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2
ATOMIC STRUCTURE
Table 2.1 Fundamental particles of atom and their
characteristics
• Atom (Given by Dalton): Matter is
made up of extremely small particles
which are indivisible in nature. It consists
of subatomic particles electron, proton
and neutrons knows as fundamental
particles. Atomic Number, Mass Number,
1. Electron (Named by Stoney):
Isotopes and Isobars
Discovered by Cathode Ray experiment • Atomic number (Z): The no. of protons or
[In crook’s tubes]. A long glass tube electron in a neutral atom or No. of protons
with two metal electrodes. At every low in an atom (or ion).
pressure when high voltage is applied a
• At mass no. (A): Total no. of protons and
flow is produced due to flow of − ve
neutron in an atom
charge particle [known as electron],
cathode rays. Cathode rays have −ve
change, travel in straight lines has
• Isotopes: Atoms of same element with
electric and magnetic field have heating
effect more penetrating effect. Charge different mass no.
on e- was found by Oil drop experiment • Isobars: Atoms of different element with
[Millikan].
same mass no.
2. Proton (Discovered by Goldstein in
• Isotones: Atoms of different element with
anode ray experiment: In a perforated
cathode tube with gas at low pressure same no. of neutron.
high voltage was passed between • Isoelectronic: Atoms, molecules or ions
electrode rays from cathode produced with same no. of e-[Ne; O2-].
green fluorescence on ZnS all. These Earlier Models
were called as anode rays. They travel
in straight line, with + ve charge, get Thomson’s Model
defected in electric and magnetic field.
• J.J. Thomson: The sphere of +ve change
3. Neutron: Fundamental particle which nucleus model of atom is 14 Chemistry
carries no charge but has mass equal to balanced by coulombic force of attraction
N atom or Proton. Discovered by James of e-. Like a Raisin Pudding Model
Chadwick.
Fig. 2.1: A pictorial representation of Thomson’s plum-
pudding model
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Rutherford’s Experiment d. Wave no.: No. of waves per unit length
denoted by
• Ruther Ford (Discovery of nucleus):
particles (+ve charge) bombarded on
gold foil. e. Velocity: Linear distance travelled by a
a. 99.9% passed without deflection: wave in one second.
Most space inside the atom is empty.
(b) Only few deflected therefore mass
of atom centrally placed called
nucleus.
b. Very few deflected back therefore mass Electromagnetic Spectrum
of atom contains +ve charge particles
[Protons].
• Energy wise order for EM radiation.
c. Atom is electrically neutral hence −ve
change particles placed outside the
nucleus and have very less mass.
• Limitations: No distribution and
energies of e-considered, could not
Line Spectrum
explain e- does not fall into the nucleus
or not; no details of line spectra of H • When the vapors of some volatile
atom. substance are allowed to fall on the flame
of a Bunsen burner and then analyzed
Electromagnetic Radiations with the help of a spectroscope. Some
• Energy emitted from any source (in specific-colored lines appear on the
forms of waves) in which electric and photographic plate which is different for
magnetic fields oscillated perpendicular different substances. For example, sodium
to each other and travelling with a or its salts emit yellow light while
velocity to light is known as EM potassium or its salts give out violet light.
radiation.
Line Spectrum of Hydrogen Atom
Characteristic Parameters of
Electromagnetic Radiations • Hydrogen spectrum: When e- in
hydrogen atom is provided energy it
a. Wavelength: the distance of one crest
gets excited to higher shell from ground
and one trough in a wave. Denoted by ‘’
state, it comes back to ground state by
emitting energy in definite values.
b. Frequency:no. of waves • “Quanta”: The emission of light energy
passing through a given point in one is known as emission spectra. It
second. corresponds to each atom depending
upon which energy shell e-is excited. It
is discontinuousspectra as ‘’ of light
c. Amplitude: The height of crest or depth
of a trough denoted by ‘a’
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radiations do not merge with each other
like is VIBGYOR (Continuous Spectra).
When e- falls from any excited state to
a. Ist energy level nf = 1, ni= 2, 3, 4, ....
Wave – Particle Duality
[Lyman series] (UV)
b. When e- to final state nf = 2, ni= 3, 4, • Debroglie equation: All material particles
5, .... [Balmer series] (VIBGYOR) possess both matter should also exhibit
wave like properties. Wave character as
c. When e- to falls to final state nf = 3 well as wave character as well as wave
ni = 4, 5, 6 [Paschem series] IR. character.
d. When e- to falls to final state nf =4 ni
= 5, 6, 7 [Bracket series] IR.
• For microscopic particles mass is very less
e. When e- to falls to final state nf =5ni therefore ‘’ more and more wave
= 6, 7, 8[Pfund series] IR. character.
• For macroscopic particles mass is large
Bohr’s Model is less therefore more particle character.
• Bohr’s theory for H [H like one e-
systems He+; Li2+]e-revolving round
the nucleus in circular path [stationery
state; [Shell]With a definite angular Heisenberg’s Uncertainty Principle
momentum [n no. of shell of e-]
• It is impossible simultaneously to determine
and with definite energy the exact position and exact velocity of a
subatomic particle.
• As n increases Z Decreases Energy of
e- becomesless -ve [Due to less,
force of Proton attraction] • For microscopic (mass very less) certainty
in position is less therefore x is more v is
• As n Decreases Z increases Energy of
less.
e- becomes More -ve [Due to more • For macroscopic (large mass) certainty in
force of attraction by protons] position is more x is less v is more.
• In infinity shell e- has zero force of Wave Mechanical Model Of Atom
attraction therefore zero energy.
• Electron energy only changes by • Erwin Schrödinger proposed the quantum
definite values E = Ef −Ei . mechanical model of the atom, which treats
electrons as matter waves. The wave
mechanical model proposed that the electrons
act like particles as well as waves of energy.
According to the fields around, the electrons
change their path and they move very fast,
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hence they are not in one place during Aufbau (or building up) Principle
any particular time. The wave
mechanical model was used for the a. e- are filled in increasing order of energy of
construction of an atom. subshell.
b. As ‘n + l’ value increases energy of e-
Significance of Quantum increases in that subshell.
c. For two subshells with some ‘n + l’ value.
Numbers
As ‘n’ value increases energy of e-
1. Principal Q. No. : It describes the increases.
distance of e- from nucleus ‘n’ i.e., Pauli’s Exclusion Principle
defines the shell no. It is denoted by
‘n’. • No two e- can have same set of 4
quantum nos. If two e- are present in
same shell, subshell, orbital they will have
2. Azimutha Q. No. : It defines the path different spin value.
of e- decided by angular momentum of Hund’s Rule
e-. Each angular momentum value
corresponds to one subshell. The no. • The pairing of e- in degenerate orbitals
of subshells in a shell is 0 to n−1. (different orbitals with same energy) will
get paired only once they have been
singly occupied. The no. of [Spherical
nodes or radical nodes] = n - l -1.
Shapes of Orbitals
All subshells are wave functions for
locating e-In the same shell energy
wise S < P < d < f
3. Magnetic Q. No. : It gives the no. of
magnetic orientations an e- can have
in a subshell. The no. of magnetic Fig. 2.18: The boundary surface diagrams (shapes) of the s, p,
orientation an e- can have in a d-orbitals
subshell. Difference between psi and psi square:
4. Spin Q. No. : An e-
is continuously
spinning on its own axis. This Q. No.
describes e- can have clockwise spin
motion or e- can have
anticlockwise spin motion . An Difference between Orbit and Orbitals:
-
orbital can have mximum two e one
with clockwise and other with
anticlockwise spin.
Electronic Configuration of
Elements
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Check Yourself Stretch Yourself
1. How many orbital’s can have the
following set of quantum numbers,
n = 3, l = 1, m1 = 0? 1. Calculate the mass and charge of
one mole of electrons.
(A) 3 (B) 1 2. Calculate the number of
electrons which will together
(C) 4 (D) 2 weigh one gram.
3. What is the value of the Bohr’s
2. Electronic configuration of the
radius for the first orbit of
outer shell of the element Gd with
hydrogen atom?
atomic number 64 is 4. Distinguish between a photon
(A) 4f45d56s1 (B) 4f35d56s2 and a quantum
5. What type of metals is used in
(C) 4f55d46s1 (D) 4f75d16s2 photoelectric cell? Give one
example.
3. Maximum number of electrons in
a subshell can be
(A) 4l + 2 (B) 4l – 2
(C) 2n2 (D) 2l + 1 Test Yourself
4. The orientation of atomic orbital’s
depends on their Question: The Vividh Bharati
station of All India Radio, Delhi,
(A) Spin quantum number broadcasts on a frequency of 1,368
(B) Magnetic quantum number kHz (kilo hertz). Calculate the
wavelength of the electromagnetic
(C) Azimuthal quantum number
radiation emitted by transmitter.
(D) Principal quantum number
Which part of the electromagnetic
5. A gas X has Cp and CV ratio as spectrum does it belong to?
1.4, at NTP 11.2 L of gas X will
Answer: Wavelength =λ
contain_______ number of atoms
Frequency = ν = 1,368 kHZ
(A) 1.2 × 1023 (B) 3.01 × 1023
c = speed of light =v×λ
(C) 2.01 × 1023 (D) 6.02 × 1023
λ=c/λ=3×108/1368×103=219
meters
It belongs the far infrared region
or radio wave region.
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Answers
Check Yourself
Answer: 1(D); 2(C); 3(A); 4(B); 5(C)
Stretch Yourself
1. Mass of one mole of electrons is 9.11×10−31×6.023×1023=5.486×10−7kg.
Charge of 1 mole of electrons is 1.602×10−19×6.023×1023=9.647×104C.
2. The number of electrons which will weigh 1 g is 10−3 \9.11×10−31=1.098×1027
Mass of one mole of electrons is 9.11×10−31×6.023×1023=5.486×10−7kg.
Charge of 1 mole of electrons is 1.602×10−19×6.023×1023=9.647×104C.
3. Hint: We know that Neil Bohr was the first to explain the general features of hydrogen
atom structure and its spectrum. Bohr’s theory can be applied on the ions containing
only one electron similar to that of hydrogen atom like Li2+, Be3+ and He+, such
species are also called hydrogen like species.
Formula used: For hydrogen like species, the radii expression from bohr’s theory is
given as: rn=a∘(n2)\Z pm
4. The smallest packet of energy of any radiation is called a quantum whereas that of light
is called photon.
5. When exposed to light, the alkali metals lose electrons. This is known as photoelectric
effect. Electrons of lithium are strongly held by nucleus as Li is smaller in size and
therefore, requires high energy to lose an electron. While on the other hand, Cs has
low ionization energy. Hence, it can easily lose electrons and cannot be utilized in
photoelectric cells.
Hence, the correct answer is 1.
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