A description of the nucleus:
Symbols for Nuclear Quantities

Symbol

Name

Explanation

A

Mass Number

the number of nucleons (protons and neutrons) in the nucleus

Z

Atomic Number

the number of protons in the nucleus

N

neutron Number

The number of neutrons in the nucleus


An Example:
Mass Number(A)

Chemical Symbol

27

Al

13

Atomic Number (Z)



Here
the symbol is for aluminum (aluminium for the rest of the world). A
typical aluminum atom has a mass number of 27 and an atomic
number of 13. Therefore it has 13 protons(atomic number) and 14
neutrons (27  13 = 14).
So the relationship is:
A = Z + N
In
fact this is how the neutron number is calculated. But what about
atoms that have the same number of protons and different numbers of
neutrons? The number of protons determines what the atom is, and some
atoms have different numbers of neutrons. These called isotopes. For example, hydrogen has three isotopes:
1



2



3



H (Hydrogen)



H (Deuterium)



H (Tritium)

1



1



1



Density of the Nucleus:
The nucleus is about 2.3 x 10^{14 }times as dense as water. So a cubic meter of nuclear material would have a mass of 2.3 x 10^{17 }kg.
Physicists today speak of the mass of the nucleus in terms of its rest energy. Yes
that is correct. Mass is energy! This concept is best
demonstrated by Einstein’s special theory of relativity. Rest
energy is equal to the mass times the square of the speed of light. Or E_{0}= m c^{2}. The Energy is expressed in MeV or million electron volts, this makes the mass units MeV/c^{2}, strange indeed.
Mass of proton, neutron and electron

Particle

kg

u

Mev/c^{2 }

proton

1.673 x 10^{27}

1.007 276

938.3

neutron

1.675 x 10^{27}

1.008 665

939.6

electron

9.109 x 10^{31}

0.000 549

0.5110


