FSc Notes Chemistry Part 1 Chapter 5 Atomic Structure Lecture 3
Radioactivity:
The phenomenon by means of which certain elements emit
radiations spontaneously (i.e. without any outside help) is known as
radio-activity.
Or
The spontaneously emission of radiations from the nuclear of
radioactive elements is known as radio—activity.
The elements which emit such radiations are known as “Radioactive
element ". The phenomenon of Radioactivity was discovered Henry Becquerel
in 1895. He observed that certain elements like uranium, thorium, radium, etc,
emit radiations which cause fogging of photographic plates. When the
radioactive rays are A.J Notes passed through an electric field or magnetic
field, they are divided into three different types of rays which are.
- Alpha rays
- Beta rays
- Gamma rays
1. Alpha Rays:
The rays which are attracted towards the negatively charged
plate of the electric field are called alpha rays. They are positively charged.
The alpha particles have positive charges. Each alpha particles has a + 2
charge like helium nucleus. These rays are good ionizers of gases. Due to their
heavy mass, the alpha rays cannot penetrate much through thin layers of solids.
They are deflected by electric and magnetic field.
2. Beta Rays:
The rays which are attracted towards positively charged
plate of the electric field are called beta particles. These particles have
negative charge. These particles are high energetic electrons like cathode
rays. As compared to alpha particles, the Beta particles have low ionizing power because
of their small size. However they have high penetrating power. Their
penetrating power is 100 times that of alpha particles. Their charge to mass ratio
is like electrons. They are deflected by electric and magnetic fields but their
angle of deflection is larger than that of alpha particles because of their smaller
size.
3. Gamma Rays:
The un-deflected rays are known as Gamma rays. They are
electromagnetic radiations, like light. They have high energy. They have high
frequency and hence high energy. They are poor ionizers of gases. On account of
their high velocity and non-material nature, they have more penetrating power
than alpha and beta particles.
Artificial Radioactivity (Discovery of Neutron).
When the nuclei of stable isotopes of non-radioactive elements
are bombarded with alpha particles, these nuclei are broken down producing new
atoms and some radiations are emitted. This is known as artificial
radioactivity.
For example:
4 Be9 + 2He4 -------------------->6C12 + 6n1 + Q
In 1932 Chadwick discovered, electrically neutral particles
in the nucleus. They were named as "Neutrons"
Experiment:
He bombarded the nucleus of Beryllium (Be) with alpha particles
and found that some high penetrating radiations got out of the nuclei of Be
atoms (i.e. artificial radioactivity). These radiations knocked protons out of
paraffin with great force. Chadwick concluded that only a neutral particle,
with mass close to that of a proton, could knock proton out of paraffin. These
particles are now called “neutrons". Neutrons are the fundamental
particles of atom in addition to protons and electrons.
Characteristics of Sub—Atomic Particles:
So far, more than 100 particles have been discovered in
atom. However proton, electron and neutron are the fundamental particles of
atom because all the physical and chemical properties of atom depend upon these
3 particles. The important characteristics of these fundamental particles are
as follow.
1. Electron:
They are negatively charged particles which are revolving
around the nucleus in an atom. The mass of a single electron is 9.1094x10(-31)
Kg. An electron has a negative charge and the quantity of its charge is
1.60218x10-19 coulomb.
2. Proton:
They are positively charged particles which are present
inside the nucleus of atom. The mass of a single proton is 1.6726x10-27 Kg.
Thus a proton is 1836 times heavier than an electron. A proton has a single +ve
charge and the quantity of this + ve charge is same as that of electron i.e.
1.60218x10(-19) C.
3. Neutron:
They are neutral particles which are present inside the
nucleus of atom. The mass of a single neutron is 1.67493x10(-27) Kg a neutron
is 1842 times heavier than an electron.
X—Rays and Atomic Number:
In 1885, W.C Roentgen discovered that when high energy
electrons ie cathode rays collide with the surface of anode, a very penetrating
kind of rays are produced. He named these rays as x- rays. x-rays are the
radiations with a very high frequency, thus have very high penetrating power.
The frequency of x-rays depend upon the material which is used as anode.
In
1913, Mosley succeeded in explaining the fact that the frequency of the emitted
rays ( x-rays ) increase with increasing the no. of protons in the nucleus of
the target element ( anode ). He noticed that the no. of protons increases by
single electron unit from elements to elements. This no. of positive charges on
the nucleus of an atom is called as atomic no ( z) of that element.
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