Bohr's Model of Atom

 

 Neils Bohr (1913) was the first to explain quantitative the general features of the structure of hydrogen atom. The model explained the atomic spectrum of the hydrogen atom and the stability of the atom 

• Electrons moves around the nucleus With fix path and fix energy. Fix path are called Orbits / stationary State / energy state / energy level.

• Energy of electron does not Change. when electron Absorbs energy It moves from Lower stationary State to higher stationary state. 

  When elecron moves From higher stationary state to lower stationary state It emits energy in the form of EMR 

• 

  Energy of radiation, absorbed or emitted is equal to difference two energy state / energy of to stationary state

∆E = Ef - Ei          ∆E = E2 - E1

∆E = energy of radiation Absorbs or emitted

Ef = final energy state

Ei = initial energy state 

• there are stationary orbits for electron. As long as electrons revolve in these stationary orbits, they do not radiate energy. The energies of electrons remain constant in stationary orbits.

Different stationary levels or stationary orbits or shells are represented by capital letters K, L, M, N  with corresponding shell number 1, 2, 3, 4, 5 etc. 

• Angular momentum of electron is quanties

  

Angular momentum = m×v×r

M= mass of electron

V=  velocity of electron

R=  radius of Orbits 

   mvr = n(h/2π)

Angular momentum Of integral multiple of h/2π

• radius of Orbit

 As the elecron of an atom revolve in one of the stationary orbital, the coulombic force of attraction of nucleus of electrons, i.e. ze2/r2 must be equal to centrifugal force of the electron mv2/r2

Hence, 

       Ze2/r2 = mv2/r ____(1)

  R= Ze2/mv2.  ______(2)

According to the Bohr's angular momentum of electron mvr is integral multiple n of h/2π

Hence, mvr = nh/2π _____(3)

                  V = nh/ 2πmr _____(4) 

  Substituting v in equation (2) 

        r = n2h2/4π2mZe2

Substituting values of h,m,z=1 for hydrogen and e the electronic charge r may be caculated as 

   r = 0.529 × n2 A° 

 Thus, 

     n = 1.        r = 0.529A°

     n= 2.          r= 2.116A°

     n= 3.          r = 4.761A°

     n= 4.          r = 8.464A°

     n= 5.          r = 13.225 A°

And so on. This also suggest that electron takes up only discrete energy levels.

Hund's rule

before we can discuss atoms containing six or more electrons, we need to define Hund's rule.

  Hund's rule states that 

 When there are two or more atomic orbitals with the same energy, an electron will occupy an empty orbital before it will pair up with another elecron.

   In this way, elecron repulsion is minimized.

     

The sixth elecron of a carbon atom, therefore, goes into an empty 2p orbitals, rather than pairing up with the elecron already occupying a 2p orbital see in the table.

.   The electronic configuration of the smallest atom

There is one more empty 2p orbital, so that is where nitrogen's seventh electron goes. The eighth electron of an oxygen atom pairing up with an electron occupying a 2p orbital rather than going into the higher-energy 3s orbital.

       The location of the electrons in the remaining elements can be assigned using these three rules.

    The elecrons in inner shells (those below the outermost shell) are called core electrons.  Core electrons do not participate in chemical bonding.the electrons in the outermost shell are called valence electrons.

    Carbon has two core electrons and four valence electrons Lithium and Sodium each have one valence electron. If you examine the periodic table, you will see that and Sodium are in the same column. Elements in the same column of the periodic table have the same number of valence electrons. Because the number of Valence electrons in the major factor determining an element's chemical properties, elements in the same column of the periodic table have similar chemical properties. Thus, the chemical behaviour of an element depends on its electronic configuration.

   

* Some important notes

• core electrons are electrons in inner shells.

• Valence electrons are electrons in the outermost shell.

 • The chemical behaviour of an element depends on its electronic configuration.