Atoms and Nuclei

Atoms are building blocks of matter which consists of Protons, neutrons, and Electrons. Protons and neutrons are in one region that is called as nucleus but an electron revolves around the nucleus in fixed orbits. The best example is watermelon in this black seeds is considered as electrons and rest of the part is the nucleus.

Let us learn more about an atom.

• Atomic Masses
• Discovery of Neutron
• Basic properties of neutron
• Composition of nucleus
• Nuclear density
• Mass- Energy Equivalence
• Size of the Nucleus
• Isotopes
• Isobars
• Isotones
• Energy equivalence of one Atomic Mass Unit
• Nuclear binding energy
• Mass defect
• Binding energy Nucleon
• Binding energy curve and its features
• Nuclear force

Notably, Proton has a positive charge and Neutron have no charge whereas electron has negative charge

Atomic Mass:

The Mass of an Atom is ‎1.66×10−27 kg and it is defined as 1/12th of the mass of carbon atom.

1 a.m.u= 1.66×10−27 kg  { a.m.u =Atomic mass unit}

Discovery of Neutron:

James Chadwick observed emission of neutral radiation in 1932 when beryllium nuclei were bombarded with alpha-particles. It was found when neutral radiation of protons from light nuclei such as helium, carbon, and nitrogen.

The principles of conservation of energy and momentum showed that if the neutral radiation consisted of photons, the energy of photons would have to be much higher than is available from the bombardment of beryllium nuclei with α particles

This can be explained by assuming that the neutral radiation consists of a new type of neutral particles called neutrons.

Basic properties of neutron

• A neutron is an electrically neutral particle
• Mass of a neutron is almost equal to the mass of a proton
• A neutron resides within the nucleus just like a proton.

Composition of nucleus

The composition of nucleus can be described as following terms

Z- atomic number= number of protons

N- Neutron number= number of neutrons

A-mass number= Z+N= total number of protons and neutrons

Size of Nucleus:  size of Nucleus R=R₀A^1/3 where A = mass number and R₀=constant.

 

Where R₀ = 1.2×10^-15m.The volume of a nucleus is ∝to the mass number.

V =(4/3)πR3, Also R ∝(A)^1/3

=> (R)³∝A

Therefore V ∝ (R) ³∝

Isotopes:

These are defined as the same number of protons but differ from each other in their number of neutrons

Isobars

All nuclides with the same mass number are called isobars.

Isotones

These are containing the same number of neutrons are called isotones

Energy Equivalence of one Atomic Mass Unit

1 a.m.u represents the average mass of nucleon and 1 a.m.u=931.25eV

Nuclear binding Energy:

We already know the nucleus is made up of neutron and protons so it may be expected that the mass of the nucleus is equal to the total mass of its individual protons and neutrons.

Mass Defect:

Mass Defect is the difference between the sum of masses of protons and neutrons forming a nucleus and actual mass of the nucleus.

Binding Energy: sometimes mass disappears in the nucleus in the form of mass defect. This loss is in the form of energy is called as binding energy.

 

BINDING ENERGY = MASS DEFECT X

Binding energy per nucleon :

It is the ratio of the binding energy of nucleus to the number nucleons.

                               =        

Binding Energy Curve and its features:

This curve is a plot of the binding energy per nucleon versus the mass number for large nuclei

This curve indicates how stable atomic nuclei are; the higher the curve the more stable the nucleus. Notice the characteristic shape, with a peak near A=60. These nuclei (which are near iron in the periodic table and are called the iron peak nuclei) are the most stable in the Universe. The shape of this curve suggests two possibilities for converting significant amounts of mass into energy. Note that this is the “upside down” version of the similar graph in a text. There the vertical scale increases downward, here it increases upward.

Fission Reactions

From the curve of binding energy, the heaviest nuclei are less stable than the nuclei near A=60. This suggests that energy can be released if heavy nuclei split apart into smaller nuclei having masses nearer A=60. This process is called fission. It is the process that powers atomic bombs and nuclear power reactors.

Fusion Reactions

The curve of binding energy suggests a second way in which energy could be released in nuclear reactions. The lightest elements (like hydrogen and helium) have nuclei that are less stable than heavier elements up to A~60. Thus, sticking two light nuclei together to form a heavier nucleus can release energy. This process is called fusion and is the process that powers hydrogen (thermonuclear) bombs and (perhaps eventually) fusion energy reactors.

In both fission and fusion reactions, the total masses after the reaction are less than those before. The “missing mass” appears as energy, with the amount given by the famous Einstein equation.

Stellar Energy Production

Both fission and fusion reactions have the potential to convert a small amount of mass into a large amount of energy and could conceivably account for the energy sources of stars. However, stars are made from light elements (mostly hydrogen and helium). Thus, fission cannot be initiated in stars as a source of energy, but fusion is quite possible if the right conditions prevail. As we shall see, these conditions can be found in the cores of stars, and thermonuclear fusion is the primary source of stellar energy.

Nuclear force

From the above session, the average mass nucleon of the  binding energy per nucleon is approximately 8 Mev

Hence to bind a nucleus together there must be a strong attractive force of totally different kind.