Atomic structure

Atomic Structure ….a brief overview…..

Important Highlights

• INTRODUCTION

• RUTHERFORD’S ATOMIC MODEL

• BOHR’S ATOMIC MODEL

• DISTRIBUTION OF EXTRA-NUCLEAR ELECTRONS IN SHELLS & SUB-SHELLS

• ELECTRONIC CONFIGURATION OF ATOM

• WAVE-NATURE OF AN ATOM

• ATOMIC NUMBER

• MASS NUMBERBESU SHIBPUR,HOWRAH

Introduction• ATOM : An atom is the smallest particle of an element, which

Can not be subdivided.Does not exist in free state. But,Participate in chemical reaction.

The structure of an atom can be broadly divided into two partsNuclear PartsExtra-Nuclear Parts

Nuclear part basically consist of 2 major particles– protons & neutrons. Apart from this there are some more tiny particles such as deuteron, neutrino, positron etc.

Extra-Nuclear parts consists of mainly electrons.BESU SHIBPUR,HOWRAH

Rutherford’s Atomic Model• Also known as “Planetary Model” of Atom.

Key Features: • An atom consists of a central nucleus.  This nucleus is

composed of positively charged protons, and electrically uncharged (neutral) neutrons.

• Negatively charged electrons revolve around the nucleus in definite orbits.

• The orbits themselves can be at any distance from the nucleus.

• In any atom, the number of protons is equal to the number of electrons, and hence it is electrically neutral.

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Drawbacks of Rutherford’s Model• It could not explain the stability of the atom. A/c to

classical theory of mechanics, during uniform revolution, any body accelerates, & an accelerating charged particle must emit radiation, and lose energy. Due to the fact, the electron must emit radiation and lose energy. As a result, the electron will follow a spiral path, and ultimately fall into nucleus .So, Rutherford has given concept of unstable atom.

• The Rutherford’s model of atom does not say anything about the arrangement of electrons in an atom.

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BOHR’S ATOMIC MODEL

RUTHERFORD’S ATOMIC MODEL

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Bohr’s Atomic Model

The main points of Bohr’s atomic model are….• Electrons revolve around the nucleus in orbits that have

fixed radius and energy. So, such orbits are also known as stationary orbit ,Energy shells etc.

• The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit and its vice-versa.

• Radiation is absorbed or emitted when an electron jumps from one orbit to another.

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Bohr postulated that the angular momentum of the electron is quantized as shown…

Where, h=6.626*10^(-34) Js

Then by using simple physical equation he deduced the expression for energy,velocity,radius,wavelength,wave-no. etc.

L=nh/2ᴨ= mvr F= Ze*e/r*r=mv*v/r

Where,L=Angular accn., Z=Atomic number, v=Orbital velocity

m=mass of electron, n=Principal Quantum Number

r=Radius of Orbit, h=Planck’s constant=6.626*10^34Js BESU SHIBPUR,HOWRAH

Deduced Formulae:

• Radius, r= (n^2/Z)*0.529 Å

• Energy, E= -(Z^2/n^2)* 13.6 eV

• Wave No.=1/wave-length=R(1/nf2-1/ni2)

• Velocity, v=(Z/n)*21.88*10^5 m/s

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e.g. Emission of a Quantum:1st Balmer Transition

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-3.4eV

Limitation of Bohr’s Atomic Model• The Bohr atomic model is applicable only for one-electron

system. e.g. H, He+, Li++, Be+++ etc.• The Bohr Model provides an incorrect value for the ground

state orbital angular momentum. • It does not predict the relative intensities of spectral lines. • The Bohr Model does not explain fine structure and hyperfine

structure in spectral lines. • It violates the Heisenberg Uncertainty Principle because it

considers electrons to have both a known radius and orbit. • It does not explain the Stark’s & Zeeman’s Effect.

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Concept of shells and subshells• Shells, n, represents the distance of an electron from

Nucleus. It is represented by K,L,M,N,O,…for n=1,2,3,4,..• Shell no., n, also represents the maximum no. of electrons

accommodated by a particular shell, which is 2n(2). i.e. for n=1, First Shell, maximum no. of electrons=2.

Similarly, for n=2, it’s 8.

n=3, it’s 18 and so on.

• Subshell represents the traversal path of any electron in 3D view or, shape of the Orbital. For any Orbit ,n, it’s value ranges from 0 to(n-1). It is represented by s,p,d,f,…for l=0,1,2,3,…

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Quantum Numbers of Wave funtionsQuantum Numbers of Wave funtionsQuantum # Symbol Values Description

Principal n 1,2,3,4,… Size & Energy of orbital

Angular Momentum

l 0,1,2,…(n-1) for each n

Shape of orbital

Magnetic ml-l…,0,…+l for each l

Relative orientation of orbitals within same l

Spin ms+1/2 or –1/2 Spin up or Spin down

Angular Momentum Quantum # l Name of Orbital

0 s (sharp)

1 p (principal)

2 d (diffuse)

3 f (fundamental)

4 g

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Electronic ConfigurationThe electrons gather around the nucleus in quantum orbitals

following four basic rules, called the Aufbau principle.• no two electrons in the atom will share the same four quantum

numbers n, l, m, and s. (Pauli’s Exclusion Principle)• electrons will first occupy orbitals of the lowest energy level.

• orbitals will be filled with the same spin number until all the orbitals are filled, before it will begin to fill in the opposite spin. (Hund’s Rule of maximum multiplicity)

• (n+l) Rule: electrons will fill orbitals by the sum of the quantum numbers n and l. In case equal values of (n+l), orbitals with the lower value of n, will be filled first.

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Electronic Configuration contd.Hence, Aufbau Principle Gives order of filling of electrons in

different subshells. It can be shown as follows….

• 1s2 • 2s22p6

• 3s23p63d10

• 4s24p64d104f14

• 5s25p65d105f14(5g18)• 6s26p66d106f14(6g18)(6h22)• 7s27p67d106f14(6g18)(6h22)(7i26)

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Electronic Configuration contd.• The electronic configuration of elements generally follows

Aufbau principle.• except some cases viz. Cu,Cr type d-block elements(due

to some stability factor).

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Wave Nature of an Atom• Wave–particle duality postulates that, all particles exhibits

both particle as well as wave characteristics.

• Louis-Victor de Broglie describes the atom considering "Dual Properties of Matter”.

• He formulated a hypothesis, claiming that all matter, not just light, has a wave-like nature; he related wavelength (denoted as λ), and momentum (denoted as p):

λ =h/pThis is the generalization of Einstein’s and Planck’s Equation.

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Atomic Number

• Atomic Number, Z: Atomic Number, Z: The number of protons in an atom.• As atoms are neutral,

So,

N(p)=N(e)=Z

Where, N(p)= number of protons N(e)=number of electrons Z=Atomic Number

. BESU SHIBPUR,HOWRAH

Mass Number

Mass Number: Mass Number: The total numbers of neutrons and protons in an atom.

So, if we know mass no. and atomic number, we can find out the number of neutrons in an atom .

Mass Number - Atomic Number=Number of neutrons

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Notation of an Atom

Mass Number

Atomic Number

65 Cu29

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References…...• http://www.answers.com/topic/electron-configuration

• http://www.sparknotes.com/chemistry/fundamentals/atomicstructure/section1.rhtml

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Thanking You…!

Presented By………Navin Kumar

COMPUTER SCIENCE & TECHNOLOGY

BENGAL ENGINEERING & SCIENCE UNIVERSITY SHIBPUR, HOWRAH

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….question please….!!

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