Basic structure of an atom and atomic models, Physics tutorial


Atom is the Smallest part of the element which can take part in the chemical reaction and retains all chemical properties of that element.

Atomic Structure:

The atom is spherical in shape. It has nucleus which is positively charged and located at centre of atom. Nucleus is composed of protons and neutrons. Negative electrons move round nucleus in energy shells. Electrons are held together by strong Columbic forces. Positively charged nucleus and negatively charged electrons combined to provide the atom a neutral charge. Atoms have a radius of 10-10 m and radius of nucleus is 10-16 m.

Charge Quantization:

Electricity comprises of charges carried by protons, electrons, neutrons etc. Electric charges are of two forms: Positive and Negative. Negative charges are known as electrons and positive charges are called protons. Charge carry by both electron and proton are exactly equal but opposite.

Minimum electric charge is denoted by the symbol e, electronic charge has the magnitude of 1.6 X 10-19 Coulombs.

That is e = 1.6 X 10-19 C.

Any physical existing charge in universe is the integral multiple of e, i.e. multiple integral of e = ne, where n is any number. So charge (e) exists in discrete form and not in continuous amount. This is referred to as charge quantization.

Mass Spectra (Singular, mass spectrum): Is the record of chemical constituents of the substance separated according to the mass and presented as the spectrum as estimated using a mass spectrometer. One version of this device is called as Bainbridge mass spectrometer. It operates on principle that the beam of ions is made to pass through velocity selector. In velocity selector charged particles all move with the same velocity. This is attained by the combination of vertically downward electric field E that is perpendicular to the magnetic field B. In velocity selector the magnetic force is qv X B and electric field is qE. When charged particles move in straight horizontal line through fields, the magnetic force is equivalent to electric field i.e. qvB = qE

v = E/B

Charged particles from velocity selector enter second magnetic field B0 which has same direction as in velocity selector. On entering second magnetic field the ions move in semicircles of radius r before striking the photographic plate. If te ions are positively charged, beam deflects upward, and if ions are negatively charged, the beam deflects upward. Whereas in circular motion, centripetal ( F = mv2/r) force is equivalent to magnetic force (qvB0)

i.e. mv2/r = B0qv

mv/r = B0q

m/q = B0r/v

but v = E/B

m/q = B0rB/E

Thus, we can find out m/q by estimating radius r of curvature and knowing fields B, B0, and E. In practice mass spectrometer estimate different isotopes of given ion, as ions all carry same charge q. In this way, mass ratios can be determined even if q is unknown.


Isotopes are elements with unequal mass number but equal atomic number (or same atomic number but different neutrons number)

E.g. 1) 21H, 31H

2) 3517cl, 3717cl etc.

Atomic models:

Atomic models are different pictures of atom that scientists presented how the atom looks like. Few of these models comprise:

  • J. J Thomson model of the atom: He sees the atom as homogeneous sphere of positive charges inside of which negatively charged electrons are joined. This model of atom was given rather unlikely name of 'plum pudding model'.
  • Rutherford's model: He sees the atom comprising of the positively charged nucleus at the centre of atom. Negatively charged electrons move round nucleus in circular orbit just like planets move round the sun.
  • Other models comprise Electron cloud and Bohr's models etc.

Bohr's model of an atom:

Bohr's model:

Bohr's model of the atom was extensively accepted as correct model of an atom. Some of its basic revolutionary suggestions of an atom are:

Electrons can revolve round nucleus in only in certain permitted orbits and while they are in the orbits they don't emit radiations (energy). Every orbit has a fixed amount of energy (has kinetic energy because of motion and potential energy due to attraction of the nucleus.)

Allowed orbits are those for which orbital angular momentum is equal to the integral multiple of ?(? = h/2π) and h is Planck's constant. L = mvr = n? where n = 1, 2, 3, 4, ------- and n is known as principal quantum number.

The electron can jump from orbit of energy E2 to another of energy E1 and energy difference is emitted as one quantum of radiation of frequency f provided by Planck's equation:

E2 - E1 = hf

Hydrogen Spectra:

These are horizontal lines drawn one above other to symbolize energy transition (or energy levels) in increasing order in the hydrogen atom. The transition is the jump from one energy level to another. Bohr derived the formula for energy of the electron in any energy level as:

En = -me4/8ε20h2n2

Where m = mass of an electron

e = Charge of an electron

ε0 = permittivity of free space or vacuum

h = Planck's constant

Recasting the above formula i.e

En = -(me4/8ε20h2)x1/n2

If m = 9.1 X 10-31 Kg

e = 1.6 X 10-19 Coloumb

ε0 = 8.85 X 10-12 farad per meter

h = 6.6 X 10-34 Js

Then En = -9(9.1 x 10-31)(1.6 x 10-19)4/8(8.85 x 10-12)2(6.6 x 10-34)]1/n2

En = -2.179 x 10-18/n2 Joules

But one electron volt (1ev) = 1.6 x 10-19joule

Thus energy of the electron in any energy level is:

En = -(2.179 x 10-18/1.6 X 10-19) X 1/n2 eV

En = -13.61875/n2ev

For n = 1, E1 = -13.62 eV

For n = 2, E2 = -3.41 eV

For n = 3, E3 = -1.51 eV and so on.

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