Lecture 1
Dr: Sahar Mousa
Electrons and nuclei
The Familiar Planetary Model of the atom
was proposed by Rutherford in 1912
All the mass of an atom was concentrated
in the positively charged (+) Nucleus.
Electrons have Negatively charged (-).
electrostatic force: it’s a force required to
attracted the electrons to the nucleus.
Nuclear structure
Atomic number (Z) ,is the no. of protons
and also the no. of electrons in a neutral
atom.
Mass number,is the no.of protons + the
no. of neutrons
The lightest atomic nucleus (hydrogen) is
1830 times more massive than an
electron.
The size of a nucleus is around 10−15 m
(1 fm)
Nuclear structure
Nuclei contain positively charged Protons and
uncharged Neutrons
Protons and neutrons are held together by an
attraction force (strong interaction).
An electrostatic ruplsion between protons
occurs inside the nucleus
The balance of the two forces controls some
important features of nuclear stability
Nuclear structure
The lighter nuclei are generally stable with
approximately equal no. of protons and neutrons
as O16 andC12.
The heavier ones have higher proportion of
neutrons as pb 208.
As Z increases the electro static repulsion
comes to be dominate.
There is a limit to the number of stable nuclei.
All elements beyond Bi (z=83)being radioactive.
• Magic number
The nuclei with even no. of either protons
or neutrons ( or both) are generally more
stable than ones with odd no.
O16 and Pb208 are example of nuclei with
magic no.
-certain magic numbers of protons or
neutrons, which give extra stability
Isotopes

are atoms of the same element that have a different
number of neutrons. Therefore, isotopes have the
following characteristics:
 Isotopes have the same atomic number (same number
of protons), but a different atomic mass number (a
different number of neutrons).
 Isotopes behave the same chemically, because they are
the same element. The only difference is that one is
heavier than the other, because of the additional
neutrons.
 For example, carbon-12 and carbon-14 are both
isotopes of carbon. Carbon-12 has 6 neutrons; carbon14 has 8 neutrons.
Isotopes
Some elements have:
only one stable isotope (e.g. 19F, 27Al, 31P).
others may have several (e.g. 1H and 2H, the
latter also being called deuterium, 12C and 13C).
Molar mass is also known as Relative atomic
mass (RAM) is determine by the Proportions
(Mixture of the isotopes of an element).
Isotopes
NMR (nuclear magnetic resonance )
technique depend on NMR of H1 and C13.
All elements have unstable radioactive
isotopes, some of these occur Naturally
and the other can be mad e artificially.
Importance and uses of isotopes
(students)
Radioactivity
Radioactive decay:
It is a process whereby unstable nuclei
change into more stable ones by emitting
particles of different kinds as Alpha, beta
and gamma (α, β and γ) radiation
Radioactivity
Gamma radiation, is high energy electro
magnetic radiation accompanies alpha
and beta decays.
Half-life is the time taken for half of a
sample to decay. It can vary from a
fraction of a second to billions of years.
All elements beyond Bi (z=83) are
radioactive and non beyond U(z=92)
occur naturally on Earth.
Radioactivity
Spontaneous fission
It occurs for heavy elements ,where the
nucleus splits into two fragments of similar
mass.