Almost all matter in the universe is concentrated within atomic nuclei composed of neutrons and protons. Radioactive decay occurs when an unstable atomic nucleus loses energy by emitting radiation such as alpha particles, beta particles, or gamma rays. The number of protons defines the element, while the total number of protons and neutrons is the mass number. Radioactive emissions are random, but the average rate of decay follows statistical laws defined by half-life.

1. 

2.  Almost all matter in the Universe is concentrated within
atomic nuclei.
 All nuclei are made up of neutrons and protons, collectively
known as nucleons, which play similar roles in maintaining
the structure of the nucleus.
 The no. of protons in the nucleus is called atomic number Z .
 The total no. of neutrons and protons is called the mass
number A .

5.  Radioactive decay (also known as nuclear
decay, radioactivity or nuclear radiation) is the process
by which an unstable atomic nucleus loses energy (in
terms of mass in its rest frame) by emitting radiation, such
as an alpha particle, beta particle with neutrino or
a gamma ray.
 Radioactive decay occurs because a nuclei is unstable. It
emits alpha particles, beta particles or gamma rays to
become more stable.

7. Natural Radioactivity
 Is spontaneous, since in
natural radioactivity, the
nuclei of heavy atom
disintegrate on their
own accord, emitting
radiations.
 Is usually shown by
heavy elements.
 Is not spontaneous, since
in it the nuclei of the
atoms have to be
bombarded by fast
moving particles like
neutrons, protons,
deuterons.
 Can be induced even in
light element.
Artificial Radioactivity

10.  Alpha Rays (or Alpha particles) = nuclei of Helium
atoms ( 2 protons, 2 neutrons).
 Beta Rays = electrons (created within the nucleus)
 Gamma Rays = high-energy photons (packets of
energy)

11.  Occurs because the strong nuclear force is unable to hold
very large nuclei together.
 The loss of 2 protons and 2 neutrons(the Helium nucleus)
changes the atom.
 Since most effective ionization therefore, most energy is lost
during travel, hence cover least distance.
 An example of Alpha decay is a smoke detector.

13.  Beta decay occurs with the emission of an electron
(e) or β particles.
No nucleons are lost and the total no. of nucleons are
the same in the daughter nucleus and parent nucleus.
With the loss of an electron, the nucleus must have an
extra positive charge.

14.  Sometimes a nucleus that is still unstable after a beta
or alpha decay is still unstable so may emit gamma
radiation to become more stable.
 This is an electromagnetic wave with a very short
wavelength.
 it does not change the number of protons or neutrons
in the nucleus.

18.  The emission of an alpha particle or beta
particle leads to a change in the composition of
a nucleus in terms of protons or neutrons.
The emission of a gamma photon does
NOT change the composition of the nucleus, it
only lowers the energy associated with
the nucleus after the radioactive decay has taken
place.

19.  ALL the nuclei of a radioactive substance have the same
probability of undergoing radioactive decay (by emitting
alpha, beta or gamma particles). It is IMPOSSIBLE to tell
which nuclei will be the next in line to decay or how long it
will take to decay.
 Since it is independent of external conditions and the
breakdown within the nucleus is completely random, i.e.,
occurrence of radioactive emission is random over space
and time.
 But, it is noticed that with many repeated observations of
the random process, the average of its radioactive
emissions does follow some statistical laws. You can
calculate approximately how much of a “basket” of
radioactive materials will decay in X time.

20. The half-life of a radioactive substance is a
characteristic constant. It measures the time it takes
for a given amount of the substance to become
reduced by half as a consequence of decay, and
therefore, the emission of radiation.
Or
The term half-life is defined as the time it takes for
one-half of the atoms of a radioactive material to
disintegrate.

21. Archeologists and geologists
use half-life to date the age
of organic objects in a
process known as carbon
dating. During beta decay,
carbon 14 becomes nitrogen
14. At the time of death
organisms stop producing
carbon 14. Since half life is a
constant, the ratio of carbon
14 to nitrogen 14 provides a
measurement of the age of a
sample.

22.  Fusion is the process that powers the sun and the stars.
It is the reaction in which two atoms of hydrogen
combine together, or fuse, to form an atom of helium.
 In the process some of the mass of the hydrogen is
converted into energy. The easiest fusion reaction to
make happen is combining deuterium (or “heavy
hydrogen) with tritium (or “heavy-heavy hydrogen”)
to make helium and a neutron.

23.  Nuclear fission is the process in which a large
nucleus splits into two smaller nuclei with the
release of energy. In other words, fission the process
in which a nucleus is divided into two or more
fragments, and neutrons and energy are released.
 The by-products include free neutrons, photons
usually in the form gamma rays, and other nuclear
fragments such as beta particles and alpha particles.

24.  Fission of heavy elements is an exothermic reaction
and can release substantial amounts of useful energy
both as gamma rays and as kinetic energy of the
fragments (heating the bulk material where fission
takes place).
 Nuclear fission produces energy for nuclear power and
to drive explosion of nuclear weapons.