Detailed explanation of Rutherford’s Nuclear Model of Atom, Atomic Number and Mass Number,Isobars and Isotopes

 Rutherford and his students (Hans Geiger and

Ernest Marsden) bombarded very thin gold

foil with α–particles. Rutherford’s famous

α–particle scattering experiment is

represented in Fig. 2.5. A stream of high

energy α–particles from a radioactive source

was directed at a thin foil (thickness ∼ 100

nm) of gold metal. The thin gold foil had a

circular fluorescent zinc sulphide screen

around it. Whenever α–particles struck the

screen, a tiny flash of light was produced at

that point.

The results of scattering experiment were

quite unexpected. According to Thomson

model of atom, the mass of each gold atom in

the foil should have been spread evenly over

the entire atom, and α– particles had enough

energy to pass directly through such a

uniform distribution of mass. It was expected

that the particles would slow down and

change directions only by a small angles as

they passed through the foil. It was observed

that :

(i) most of the α– particles passed through

the gold foil undeflected.

(ii) a small fraction of the α–particles was

deflected by small angles.

(iii) a very few α– particles (∼1 in 20,000)

bounced back, that is, were deflected by

nearly 180°

.

On the basis of the observations,

Rutherford drew the following conclusions

regarding the structure of atom :

(i) Most of the space in the atom is empty

as most of the α–particles passed

through the foil undeflected.

(ii) A few positively charged α– particles were

deflected. The deflection must be due to

enormous repulsive force showing that

the positive charge of the atom is not

spread throughout the atom as Thomson

had presumed. The positive charge has

to be concentrated in a very small volume

that repelled and deflected the positively

charged α– particles.

(iii) Calculations by Rutherford showed that

the volume occupied by the nucleus is

negligibly small as compared to the total

volume of the atom. The radius of the

atom is about 10–10 m, while that of

nucleus is 10–15 m. One can appreciate

this difference in size by realising that if

a cricket ball represents a nucleus, then

the radius of atom would be about 5 km.

On the basis of above observations and

conclusions, Rutherfor d proposed the

nuclear model of atom (after the discovery of

protons). According to this model :

(i) The positive charge and most of the mass

of the atom was densely concentrated

in extremely small region. This very small

portion of the atom was called nucleus

by Rutherford.

(ii) The nucleus is surrounded by electrons

that move around the nucleus with a

very high speed in circular paths called

orbits. Thus, Rutherford’s model of atom

resembles the solar system in which the

nucleus plays the role of sun and the

electrons that of revolving planets.

(iii) Electrons and the nucleus are held

together by electrostatic forces of

attraction

2. Atomic Number and Mass Number

The presence of positive charge on the

nucleus is due to the protons in the nucleus.

As established earlier, the charge on the

proton is equal but opposite to that of

electron. The number of protons present in

the nucleus is equal to atomic number (Z ).

For example, the number of protons in the

hydrogen nucleus is 1, in sodium atom it is

11, therefore their atomic numbers are 1 and

11 respectively. In order to keep the electrical

neutrality, the number of electrons in an

atom is equal to the number of protons

(atomic number, Z ). For example, number of

electrons in hydrogen atom and sodium atom

are 1 and 11 respectively.

Atomic number (Z) = number of protons in

 the nucleus of an atom

 = number of electrons

 in a nuetral atom (2.3)

While the positive charge of the nucleus

is due to protons, the mass of the nucleus,

due to protons and neutrons. As discussed

earlier protons and neutrons present in the

nucleus are collectively known as nucleons.

The total number of nucleons is termed as

mass number (A) of the atom.

mass number (A) = number of protons (Z)

 + number of

 neutrons (n) 

3. Isobars and Isotopes

The composition of any atom can be

represented by using the normal element

symbol (X) with super-script on the left hand

side as the atomic mass number (A) and

subscript (Z) on the left hand side as the

atomic number 

Isobars are the atoms with same mass

number but different atomic number for

example, ⁶C¹⁴ and ⁷N¹⁴. On the other hand,

atoms with identical atomic number but

different atomic mass number are known as

Isotopes. In other words (according to

equation 2.4), it is evident that difference

between the isotopes is due to the presence

of different number of neutrons present in

the nucleus. For example, considering of

hydrogen atom again, 99.985% of hydrogen

atoms contain only one proton. This isotope

is called protium( ¹H¹). Rest of the percentage

of hydrogen atom contains two other isotopes,

the one containing 1 proton and 1 neutron

is called deuterium (

¹D², 0.015%) and the

other one possessing 1 proton and 2 neutrons

is called tritium (¹T³). The latter isotope is

found in trace amounts on the earth. Other

examples of commonly occuring isotopes are:

carbon atoms containing 6, 7 and 8 neutrons

besides 6 protons 

atoms containing 18 and 20 neutrons besides

17 protons.

Lastly an important point to mention

regarding isotopes is that chemical properties

of atoms are controlled by the number of

electrons, which are determined by the

number of protons in the nucleus. Number of

neutrons present in the nucleus have very

little effect on the chemical properties of an

element. Therefore, all the isotopes of a given

element show same chemical behaviour.

"This Content Sponsored by Genreviews.Online

Genreviews.online is One of the Review Portal Site

Website Link: https://genreviews.online/

Sponsor Content: #genreviews.online, #genreviews, #productreviews, #bestreviews, #reviewportal"