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Universe from Elementary Particles Universe from elementary particles Yes! It all started with a bang!, known as Big-Bang about 13.7 billion years ago. Man has evolved both physically and intellectually over the years to trace back the reason for the bang. This big bang created all the elementary particles and the interaction between them that lead to the formation of the universe as we observe. What makes these elementary particles to be the building blocks of nucleus of an atom? What is the structure and behaviour of nuclei of various elements of matter? To answer these questions we need to study this topic in a detailed manner.

Learning objectives

After completing the topic, the student will be able to:

  • Understand the structure of nucleus and analyze its components, related to all the atoms of the known elements.
  • Determine the size, mass and nuclear density of the nucleus for any given atom depending on the number of its constituents.
  • Discuss and differentiate between the isotopes, isobars and isotones of the atomic elements.
  • Explore the nuclear forces and its characteristic properties that bind the components of the nucleus to build a stable atom.
  • Discuss and relate mass defect to explain the stability of nucleus; also determine the most stable nuclei (of a particular element) among all the elements.
Nuclear energy power station & nuclear medicine imaging Nuclear energy power station & nuclear medicine imaging Nuclear energy is used to produce electricity and also in medicine imaging to identify the defects of a patient(NMR spectroscopy)
Introduction

The atom is almost all empty space. If we had an atom and wished to see the nucleus, we would have to magnify it until the atom was the size of a large room, and then the nucleus would be a large speck which you could just make out with your eye. The radius of the nucleus is 1,00,000 times smaller than the atom and in terms of volume, atom is a thousand million million times bigger than nucleus.

But nearly all the weight of the atom is in the infinitesimal nucleus. As neutrons and protons in nucleus are very nearly of the same mass as each other and each is about 2000 times as massive as an electron, the total number of protons and neutrons in the nucleus determines all but a small fraction of the mass of an atom.

The energy of the nucleus reveals itself in many ways; nuclear warheads that can destroy cities, submarines driven by nuclear reactors, nuclear power plants and in destroying cancerous cells. Nuclear energy comes from the conversion of mass and can result from nuclear reactions like fission (the splitting of the nucleus) and fusion (the coming together of nuclei).

In case of nuclear fission, nucleus splits into two roughly equal fragments and two or three more neutrons. The sum of these masses is less than that of the original nucleus, and the difference in mass is converted into energy of the motion of fragments which can lead to a nuclear chain reaction in certain conditions. The energy released as a result of nuclear chain reaction is used to run commercial nuclear reactors and generate electricity.

Nuclear medicine is a branch of medicine that uses the nuclear properties of matter in diagnosis and therapy. Nuclear medicine is a safe, painless and cost–effective technique to image the body and treat disease. In diagnosis, radioactive substances are administered to patients and the radiation emitted is measured. In therapy, radio nuclides are administered to treat disease or provide palliative pain relief. Nuclear medicine imaging procedures often identify abnormalities very early in the progress of a disease ‐ long before many medical problems are apparent with other diagnostic tests.

The OPAL detector at CERN The OPAL detector at CERN A technician works on one of two end caps of the OPAL detector at CERN, the European centre for particle physics near Geneva. OPAL is one of 4 giant particle detectors at the LEP collider. LEP collides electrons & positrons accelerated to an energy of 50 GeV in a circular tunnel 100m underground & 27km in circumference. OPAL is a cylindrical assembly of many types of apparatus which fit together like layers of an onion around the point where the particles collide.

Click to watch video lesson
Nuclear energy and nuclear reactions

Nuclear physics is unfortunately identified with applications of destruction such as submarines driven by powerful nuclear reactors which can travel around the world and sustain a crew for months without surfacing. Nuclear warheads have the power to destroy cities. But, put to right use we can find many important applications for human welfare. Nuclear energy is used to destroy cancerous cells. Nuclei when injected into the body, allow physicians to trace the movement of atoms and make diagnosis. The sunlight that nourishes our planet comes from nuclear reactions inside the Sun.

Nuclear energy can be understood simply as energy that comes from conversion of mass. The nucleus of the atom which is dense and heavy accounts for most of the atom′s weight and almost none of its volume. The nucleus which is responsible for nuclear energy and radioactivity behaves independent of the atom′s electrons which control chemical bonding.

The nuclei of most familiar atoms are stable and do not change, but some of them disintegrate and emit energetic particles that we call radiation. The radiation occurs when some part of the original mass converts to energy. Energy can result from nuclear reactions like splitting of the nucleus (fission) and coming together on nuclei (fusion). In Fission as well as fusion energy comes from conversion of mass into energy.

Neutron diffraction spectrometer Neutron diffraction spectrometer This is the spectrometer which was first used at Ryerson Laboratory, at the University of Chicago, USA, in the scattering of X‐rays by gases for free atom form factor determinations. In 1944 it was taken to Oak Ridge and set up at the face of the Graphite Reactor. There, it was used in 1945 by US physicist Ernest Omar Wollan (1902‐1984) to develop the techniques of neutron diffraction spectroscopy. This technique is used to determine the structure of materials in a similar way to X‐ray diffraction.
Composition and size of nucleus

The central core of the Altatom which contains all of the atom’s positive charge and most of its mass is known as atomic nucleus. The size of the nucleus is 10,000 times smaller than the size of the atom, but more than 99.9% of the whole mass of the atom is concentrated in the nucleus. Therefore, a nucleus occupies a very small space in the atom consisting of protons and neutrons.

Since neutron is a neutral particle, it has high penetrating power and very low ionising power. Further, electric and magnetic fields have no effect on it. These two constituents of a nucleus (protons and neutrons) are called nucleons. Although the hydrogen nucleus consists of a single proton alone, the nuclei of other elements consist of both neutrons and protons. The different types of nuclei are often called nuclides

Proton is a positively charged particle and has a charge equal to that of an electron. However, the mass of a proton is about 1836 times that of the electron. Charge on proton, e = + 1.6 × 10−19 C and mass of proton, mp = 1.6726 × 10−27 kg = 1.007825 a.m.u.

Neutron is a neutral particle and has no charge. Its mass mn = 1.6750 × 10−27 kg = 1.008665 a.m.u. Atomic mass unit (a.m.u.) is a smaller unit of mass developed to make the study of nuclear masses for convenience. One atomic mass unit is defined as one–twelfth (1/12) of the mass of a atom. According to Avogadro′s hypothesis, there are 6.023 × 1023 atoms in 12g of carbon.

The nucleus is believed to be spherical and hence its size is usually given in terms of radius. It has been found experimentally that the volume of a nucleus is directly proportional to its mass number A. If R is the radius of nucleus, its volume = (4/3) πR3, R = R0A1/3, where R0 is a constant whose value is found to be 1.2 × 10−15 m. The radius of the nucleus depends upon the mass number A and therefore, atomic nuclei of different elements have different sizes.

As the radius of a nucleus is extremely small, is order of 10−15m. In a usual practice express the nuclear radius in a smaller unit of length called fermi, named after the famous physicist Enrico Fermi. (1 Fermi (1fm) = 10−15 m).

Example: Find the nuclear radius of (a) lead and (b) oxygen. R0= 1.2 × 10−15 m

(a) Lead. Mass number, A = 208

∴ Nuclear radius, R = R0 A1/3= 1.2 × 10−15 × (208)1/3 = 7.1 × 10−15 m =7.1 fm

(b) Oxygen. Mass number, A = 16

∴ Nuclear radius, R = R0 A1/3 = 1.2 × 10−15 × (16)1/3 = 3.0 × 10−15 m = 3.0 fm


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