CHAPTER 2-STRUTURE OF ATOMS

v RUTHERFORD NUCLEAR MODEL OF ATOM –features

·         The entire mass and the positive charge are concentrated at the centre known as nucleus.

·         The negatively charged electron which encircle the nucleus.

·         Most of the space is empty.

Ø FAILURE OF RUTHERFORD MODEL

·         By circling around the nucleus it continuously loss energy and fall to nucleus and there will be no atom.

·         Inability to explain atomic spectra of elements.

v PHOTOELECTRIC EFFECT

·         The phenomenon of ejection of electron from the surface of metals like potassium and caesium, when the frequency (suitable) strikes on it.

Ø LAW OF PHOTOELECTRIC EFFECT

·         Only radiation of certain minimum frequency causes this effect. This frequency is called threshold frequency.

·         When the radiation of energy hɤ falls on the metal part is used to provide work function (threshold energy) and remaining to impart kinetic energy(K.E).

·         Number of electron ejected is proportional to the intensity of radiation.

·         K.E α frequency

v BOHR’S MODEL OF ATOM

·         An atom consists of a small, heavy negatively charged nucleus in the centre and the electron revolves around it in circular orbits.

·         Out of a large number of circular orbits theoretical possible, the electron revolves only in those orbits which here a fixed value of the energy.

·         The electron in an atom can have only certain definite values of energy.

·         Only that circular path is permissible which satisfy the quantum condition .

·         As long as an electron is revolving in a circular path it does not emit or absorb energy.

·         When an electron jumps from lower energy to higher energy, energy is absorbed and vice versa.

Ø LIMITATIONS OF BOHR MODEL

·         Inability to explain line spectra of multi electron atoms.

·         Inability to explain splitting of lines in a magnetic field (Zeeman effect) and in a electric field (Stark effect).

·         Inability to explain de Broglie matter wave concept of electron.

·         It contradicts with Heisenberg’s uncertainty  principle,

v HEISENBERG’S UNCERTAINTY PRINCIPLE

It states that it is impossible to determine exactly both position and momentum of moving microscopic particle simultaneously.

v DUAL NATURE OF ELECTRON

Ø DE BROGILE CONCEPT

·         Every matter exhibits both wave and particle nature. From Planck’s relation.

v QUANTUM NUMBERS

    Set of four numbers with the help of which we can get complete information about all electrons in an atom.

Ø PRINCIPLE QN (n)

                     Average distance and energy.  n can be 1,2,3 or k,l,m,n etc.

Ø AZIMUTHAL QN (l)

                         It represents sub shell.

                         It can have the values 0 to (n-1) value.

                         Representation—s,p,d…….

Ø MAGNETIC QN (m)

                        Orientation of electron cloud .

          Can have values (2l+1) .

Ø SPIN QN (s)

                        Represents revolution 

v SHAPES OF ORBITALS

v RULES

Ø PAULI’S EXCLUSION

              No two electrons can have the same value for all 4 quantum number.

Ø AUFBAU

Electrons are filled in the orbital in the increasing order of energies ,[(n+l)] value.

Ø HUND’S RULE

Paring take place only after all the orbitals are singly occupied.