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Protons and Neutrons

The smallest unit of any matter which involve in all chemical reaction is known as atom. Apart from Nobel gases any atom cannot exist in free State. Hence, atoms involve in chemical reactions and form molecule which can exist in free state.

Each atom is made up of certain particles called as subatomic particles. There are mainly three subatomic particles; electron, proton and neutron.
In any atom, electrons revolve in a certain path around a hard, dense and smallest center called as nucleus. Nucleus consist two types of subatomic particles that is protons and neutrons. In 1809, John Dalton purposed that the atom is an indivisible particle of matter.But the discovery of radioactive elements put a question mark of Dalton atomic concept.

Professor Henri Becquerel proved that atom is not an indivisible unit but made up some fundamental particles which are protons, electrons and neutrons. Apart from these fundamental particles there are some other subatomic particles like positron, meson, neutrino, anti-neutrino etc. The discovery of cathode rays gave all the necessary information regarding electrons by J.J. Thomson and William crooks.

 

Protons

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Proton is a positively charged particle present in the nucleus with neutral neutrons in an atom.
  1. The discovery of proton was the part of study of canal rays or positive rays.
  2. In 1886 Eugene Goldstein discovered positive rays through a gas discharge experiment.
  3. He used a gas discharge tube also known as crooks tube with a perforated cathode.
  4. When gas discharged at a very high electrical potential between the cathode and anode a faint luminous "rays" are seen which are extending from the holes in the back of the cathode.
  5. Since these rays moving in a direction opposite to the "cathode rays," which are negatively charged, hence they called as positive rays moving towards cathode. Another name given by Goldstein to positive rays was "Kanalstrahlen¬Ě" or "channel rays" or "canal rays".
Characteristic of positive rays
  • Positive rays move in the opposite direction to cathode rays.
  • They always travel in a straight line and effect photographic plates.
  • These rays are affected by electrical and magnetic field which proves that they have some charge.
  • Since they can penetrate only a very thin film, hence their penetration power is less than cathode rays.
  • They can create fluorescence and phosphorescence.
  • The velocity of canal rays is less than cathode rays.
  • The nature of canal rays depends upon the nature of gas filled in gas discharged tube.

For example the positive and negative particles produced from some gases are as follows.

Gas Positive Rays
Cathode Rays
H
H+
e-
O
O+
e-
O2
O2+ e-
O2
O22+
2e-

Cathode Ray Tube

J.J. Thomson and Wilhelm Wien studied further about positive rays and concluded that canal rays are made of positively charged particles so moving towards cathode which is a negatively charged electrode. The magnitude of charge by mass ratio is depends on the nature of gas present in discharged tube.

The mass of lightest particle of canal rays is equals to the mass of one hydrogen atom or 1837 times heavier than the mass of electrons. The magnitude of charge by mass ratio (e/m) for positive charge particle formed from hydrogen gas that is hydrogen ion (H+) is 9.578 x 104 coulomb /gm. Here the charge (e) considered as 1.6 x 10-19 and mass as 1.6725 x 10-24 gm. This positively charged particle is called as proton.

Proton is a fundamental particle of an atom with positive charge discovered by Goldstein and named by Thomson and Wine. Protons located in nucleus along with the neutrons. In a neutral atom, the number of protons is equals to the number of electrons and also termed as atomic number.

The chemical and physical properties of an element are periodic function of their atomic number. Protons and neutrons are termed as nucleons and responsible for the mass of atom. The total sum of number of protons and neutrons is known as mass number of atom.

Some properties of proton are as follows.
  • e/m ration= 9.758 x104 coulomb /gm
  • Charge = 1.602 x 10-19 coulomb = 4.8 x 10-10 e.s.u
  • Mass = 1.6725 x 10-24gm
= 1.6725 x 10-27 kg
= 1.6725 x 10-29 quintal
= 1837 times of mass of electron or mass of one hydrogen atom =1.00757 a.m.u.
  • Gram molar mass of proton= 1.6725 x 10-24 x 6.023 x1023 (Na) =1.008
Atom

Neutrons

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The discovery of electron and proton in an atom as negatively and positively charged particle leads to the existence of some other particle also.
  1. In 1920 Rutherford (1920) had predicted the existence of some neutral fundamental particle which can contribute in mass of atom but not in charge.
  2. Since electrons have negligible mass, so the mass of atom must be equals to the mass of protons present in atom.
  3. But it observed more that the expected value. Hence there must be neutral particle with some magnitude of mass more than electron.
  4. In 1932, Chadwick bombarded the element Beryllium with alpha particles and observed the emission of a radiation which have high penetration power but electrically neutral as they were unaffected by magnetic and electric fields.
  5. There mass was approximate equals to the mass of protons.
These particles named as neutrons written as on1. In place of beryllium, the element aluminium can also be used.

Alpha Particles

The nuclear reactionsof the formation of neutrons are as follows.

4Be9 +2He4 $\rightarrow$ 6C12 +0n1
13Al27 + 2He4
$\rightarrow$ 15P31 + 0n1

Neutrons are fundamental particles present in nucleus in all atoms excluding hydrogen atom.
They are neutral particles with almost same mass as for protons.
Because of the presence of different numbers of neutrons, isotopes are formed.
Some other properties of neutrons are;

Mass = 1.6748 x 10-24gm =1.00893 a.m.u
Gram molar mass = 1.00893
Density = 1 x 1012 Kg/cc


Generally neutrons considered as unstable particles which disintegrated to proton, electron and antineutrino. The half life of neutron is taken as 20 minute.

0n1 $\rightarrow$ +1p1 +-1e0 +0q0
In any atom, protons and neutrons are packed together in a very small space in the nucleus.
  • There must be some force of attraction between these particles to maintain the existence of nucleus.
  • The columbic force of attraction is not enough to hold many particles in such a small space, as the repulsion force between protons would render the nucleus highly unstable and tend to disintegration.
  • Hence, the force of attraction between nucleons which is stronger than the repulsive force between protons is called as nuclear forces.
  • Nuclear forces operate within a small distance around 0.10 fermi only.
The nuclear forces generate due to exchange of meson particles between nucleons.
n + e- $\rightarrow$ p
p + e-
$\rightarrow$ n

While the exchange of neutral meson particles takes place between same types of particles. The exchange of mesons between particles creates exchange forces, which bounded particles in nucleus. The stability of any nucleus is depends on the ratio of number of neutron and proton present in atom.

Those nucleus with n/p ratio one is highly stable. Elements with high n/p ratio are unstable an radioactive in nature. Elements till atomic number 20 are stable with n/p =1, as atomic number increases, n/p ratio increases and instability increases.

Elements
Mass No. [A]
Atomic No. [Z]
No. of protons p = Z
No. of Neutrons n = (A - Z)
n / p Ratio
Light Elements
Sodium
11Na23
23 11
11
12
$\frac{12}{11}$ = 1.09
Potassium
19K39
39 19
19
20
$\frac{20}{19}$ = 1.05
Heavy Elements Uranium
92U238
238 92
92
146
$\frac{146}{92}$ = 1.59
Polonium
84Po215
215 84
84
131
$\frac{131}{84}$ = 1.56


Mass defect and binding energy of nucleus


Since the mass of electrons is negligible, hence, the mass of any atom must be equals to the sum of mass of protons and mass of neutrons. But the actual mass of an element is less than the sum of mass of proton and neutrons. This difference is called as mass defect.

This is because, when a stable atom formed due to the combination of nuclear particles (protons and neutrons), there is some lose of mass in the form of energy.
Hence, the mass defect is equivalent to the energy released in the formation of nucleus from nuclear particles.
  • This energy is termed as binding energy of the nucleus.
  • The binding energy per nucleon is the measure of the stability of the nucleus.
  • The greater the value of binding energy makes a more stable nucleus.
For example, the formation of energy released is 127 Mev.

8p +8n $\rightarrow$ 8O16+ 127 Mev

Hence, same amount of energy required to break the oxygen nucleus into its constituent particles.

8O16 +127 Mev $\rightarrow$ 8p +8n

The relation between mass defect and binding energy is

BEN = $\Delta$m x 931 Mev
Here, BEN = binding energy
$\Delta$m = mass defect
1Mev =1.602 x 10-6 erg


The sum of number of neutron and proton in a nucleus is called as nucleon.
Per nucleon binding energy is equals to; BEN = Binding energy / Number of nucleon.

Binding Energy Curve

Generally the magnitude of binding energy of nucleons in the nucleus of an atom is around 8 MeV per nucleon. As the atomic number increases, the atoms become heavier and for heavier atoms like uranium, the binding energy per nucleon is less negative, hence it becomes unstable and undergoes into two nuclei of medium mass number.

In the same way binding energy of the light nuclei like deuterium and tritium is less negative than that of the helium nucleus. Therefore, energy is released during the fusion of these elements and formation of helium take place.
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More topics in Protons and Neutrons
Alpha Beta Gamma Particles Electromagnetic Repulsion
Transmutation Natural Transmutation
Electromagnetic Force Alpha Beta Gamma Rays
Artificial Transmutation
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