Wednesday, 16 October, 2019

Atomic structure

Solved Examples on Atomic Structure - Study Material for IIT JEE ,ATOMIC STRUCTURE - SOLVE PAPER - NEET/AIIMS/MCAT EXAM

Atomic structure

1. Introduction

(a) The word atom was first introduced by Ostwald (1803 – 1807) in scientific world.
(b) According to him matter is ultimately made up of extremely small indivisible particles called atoms.
(c) It takes part in chemical reactions.
(d) Atom is neither created nor destroyed
2. Dalton’s Atomic Theory
Dalton proposed the atomic theory on the basis of the law of conservation of mass and law of definite proportions. He also proposed the law of multiple proportion as a logical consequence of this theory. The salient features of this theory are
(a) Each element is composed by extremely small particles called atoms.
(b) Atoms of a particular element are all alike but differ with the atoms of other elements.
(c) Atom of each element is an ultimate particle, and has a characteristic mass but is structureless.
(d) Atom is indestructible i.e. it can neither be destroyed nor created by simple chemical reactions.
(e) Atom of an element takes part in chemical reaction to form molecule.
(f) In a given compound, the relative number and kind of atom are same.
(g) Atoms of different elements combine in fixed ratio of small whole numbers to form compound atoms (now called molecules).



2.1 Merits and Demerits of Dalton’s theory :
2.1.1 Merits :
(a) Dalton’s theory explains the law of conservation of mass and some other laws of chemical combination.
(b) Atoms of elements take part in chemical reaction is true till today.
2.1.2 Demerits :
(a) There is no mention of atomic weights of elements.
(b) He could not explain that why do atoms of same element combined with each other.
(c) The law of definite proportion fails if different isotopes are used.
3. fundamental particles
3.1 Properties of electron
(a) Electron was discovered by Sir J.J. Thomson
(b) The charge on the electron is 1.6 × 10–19 coulomb/gm (Millikan)
(c) The molar mass of electron is 5.48 × 10–4 gm/mole
(d) The mass of electron in motion is expressed as
m´ =
where m’ = mass of the electron in motion
m = rest mass, v = velocity of the electron
c = velocity of light
(e) In 1897, J.J. Thomson determined the e/m value (charge/mass) of the electron by studying the deflections of cathode rays in electric and magnetic fields. The value of e/m has been found to be –1.7588 × 108 coulomb
(f) The first precise measurement of the charge on the electron was made by Robert A. Millikan. in 1909 by oil drop experiment. Its value was found to be – 1.6022 x 10–19 coulomb.
(g) The mass of electron can be calculated from the value of e/m and the value of e which is 9.1096 × 10–31 Kg.
3.1.1 Cathode rays

(a) The electron was discovered as a result of the studies of the passage of electricity through gases at extremely low pressures known as discharge tube experiments.
(b) When a high voltage of the order of 10,000 volts or more was impressed across the electrodes, some sort of invisible rays moved from the negative electrode to the positive electrodes these rays are called as cathode rays
(c) Cathode rays have the following properties.
(i) Path of travelling is straight from the cathode with a very high velocity
As it produces shadow of an object placed in its path

(ii) Cathode rays produce mechanical effects. If a small pedal wheel is placed between the electrodes, it rotates. This indicates that the cathode rays consist of material part

(iii) When electric and magnetic fields are applied to the cathode rays in the discharge tube, the rays are deflected thus establishing that they consist of charged particles.

(iv) Cathode rays produce X-rays when they strike against hard metals like tungsten, copper etc.
(v) When the cathode rays are allowed to strike a thin metal foil, it gets heated up. Thus the cathode rays possess heating effect.
(vi) They produce a green glow when strike the glass wall beyond the anode. Light is emitted when they strike the zinc sulphide screen.
(vii) Cathode rays penetrate. Through thin sheets of aluminium and other metals.
(viii) They affect the photographic plates
(ix) The ratio of charge to mass i.e. charge/mass is same for all the cathode rays irrespective of the gas used in the tube.
3.2 Properties of proton
(a) Proton was discovered by Goldestein
(b) Proton carries a charge of +1.602 x 10–19 coulomb, i.e., one unit positive charge.
(c) Mass of proton is 1.672 x 10–27 kg or 1.0072 amu
(d) A proton is defined as a sub-atomic particle which has a mass nearly 1 amu and a charge of +1 unit
3.2.1 Positive Rays-Discovery of Proton
(a) The existence of positively charged particles in an atom was shown by E. Goldstein in 1886
(b) He repeated the same discharge tube experiments by using a perforated cathode.
(c) It was observed that when a high potential difference was applied between the electrodes, not only cathode rays were produced but also a new type of rays were produced simultaneously from anode moving towards cathode and passed through the holes or canal of the cathode. These termed as canal ray or cathode ray

 




(d) Characteristics of Anode Rays are as follows.
(i) These rays travel in straight lines and cast shadow of the object placed in their path.
(ii) The anode rays are deflected by the magnetic and electric fields like cathode rays but direction is different that mean these rays are positively charged.
(iii) These rays have kinetic energy and produces heating effect also.
(iv) The e/m ratio of for these rays is smaller than that of electrons
(v) Unlike cathode rays, their e/m value is dependent upon the nature of the gas taken in the tube.
(vi) These rays produce flashes of light on Zn-S screen
(vii) These rays can pass through thin metal foils
(viii)They are capable to produce ionisation in gases
(ix) They can produce physical and chemical changes.
3.3 Properties of neutron
(a) This was discovered 20 years after the structure of atom was elucidated by Rutherford.
(b) It has been found that for all atoms except hydrogen atomic mass is more than the atomic number. Thus Rutherford (1920) suggested that in an atom, there must be present at least a third type of fundamental particle.
(c) It should be electrically neutral and posses mass nearly equal to that of proton. He proposed its name as neutron.
(d) Chadwick (1932), bombarded beryllium with a stream of a-particles and observed electrically and magnetically neutral radiations.
(e) There were neutral particles which was called neutron. Nuclear reaction is as follows
4Be9 + 2He4 ¾¾® 6C12 + 0n1
(f) A neutron is a subatomic particle which has a mass 1.675 x 10–24g, approximately 1 amu, or nearly equal to the mass of proton on hydrogen atom and carrying no electrical charge.
Fundamental Particles

Ex.1 For cathode rays’ the value of e/m –
(A) Is independent of the nature of the cathode and the gas filled in the discharge tube
(B) Is constant
(C) Is –1.7588 x 108 coulombs/g
(D) All of the above are correct Ans.(D)
Sol. Cathode rays consists of electrons which are fundamental particles of matter.
Ex.2 Which has highest e/m ratio –
(A) He2+ (B) H+
(C) He1+ (D) H Ans.(B)
Sol. Mass of H+ is minimum
Ex.3 Arrange the following particles in increasing order of values of e/m ratio : Electron (e), proton (p), neutron (n) and a-particle (a) –
(A) n, p, e, a (B) n, a, p, e (C) n, p, a, e (D) e, p, n, a Ans.(2)
Sol. Electron Proton Neutron a-particle
e 1 unit 1 unit zero 2 units
m 1/1837 unit 1 unit 1 unit 4-units
e/m 1837 1 zero 1/2
Ex.4 Mass of neutron is …….. times the mass of electron –
(A) 1840 (B) 1480
(C) 2000 (D) None Ans.(1)
Sol. Mass of neutron = 1.675 x 10–27 kg, mass of electron = 9.108 x 10–31 kg.
4. Non Fundamental particles
4.1 Positron :
(a) It is also called positive electron and symbolised as 1e0 or e+.
(b) It was discovered by Anderson in 1932.
(c) It is the positive counterpart of electron.
(d) Mass of positron is same as electron m = 9.1 × 10–28 g.
(e) Charge of positron is same but opposite signed as electron e = –1.6 × 10–19 C.
(f) It is very unstable and combines with electron producing g rays.
4.2 Neutrino and Antineutrino :
These are particles of approximately zero masses and zero charge.
4.3 Antiproton :
(a) It was discovered by Seagre.
(b) Mass of this particle is equal to 1.673 × 10–24 g.
(c) Charge of Antiproton is –1.6 × 10–19 C.



4.4 Meson (p) :

(a) It was discovered by Yukawa in 1935.
(b) It may possess 3 types of charges.
(c) On the Basis of charge, the meson is of three types, p-meson, µ-meson and neutral meson (pº).
(d) p-mesons are called pions.
(e) It tells about the stability of nucleus.
(f) The mass of this particle is 200 times of electron i.e. It is heavier than electron but lighter than proton.
5. Thomson’s Model

It states the arrangement of electrons and protons in an atom. The main principles are
(a) After discovery of electron and proton attempts were made to find out their arrangement in an atom. The first simple model was proposed by J.J. Thomson known as Thomson’s atomic model.
(b) He proposed that the positive charge is spread over a sphere of the size of the atom (i.e. 10–8 cm radius) in which electrons are embedded to make the atom as whole neutral.
(c) This model could not explain the experimental results of Rutherfords a-particle scattering, therefore it was rejected.
6. Rutherford’s model
Rutherford carried out experiment on the bombardment of atoms by high speed positively charged a – particles emitted from radium and gave the following observations, which was based on his experiment.

(a) Most of the a – particles (nearly 99%) continued with their straight path.
(b) Some of the a – particles passed very close to the centre of the atom and deflected by small angles.
(c) Very few particles thrown back (180º) .

6.1 Main features :
(a) Most of the a – particles were continued their straight path that means most of the space of the atom is empty.
(b) The centre of an atom has a positively charged body called nucleus which repel positively charged a – particles and thus explained the scattering phenomenon.
(c) Whole mass of an atom is concentrated in its nucleus and very few throw back means the size of the nucleus is very small 10–13 cm. It showed that the nucleus is 10–5 times small in size as compared to the total size of atom.
(d) The size and volume of the nucleus is very small as compared to the total size and volume of atom.
(e) As atomic number increases, the angle of deflection (q) increases.
6.2 Drawbacks of Rutherford’s model :

(a) According to classical electromagnetic theory, when an electron moves around the nucleus under the influence of the attractive force, the electron loses its energy continuously and move closer and closer to the nucleus in a spiral path, the ultimate result will be that it will fall into the nucleus but it can’t be possible because an atom is quite stable.
(b) If an electron loses energy continuously, the observed spectrum should be continuous but the actual observed spectrum consist of discontinuous well defined lines of definite frequencies.
Rutherford’s Experiment

Ex.5 Rutherford’s scattering experiment is related to reveal structure of –
(A) Nucleus (B) Atom
(C) Electron (D) Neutron
Ans . (B)
Sol. To reveal structure of atom
Ex.6 When the atoms of gold sheet are bombarded with a beam of a-particles, only a few a-particles get deflected whereas most of them go straight undeflected. This is because –
(A) The force of attraction on the a-particles by the oppositely charged electron is not sufficient
(B) The nucleus occupies much smaller volume as compared to the volume of atom
(C) The force of repulsion on fast moving a-particles is very small
(D) The neutrons in the nucleus do not have any effect on a-particles.
Sol. It was the logical conclusion of his experiment.



7. Moseley’s Experiment
7.1 Atomic number (Z) :
The number of positive charge carried by the nucleus of an atom is termed as atomic no. (Z) or the number of protons in an atom of an element is equal to its atomic number. Since an atom is electrically neutral it contains an equal number of extra nuclear electrons. Thus –
Atomic No. = Number of unit positive charge in nucleus = Number of protons
= Number of electrons.
7.2 Mass number or Neucleon number (A) :
The mass number being the sum of the number of protons and neutrons in the nucleus, which is always a whole number.
A = P + n
or
A = Z + n
where :
A = Mass number
P = Number of protons
n = Number of neutrons
Z = Atomic number
On the another side of that statement since mass of a proton or a neutron is not a whole number (on atomic weight scale), atomic weight is not necessarily a whole number.
For example : The isotopes of oxygen having mass number 17 and 18, have atomic weights equal to 17.00045 and 18.0037 respectively.

8. Some important definitions

(a) Nodal Surface :
The place find in between two ‘s’ orbitals where the value of electron density is equal to zero called Nodal surface.
The number of Nodal surfaces in an atom = (n – 1), where ‘n’ is the number of total shell in an atom.
(b) Nodal Plane :
The place for ‘p’ and ‘d’ orbitals where the value of electron density is equal to zero called Nodal Plane.
For px = yz
py = xz
pz = xy
For dxy = yz, zx
dyz = xy, xz dx2–y2 = 0
dzx = xy, yz
dz2 = 0
(c) Nodal Point :
The nucleus of an atom called Nodal Point.
(d) Isodiapheres :
The elements which have same value of (n – p) is called Isodiapheres.
eg. 7N14 8O16
Values of (n – p) 0 0
(e) Isotone :
Elements which contain same no. of neutron is called Isotone.
eg. 14Si30 15P31 16S32
number of neutrons 16 16
(f) Isotopes :
(i) First proposed by soddy.
(ii) The isotopes have same atomic number but different atomic weight.
(iii) They have same chemical properties because they have same atomic number.
(iv) They have different physical properties because they have different atomic masses.
eg. 1H1 1H2 1H3
Protonium deuterium Tritium
Z = 1 1 1
A = 1 2 3
(g) Isobar :
The two different atoms which have same atomic masses but different atomic number is called as Isobar.
eg. 18Ar40 19K40 20Ca40
Atomic 40 40 40
mass
Atomic 18 19 20
number
(h) Isomorphous :
The two different type of compound which contain same crystalline structure called Isomorphous and this property called Isomorphism.
eg. FeSO4 . 7H2O ZnSO4 . 7H2O
Green vitriol White vitriol
Hepta hydrate Hepta hydrate Ferrous sulphate Zinc Sulphate
(i) Isomers :
Species which have same molecular formula but different structural formula is called Isomer and this type of property is called Isomerism.
eg. C2H6O ® C2H5 OH & CH3 – O – CH3
(j) Isoelectronic :
Ion or atom or molecule or species which have the same number of electron is called Isoelectronic species.
eg. 17Cl– 18Ar 19K+ 20Ca+2
No. of 18 18 18 18
electron
eg. CN– CO
No. of electron 14 14
(k) Isosters :
Substance which have same number of electron and atoms called Isosters.
eg. CO2 N2O
22 22
(l) Kernel : Orbit which present after removing the outer most orbit of that atom is called kernel and electrons which is present that orbit called kernel electrons.
eg. Mg = 1s2 2s2 2p6, 3s2
Total kernel electron = 2 + 2 + 6 = 10
(m) Core :
(i) The outer most shell of an any atom called Core and the number of electron present of that shell is called Core electron.
eg. Cl = 1s2 2s2 2p6 3s2 3p5
Core electron = 2 + 5 = 7
(ii) If the core is unstable for an atom then that atom shows variable valency.
(n) Photoelectric effect :
When a beam of light of high frequency is strike on a metal surface in vacuum condition, electrons are emitted from the metal surface. This phenomenon is called photoelectric effect and the emitted electron is called photoelectrons.
Total energy = ½mv2 + w
{½ mv2 = kinetic energy
w = Threshold energy or work function}
(o) Threshold energy : The minimum energy required to emit an electron on the metal surface called threshold energy.
(p) The value of for n, p, a, & electron is equal to –
for n = 0
for a = = 4.8 × 105
for p = = 9.58 × 104
for e– = = 1.76 × 108
Note : When an electron is in the stationary state then the value of magnetic field for that electron is equal to zero.
(q) Promotion :

The transfer of electron between subshells in an orbit is called promotion. While the transfer of one energy level to another is called transition. After the completion of promotion the transition process is occurred.
eg. First promotion of an electron is 2s (n + l = 2 + 0 = 2) to 2p (n + l = 2 + 1 = 3) sub-shell and their transition to 2nd orbit to 3rd orbit or 2p to 3s.
9. Extra stability of half filled and completely filled orbitals
Half-filled and completely filled sub-shells have extra stability due to the following reasons.
9.1 Symmetry of orbitals :
(a) It is a well known fact that symmetry leads to stability.
(b) Thus, if the shift of an electron from one orbital to another orbital differing slightly in energy results in the symmetrical electronic configuration. it becomes more stable
(c) For example p3, d5, f7 confiogurations are more stable than their near ones
9.2 Exchange Energy
(a) The e¯ in various subshells can exchange their positions, since e¯ in the same subshell have equal energies.
(b) The energy is released during the exchange process with in the same subshell.
(c) In case of half filled and completely filled orbitals, the exchange energy is maximum and is greater then the loss of orbital energy due to the transfer of electron from a higher to a lower sublevel e.g. from 4s to 3d orbitals in case of Cu and Cr
(d) The greater the number of possible exchanges between the electrons of parallel spins present in the degenerate orbitals, the higher would be the amount of energy released and more will be the stability
(e) Let us count the number of exchange that are possible in d4 and d5 configuration among electrons with parallel spins :

d4 (1)
3 exchanges by 1st e¯
(2)
2 Exhanges by 2nd e¯
(3)
only 1 exchange by 3rd e¯
Total number of possible exchanges
= 3 + 2 + 1 = 6
d5 (1)
4 exchanges by 1st e¯
(2)
3 exchanges by 2nd e¯
(3)
2 exchanges by 3rd e¯
(D)
1 exchange by 4th e¯
Total number of possible exchanges
= 4 + 3 + 2 + 1 = 10

10. Electronic configuration of elements

 

 

element K L M 4s 4p 4d,4f,5d,5f,6s,6p,6d,6f,7f

H 1 1
He 2 2
Li 3 2 1
Be 4 2 2
B 5 2 2 1
C 6 2 2 2
N 7 2 2 3
O 8 2 2 4
F 9 2 2 5
Ne 10 2 2 6
Na 11 2 2 6 1
Mg 12 2 2 6 2
Al 13 2 2 6 2 1
Si 14 2 2 6 2 2
P 15 2 2 6 2 3
S 16 2 2 6 2 4
Cl 17 2 2 6 2 5
Ar 18 2 2 6 2 6
K 19 2 2 6 2 6 1
Ca 20 2 2 6 2 6 2
Sc 21 2 2 6 2 6 1 2
Ti 22 2 2 6 2 6 2 2
V 23 2 2 6 2 6 3 2
Cr 24 2 2 6 2 6 5 1
Mn 25 2 2 6 2 6 5 2
Fe 26 2 2 6 2 6 6 2
Co 27 2 2 6 2 6 7 2
Ni 28 2 2 6 2 6 8 2
*Cu 29 2 2 6 2 6 10 1
Zn 30 2 2 6 2 6 10 2
Ga 31 2 2 6 2 6 10 2 1
Ge 32 2 2 6 2 6 10 2 2
As 33 2 2 6 2 6 10 2 3
Se 34 2 2 6 2 6 10 2 4
Br 35 2 2 6 2 6 10 2 5
Kr 36 2 2 6 2 6 10 2 6
Rb 37 2 2 6 2 6 10 2 6 1
Sr 38 2 2 6 2 6 10 2 6 2
Y 39 2 2 6 2 6 10 2 6 1 2
Zr 40 2 2 6 2 6 10 2 6 2 2
Nb 41 2 2 6 2 6 10 2 6 4 1
Mo 42 2 2 6 2 6 10 2 6 5 1
Tc 43 2 2 6 2 6 10 2 6 5 2
Ru 44 2 2 6 2 6 10 2 6 7 1
Rh 45 2 2 6 2 6 10 2 6 8 1
Pd 46 2 2 6 2 6 10 2 6 10
Ag 47 2 2 6 2 6 10 2 6 10 1
Cd 48 2 2 6 2 6 10 2 6 10 2
In 49 2 2 6 2 6 10 2 6 10 2 1
Sn 50 2 2 6 2 6 10 2 6 10 2 2
Sb 51 2 2 6 2 6 10 2 6 10 2 3
Te 52 2 2 6 2 6 10 2 6 10 2 4
I 53 2 2 6 2 6 10 2 6 10 2 5
Xe 54 2 2 6 2 6 10 2 6 10 2 6

Cs 55 2 8 18 2 6 10 2 6 1
Ba 56 2 8 18 2 6 10 2 6 2
*La 57 2 8 18 2 6 10 2 6 1 2
Ce 58 2 8 18 2 6 10 1 2 6 1 2
Pr 59 2 8 18 2 6 10 3 2 6 2
Nd 60 2 8 18 2 6 10 4 2 6 2
Pm 61 2 8 18 2 6 10 5 2 6 2
Sm 62 2 8 18 2 6 10 6 2 6 2
Eu 63 2 8 18 2 6 10 7 2 6 2
Gd 64 2 8 18 2 6 10 7 2 6 1 2
Tb 65 2 8 18 2 6 10 9 2 6 2
Dy 66 2 8 18 2 6 10 10 2 6 2
Ho 67 2 8 18 2 6 10 11 2 6 2
Er 68 2 8 18 2 6 10 12 2 6 2
Tm 69 2 8 18 2 6 10 13 2 6 2
Yb 70 2 8 18 2 6 10 14 2 6 2
Lu 71 2 8 18 2 6 10 14 2 6 1 2
Hf 72 2 8 18 2 6 10 14 2 6 2 2
Ta 73 2 8 18 2 6 10 14 2 6 3 2
W 74 2 8 18 2 6 10 14 2 6 4 2
Re 75 2 8 18 2 6 10 14 2 6 5 2
Os 76 2 8 18 2 6 10 14 2 6 6 2
Ir 77 2 8 18 2 6 10 14 2 6 7 2
*Pt 78 2 8 18 2 6 10 14 2 6 9 1
*Au 79 2 8 18 2 6 10 14 2 6 10 1
Hg 80 2 8 18 2 6 10 14 2 6 10 2
Ti 81 2 8 18 2 6 10 14 2 6 10 2 1
Pb 82 2 8 18 2 6 10 14 2 6 10 2 2
Bi 83 2 8 18 2 6 10 14 2 6 10 2 3
Po 84 2 8 18 2 6 10 14 2 6 10 2 4
At 85 2 8 18 2 6 10 14 2 6 10 2 5
Rn 86 2 8 18 2 6 10 14 2 6 10 2 6
Fr 87 2 8 18 2 6 10 14 2 6 10 2 6 1
Ra 88 2 8 18 2 6 10 14 2 6 10 2 6 2
*Ac 89 2 8 18 2 6 10 14 2 6 10 2 6 1 2
*Th 90 2 8 18 2 6 10 14 2 6 10 0 2 6 2 2
*Pa 91 2 8 18 2 6 10 14 2 6 10 2 2 6 1 2
*u 92 2 8 18 2 6 10 14 2 6 10 3 2 6 1 2
Np 93 2 8 18 2 6 10 14 2 6 10 4 2 6 1 2
Pu 94 2 8 18 2 6 10 14 2 6 10 6 2 6 2
Am 95 2 8 18 2 6 10 14 2 6 10 7 2 6 2

Cm 96 2 8 18 2 6 10 14 2 6 10 7 2 6 1 2
bk 97 2 8 18 2 6 10 14 2 6 10 8 2 6 1 2
Cf 98 2 8 18 2 6 10 14 2 6 10 10 2 6 2
Fs 99 2 8 18 2 6 10 14 2 6 10 11 2 6 2
Fm 100 2 8 18 2 6 10 14 2 6 10 12 2 6 2
Md 101 2 8 18 2 6 10 14 2 6 10 13 2 6 2
No 102 2 8 18 2 6 10 14 2 6 10 14 2 6 2
Lw 103 2 8 18 2 6 10 14 2 6 10 14 2 6 1 2

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