Basic Atomic structure

ATOMIC STRUCTURE

Vinay DesaiM.Sc Radiation Physics

KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY

• Every matter consists of basic entities called elements.

• Each element is composed of smallest particles called ‘ATOM’.

• Atom- the name is derived from Greek language Atomos means ‘Not to be cut’.

ATOM:

Atom and ‘DEMOCRITUS’

• DEMOCRITUS was a Greek philosopher who began search for description of matter more than 2400 years ago (4th century B.C.)

• Democritus was the person who first suggested the existence of ATOM & coined the name ÁTOMOS’ means Not to be cut or Indivisible.

Atomic structure

• Atom consists of positively charged NUCLEUS at the centre and negatively charged Electrons revolving around it.

• Radius of an atom -- 10-10 m.• Radius of the nucleus -- 10-15 m.• Nucleus consists of Protons and

Neutrons together called Nucleons.• Most of the mass of an Atom is

possessed by Nucleus.

Representation of an Atom:

Example:

235U92 X-Uranium A-235 Z-92

Theories of Atomic models:

John

Dalton • Matter is made of indivisible

atoms, they are indestructible.• All atoms of a given (same)

element are identical in their physical and chemical properties.

• Atoms of different elements differ in their physical and chemical properties.

• Atoms of different elements combine in simple whole-numbers ratios to form Molecules

• Chemical reactions consist of the combination separation or rearrangement of atoms

• Limitations: It could not explain

• Why and how do atoms combine together to form compound atoms (molecules)

• The nature of forces which hold atoms together in compound atoms

• Why atoms cannot exist in free state and why compound atoms can exist freely.

Theories of Atomic models:J.J.THOMPSONPlum pudding

model(1904)• Negative particles are evenly scattered

throughout an atom with a positively charged mass of matter.

• Similar to that of chocolate chip icecream

• Later proved to be incorrect.Limitations:• It could not explain the result of scattering experiment

explained by rhe Rutherford experiment.• It did not have any experimental evidence in its support .

Thermionic emission, photoelectric emission and ionization were explained on this basis.


RUTHERFORD

Rutherford Gold foil experiment setup:

• If a thin foil metal is placed in the path of the beam, the image becomes diffuse.

• This due to the scattering of α- particles by the atoms of the foil.• The particles scattered in various directions were counted by

scintillation counter• It was found that although most of the particles scattered through

angles of the order of 10 or less• But a small number say about 1 in every 10,000 scattered

through 900 or even 1800

• The experiment is known as Rutherford's alpha particle scattering experiment.

Results of the Rutherford experiment

(a) The results that the metal foil experiment would have yielded if theplum pudding model had been

correct.

(b) Actual results

The whole of the positive charge of atom must be concentrated in a very small space

Atom is mostly hollow inside Since α- particles are positively charged, the part of the atom

deflecting them must also be positive Most of the mass of the atom is concentrated in the nucleus In this model, the mass of the atom (leaving the mass of its

electrons) and its whole positive charge are concentrated at the centre of the atom in a nucleus of radius 10-15 m

The electrons are distributed around the nucleus in a hollow sphere of radius 10-10 m

Conclusion of Rutherford experiment

Regarding stability of atom• Electrons revolving around the nucleus have

centripetal acceleration• According to electrodynamics, accelerated charged

particles radiate energy in the form of electromagnetic waves

• Hence electromagnetic waves should be continuously radiated by the revolving electrons

• Due to this continuous loss of energy of the electrons, the radii of their orbits should be continuously decreasing and ultimately the electron should fall into the nucleus

• Thus atom cannot remain stable

• Rutherford’s model also failed to explain the Line spectrum.

Drawback’s of Rutherford’s Model

Niels David Bohr


Bohr’s theory (Postulates)

1. Fixed circular orbits :The electrons move around the nucleus in concentric circular orbits .

2. While revolving in stable orbits, the electrons do not radiate energy in spite of their acceleration towards the centre of the orbit.

3. Each of the fixed orbits is associated with a definite amount of energy called stationary energy.The energy levels are numbered as 1, 2, 3, 4… or

designated as K,L,M, N …4. Jumping of an electron from one energy level to the other (ground state and excited state) .

5. Principle of quantization of angular momentum of the moving electron an electron can move only in that orbit in which the angular momentum of the electron around the nucleus is an integral multiple of h/2π.

• NIELS DAVID BOHR A Danish physicist who developed Bohr model of

atomic structure, in which he introduced the theory of electrons orbiting

around the nucleus.

I. No explanation for the spectra of multi electron systems:

Eg: He, LiII. No explanation of fine spectrum of atoms: III. No explanation for Zeeman and Stark effect :

effect of electric and magnetic fields on the spectral atoms.

• When a magnetic field is applied on an atom, its usually observed spectral lines split. This effect is known as Zeeman’s effect

• Spectral lines also get split in the presence of electric field. This effect is known as Stark effect.

Limitations of Bohr’s Postulates

Isotopes - Elements having same atomic number (protons) , but different mass numbers (nucleons).

eg: 12C, 13C and 14C

are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. The atomic number of all carbon isotopes is 6.

Isobars - Elements having same mass number , but different atomic numbers.

eg: 40S , 40Cl , 40Ar , 40K, and 40Ca are isobars containing 40 nucleons; however, they contain

varying atomic number.

Classification of atoms:

Isomers - Molecules with the same molecular formula but different chemical structures. That is, isomers contain the same number of atoms of each element, but have different arrangements of their atoms in space.

eg: 131mXe54 is an isomer of 131 Xe54

m stands for meta-stable state

isomers represents identical atoms but they differ in nuclear energy states.

Isotones -Nuclides having very same neutron number N , but different proton number Z.

eg:boron-12 Carbon-13 nuclei both contain 7 neutrons

NUCLEAR STABILTY:

The Strong Force is exerted by anything with mass to attract other masses together and works within a very short distance.

Neutrons has no charge, but have the strong force to bring other nucleons together.

As a general rule, a nucleus will need a neutron/proton ratio of 3:2 (or 1.5:1) in order to stay together. This rule is more precise for larger nuclei. Of all known isotopes of natural elements (about 1500), only 250 of them are stable

.All of these stable isotopes have an atomic number in between 1 and 83.

The mass of a nucleus will be less than the mass of all of the protons and neutrons making it up. The difference is called the mass defect, which is converted into energy if the nucleus is broken up.

The amount of energy that keeps a nucleons together is called the Binding Energy. This amount of energy is higher for nuclei that are stable than it would be for unstable nuclei. (Joules)

Binding energy can be calculated by the formula E=mc2

(Einstein,s principle of equivalence of mass & energy relation)

Where,

c= speed of light ,m=mass & E = Energy

Eg: If a mass of 1 kg is converted into energy, m=1kg speed of light is 3x108 m/s E=1 kg x (3x108 m/s)2

E= 9 x 1016 J

Also, Mass of electron at rest in terms of energy equivalent is

given by, m=9.1x10-31kg and speed of light is 3x108 m/s

E = 9.1x10-31 x (3x108)2

E=8.19x10-14 J E=0.511 MeV

Masses of atoms & atomic particles are conveniently given in terms of amu.

An amu is defined as 1/12 of the mass of the 12C6 Carbon atom.

1 amu= 1.66x10-27 kgMass expressed in terms of amu is known as atomic mass or atomic weight.

Atomic mass and Energy Units

Charge and Mass of sub-atomic particles

Particle Charge of particle Mass of the particle

Proton 1.602176 x 10-19 Coulomb

1.00727 amu

Neutron Electrically neutral 1.00866 amuElectron 1.602176 x 10-19

Coulomb0.00548 amu

• Number of Protons = Number of Neutrons• 1 amu = 1/12th of mass of an carbon atom.• 1 amu = 1.66 x10-27 kg

Avagadro's law- Every gram atomic weight of a substance contains same number of atoms, the number is referred as Avagadro's number

Value of Avagadro's number 6.0221x1023 atoms per gram atomic weight

Gram atomic weight

Basic unit of energy is joule (J) 1 Joule is the work done when force of 1

Newton is acting through a distance of 1 meter.

Another energy unit in Nuclear physics is electron volt (eV)

1 eV is defined as the Kinetic energy acquired by an electron in passing through a potential difference of 1V.

1eV=1Vx1.602x10-19 C 1eV = 1.602 x 10-19 J Product of Potential difference and Charge of the electron

Atomic energy unit - Joule

DISTRIBUTION OF ORBITAL ELECTRONS:

Postulates of Bohrs theory :a) Electrons can exist in only those orbits for which angular

momentum of electron is an integral multiple of h/2Π (h= plancks constant 6.62x10-34 )b) No energy is gained or lost while an electron remains in

any one of the permissible orbits.

According to the model proposed by Niels Bohr in 1913Electrons revolve around the nucleus in specific orbits.They are prevented from leaving the atom by the necessary centripetal force of attraction between the positively charged nucleus and negatively charged electron. Bohr’s Atomic model

Arrangement of electrons in orbitals

Innermost orbit is called as K-shell.Followed by orbital’s called L-shell, M-shell and N-shell.The maximum no. of electrons in an orbital is given by the formula 2n2.

Eg: 1) Hydrogen atom has 1 electron in K-shell 2)Helium atom has 2 electrons in K-shell 3)Oxygen atom has 8 electrons (2 in K-shell, 6 in L-shell)

NUCLEAR FORCES:

There are four types of forces in nature1)Strong nuclear force 2)Electromagnetic force 3)Weak nuclear force4)Gravitational force

Strong nuclear force -Short range force(~10-15 m )

Energy level diagram of a particle in a nucleus

A)Particle with no charge

B)Particle with positive charge

U(r) - Potential energy r - distance from centre of nucleus B- barrier height R- Nuclear radius

Energy level diagram of decay of 60Co27 nucleus.

Nuclear energy levels:

60Co27 nucleus firstly emits β- particle with emission of photons

60Co27 60 Ni28 + e- + ν

Due to Nuclear transformation an neutron disintegrates in to a proton,an electron & a neutrino

Thank you…

Vinay DesaiM.Sc Radiation Physics

Radiation Physics DepartmentKIDWAI MEMORIAL INSTITUTE OF

ONCOLOGYBengaluru

E-mail:- [email protected]

Basic Atomic structure

Science