwatermelon model Thomson model of atom was iscarded because it could not explain certainexperimental results like the scattering of ?- particles by thin metal foils.
Observations from ?- particles scattering experiment by Rutherford:
a. Most of the ?- particles passed through gold foil un deflected
b. A small fraction of ?- particles got deflected through small angles
c. Very few ?- particles did not pass through foil but suffered large deflection nearly180 o
Conclusions Rutherford drew from ?- particles scattering experiment:
a. �Since most of the ?-particles passed through foil undeflected, it means most of the �space in atom is empty
b. �Since some of the ?-particles are deflected to certain angles, it means that there is positively mass present in atom
c. �Since only some of the ?-particles suffered large deflections, the positively charged mass must be occupying very small space
d. �Strong deflections or even bouncing back of ?-particles from metal foil were due to �direct collision with positively charged mass in atom
Rutherford�s model of atom �:
This model explained that atom consists ofnucleus which is concentrated in a very small volume. The nucleus comprisesof protons and neutrons. The electrons revolve around the nucleus in fixedorbits. Electrons and nucleus are held together by electrostatic forces ofattraction.
Drawbacks of Rutherford�s model of atom :
a. �According to Rutherford�s model of atom, electrons which are negatively
charged particles revolve around the nucleus in fixed orbits. Thus,
b. �theelectrons undergo acceleration. According to electromagnetic theory of Maxwell, a charged particle undergoing acceleration should emitelectromagnetic radiation. Thus, an electron in an orbit should emitradiation. Thus, the orbit should shrink. But this does not happen.c. �The model does not give any information about how electrons aredistributed around nucleus and what are energies of these electrons
� Isotopes: These are the atoms of the same element having the same atomicnumber but different mass number.e g 1 H1 ,1 H2 ,1 H3
� Isobars: Isobars are the atoms of different elements having the same massnumber but different atomic number.e g 18 Ar 40 , 20 Ca 40
� Isoelectronic species : These are those species which have the same numberof electrons.
Electromagnetic radiations:
The radiations which are associated withelectrical and magnetic fields are called electromagnetic radiations. When anelectrically charged particle moves under acceleration, alternating electricaland magnetic fields are produced and transmitted. These fields aretransmitted in the form of waves. These waves are called electromagneticwaves or electromagnetic radiations.
Properties of electromagnetic radiations:
a.