Head Of Department

Physics Department

R.K Arya College, Nawanshahar

 Shaheed Bhagat Singh Nagar, Punjab, India 

Abstarct

In nineteenth century it was considered that all material substances are composed of atoms of elements. In this chapter I have explained about structure of atom. Initially it was J.J Thomson who gave Plum pudding model that explained structure of atom later on it was Rutherford in 1911 by his scattering experiment explained the existence of Nucleus and he also explained existence of electron in a orbit. In this chapter I also give some brief information about Nucleus and its properties. At the end I have given explained the Radioactivity phenomenon. In this book chapter I have also explained Law of radioactive decay. 

Keywords : Nucleus , Radioactivity and Curie 

Introduction

Initially it was J.J Thomson who explained the structure of atom in 1907.his model was known as plum pudding model or watermelon model. According to Thomson model an atom consist of equal number of positive and negative charges of uniform density and the radius of the atom was of the order 10^-10m.Thomson proposed that in an atom, the positive and negative charges are distributed uniformly within its spherical volume.

Rutherford in 1911 performed an α scattering experiment by atoms in which he found that Most of these particles after passing through gold foil remains undeflected, few α particles deflected through small angles. And few particles were deflected through large angles greater than 90⁰.Rutherford concluded that an atom consists of a small central hard core called nucleus and it carries a positive charge and electrons revolve around it in circular orbits.

Rutherford’s Nuclear Model of Atom

1. The entire mass and positive charge of an atom is concentrated in a very small region called nucleus of atom.

2.  The size of nucleus is of the order 10^-14m to 10^-15m.

3.  Electrons revolve around the nucleus in circular orbits.

4.  Atom is electrically neutral.

Discovery of Neutron

In 1932 by James Chadwick observed emission of neutral radiation when beryllium nuclei were bombarded with alpha-particles (or helium nuclei). It was found that this neutral radiation could knock out protons from light nuclei such as those of helium, carbon and nitrogen. The only neutral radiation known at that time was photons (electromagnetic radiation). Application of 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 phenomenon can be explained by assuming that the neutral radiation consists of a new type of neutral particles called neutrons. 

Composition of Nucleus

A nucleus of an atom consists of positively charged particles called protons and neutral particles called neutrons.

The number of protons in a nucleus is known as atomic number and is represented as Z

The sum of protons(z) number and neutrons(N) number is known as mass number and is represented as A.

 A= Z+ N

Protons and neutrons combine together known as nucleons, which is represented as

zX^A

x- symbol of the element

Z- atomic number of the element

A – mass number of the element

Properties of Nucleus

1Size of the Nucleus

Nucleus is spherical in nature, the nuclear volume is proportional to the number of its constituent particles or its mass number A

4/3 πR³∞ A

R= R₀A^1/3 where R is the radius of the nucleus and R₀ is a constant for all nuclei .its average value is 

R_{0= 1.4x 10}^-15m

2)Nuclear Density

The quantity which describes both mass and size of the nucleus is called density. Nuclear density is defined as mass per unit volume of the nucleus. The density of nucleus is a constant, for all nuclei and is 

p = 2.9 × 10¹⁷ kg m^–3 

3. Atomic Masses

One atomic mass unit is equal to one twelfth of the mass of neutral Carbon-12 atom (₆C¹²)

1 a.m.u. = 1.66 × 10^‒27 kg

4. Nuclear Charge

Moseley from his work on X-ray spectra estimated nuclear charge ,however it was Chadwick in 1920 who succeeded in measuring nuclear Charge directly.The Rutherford’s scattering formula was used to determine Ze (Nuclear Charge) where Z is atomic number or number of protons .as neutrons are neutral so they have no charge 

5. Parity and Statistics

Parity of a nucleus is associated with te value of orbital angular momentum L ,if L is even parity is also even and if L is odd ,parity is also odd.

Statistics -All nuclei with odd mass number A obey Fermi – Dirac statisctics and those with even A obey Bose- Einstein Statistics. The statistics of atomic nuclei is determined experimentally from rotational Raman spectra of homogeneous molecules.

6. Mass Defect

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

∆m = ZM_p + (A- Z) M_n – M_nucleus
M_p -- Mass of proton in nucleus
M_n - Mass of neutron in the nucleus
M_nucleus – mass of nucleus
N- no of neutrons
Z – no of protons
A-    Total number of nucleons

Binding Energy

Binding energy is the energy needed to hold or bind the  nucleons together inside the nucleus. Nuclear binding energy is the energy required to split a nucleus of an atom into its components. The mass defect of a nucleus represents the mass of the energy binding the nucleus, and is the difference between the mass of a nucleus and the sum of the masses of the nucleons of which it is composed the binding energy is the energy needed to put in to split the nucleus into individual protons and neutrons. To find the binding energy, add the masses of the individual protons, neutrons, and electrons, subtract the mass of the atom, and convert that mass difference to energy

Nuclear Forces

In the begning to explain the nucleus stability it was assumed that two types of forces in the nucleus

i) The Coulomb’s repulsive force ii) The gravitational attractive force

The stability of Nucleus is because gravitational attractive force is greater than the Coulomb’s force .But it was found that the magnitude of Coulomb’s repulsive force is much more greater than gravitational attractive force .so for the stability of a nucleus a new force called nuclear force was defined and this force was assumed to be stronger attractive force and its magnitude is greater than the Coulomb’s repulsive force.

Nuclear forces are non-central forces and also they are charge independent.

Isotopes, Isotones and Isobars

Isotopes – The atoms of an element which have the same atomic number but different mass number are called isotopes’ the number of protons inside the nucleus or number of electrons outside the nucleus of such atom is the same but number of neutrons inside the nucleus is not same.for example ₈O¹⁶, ₈O¹⁷,₈O¹⁸

Isotopes have identical chemical properties.

Isotones

Atoms whose nuclei have same number of neutrons are called isotones

Isobars

Atoms of same mass number but different atomic number are called isobars they have same number of nucleons in their nuclei so they have same atomic weight. Chemical properties of isobars are different.

Radioactivity

Radioactivity was discovered by Becqueral in 1890 who observed that radiations emitted by double phosphate of uranium and potassium capable of darkening a photographic plate enveloped in a black paper.

After Becqueral , Curie discovered the radioactivity of thorium and confirmed the Becqueral’s results.Madam Curie and Pierre Curie discovered the new radioactive elements polonium(z=84) and radium(Z= 88)which were more active than uranium.

Rutherford showed that uranium emitted two kinds of rays. One of these is α rays that was easily absorbed and the was β- rays having more penetration power.Later a third type of rays called γ – rays which was found to be more penetrating than the  α rays and  β- rays. 

α rays -

These are strongly ionising and weakly penetrating radiations and these are deflected by magnetic and electric fields as positively charged particles.these are doubly charged helium atoms ₂He⁴.They move with velocity of 10⁹ cm/sec. 

β- rays

 These are more penetrating than α rays and are less strongly ionising.These are deflected by electric and magnetic fields as negatively charged particles .These are electrons with velocity approaching velocity of light.

γ – rays

These are highly penetrating and weakly ionizing radiations and remain anaffected in electric and magnetic field.these are electromagnetic radiations of wavelength shorter than that of X- rays. 

Law of Radioactive decay 

The law of disintegration of radioactive nuclei was given by Rutherford and Soddy in 1902.for given nucleus the probability of radioactive decay per unit time must be constant .it implies that number of decays in a given time interval should be determined by the number of radioactive nuclei present at that time.If at time t , there are N nuclei present than we have

dN/dt = -λN 

where λ is known as decay constant and negative sign shows that number of nuclei is decreasing with time . 

Units of Radioactivity

The unit of radioactivity was defined as the number of disintegrations which takes place in 1gm of ^226Ra by α particle emission in one second. The unit has been given the name Curie and has value 3.7x 10¹⁰ disintegrations per second.

1 curie = 3.7x 10¹⁰ disintegrations /s

If the number of atoms in a radioactive substance disintegrates at the above rate, it is said to have radioactivity of 1 curie (Ci).