THE NUCLEAR ATOM
Atomic structure
An atom consists of a small
central nucleus composed
of protons and neutrons
surrounded by electrons.
An atom will always have
the same number of
electrons as protons.
A Lithium atom
protons
neutrons
electrons
Atomic and mass number
The atomic number (or
proton number) of an
atom is equal to the
number of protons in its
nucleus.
The mass number (or
nucleon number) of an
atom is equal to the
number of protons plus
neutrons in its nucleus.
protons = 3
neutrons = 4
electrons = 3
This Lithium atom has:
atomic number = 3
mass number = 7
Properties of protons, neutrons and electrons
Position in
the atom
Relative
mass
Relative
electric
charge
PROTON
nucleus
!
+1
NEUTRON
nucleus
!
0
ELECTRON
outside
nucleus
"#"""$
-1
Nuclear notation
An isotope of carbon consists of 6 protons and 8 neutrons.
This can be written as:
carbon 14
Number of protons
PLUS neutrons
(Mass number)
OR:
Number of protons
(Atomic number)
14
C
6
Chemical
symbol
Isotopes
The atoms of an element always have the same number
of protons.
Isotopes are atoms of the same element with different
numbers of neutrons.
The three isotopes of hydrogen
hydrogen 1
hydrogen 2
(deuterium)
neutrons
hydrogen 3
(tritium)
Note: The number after ‘hydrogen’ is the mass number of the isotope.
Question 1
An isotope of uranium (chemical symbol U)
consists of 92 protons and 143 neutrons. Give
the two different ways of notating this isotope.
The mass number of the Uranium isotope:
= 92 + 143 = 235
uranium 235
AND
235
U
92
Question 2
Determine the number of protons and neutrons in
the isotopes notated below:
(a) 13
7
(c) 197
79
N
protons = 7
neutrons = 6
Au
p = 79
n = 118
(b) 60
27
Co
(d) 239
94
Pu
p = 27
n = 33
p = 94
n = 145
Note: Apart from the smallest atoms, most nuclei
have more neutrons than protons.
Ionisation
Ionisation occurs when an
atom loses or gains one or
more electrons.
When an atom loses electrons
it becomes a positive ion.
When an atom gains electrons
it becomes a negative ion.
Lithium atom
(uncharged)
Lithium ion
(positively charged)
Models of the atom
In about 500 BC The Greek
philosopher Democritus suggested
that all matter was made up of tiny,
indivisible particles. The word
‘atom’ meaning ‘cannot be cut or
divided’
In 1803, Thomas Dalton proposed
that atoms were tiny hard spheres.
He further proposed that an element
was made up of identical atoms all
have the same mass but each different
element was made up of its own type
of atom.
Thomas Dalton
1766-1844
Electrons and sub-atomic particles
In 1897 J.J. Thomson
discovered the electron.
He found that this was a
negatively charged
particle that was much
smaller than an atom.
This discovery showed
that atoms were not
indivisible but were made
up of smaller sub-atomic
particles.
J.J. Thomson
1856-1940
The Plum Pudding Atomic Model
Based on his discovery of the
electron J.J. Thomson
suggested that atoms
consisted of a ball of:
- positively charged matter
- embedded with negatively
charged electrons like the
plums in a pudding
This became known as the
plum pudding model of the
atom.
The ‘Plum Pudding’ Model
Geiger & Marsden’s alpha particle scattering experiment
In 1909 Hans Geiger and
Ernest Marsden performed an
experiment that involved firing
alpha particles onto a very
thin foil of gold.
The results from this
experiment would show that
the plum pudding of the atom
was incorrect.
Geiger and Marsden
Alpha particle scattering by gold foil
Alpha particles are positively charged.
If repelled back from the gold foil,
there must be a positive charge
(positive charge of gold atom). Only
those which head straight to the
nucleus are reflected straight back.
The closer they are to the nucleus, the
more they are deflected or scattered.
Most alpha particles passed straight
through ( the nucleus is tiny compared
to the atom, an atom is mainly empty
space.
How the results can be explained
1.Deflections occur because
there is a force between a
charged nucleus and the
positively charged alpha
particles.
2.Most of the alpha particles do
not go near enough to the
nucleus to be deflected.
3.Backwards deflections occur
when the alpha particles make
near head on collisions with a
positively charged nucleus.
Rutherford’s Atomic Model
Based on the results of Geiger and Marsden’s experiment Ernest Rutherford
suggested that an atom consists of a a tiny positively charged nucleus
surrounded by negatively charged electrons.
Lord Rutherford
1871 - 1937
How their results supported
Rutherford’s atomic model
1.The relatively small number of
deflections indicates that most of the
atom is empty space with only a very
small nucleus.
2.The backward deflections can only
occur if the nucleus is positively charged
and contains most of the atom’s mass.
3.The ‘plum pudding’ model would not
produce backward deflections
EINSTEIN’s EQUATION
The actual mass of the atomic nucleus is always less than the mass of protons and
neutrons present in the nucleus. When a nucleus is formed, energy is released.
This energy is removed in the form of a reduction in total mass. This missing mass
is known as the ‘mass defect’ and it accounts for the energy released
The mass defect (𝚫M) can be calculated by subtracting the original atomic mass
(MA) from the sum of the mass of protons (mp= 1.00728 amu) and neutrons (mn=
1.00867 amu) present in the nucleus.
𝚫M = (Zmp + Nmn) – MA
E = mc2.
𝚫M – mass defect
MA – mass of the nucleus
mp – mass of a proton, i.e. 1.00728 amu
mn – mass of a neutron, i.e. 1.00867 amu
Z – number of protons
N – number of neutrons
Energy released = mass lost x the speed of light squared