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Atomic Number.
The number of protons in the nucleus is called the atomic number.
For sodium
(Na) the atomic number is
11.
The atomic number tells you what the element is (see the
periodic
table).
The atomic number of an element
never changes.
Mass Number.
The number of protons
plus the number of neutrons is called the
mass number.
Sodium has 11
protons and 12 neutrons. The
mass number is 11 +
12 = 23.
The mass number is the total
number of particles in the nucleus.
The mass number and the atomic
number are written above and below
the chemical
symbol for the element, as
shown.
Isotopes.
Atoms
of the same element may
have different numbers of neutrons.
These are called isotopes.
For example, Chlorine (atomic number
17) may have 18 or 20
neutrons.
The mass number will be either 17 + 18 =
35, or 17
+ 20 = 37.
Isotopes of the same element will have the same chemical properties
because the number of protons and electrons will be the same.
Relative Atomic Mass
(can be written as RAM or Ar).
In a naturally occurring sample of Chlorine,
75% of the atoms will have a mass number of
35,
25% will have a mass number of
37.
The average mass number for the
sample will be
[(75
x 35) + (25 x 37)] ÷100
= 35·5
The average mass
number is called the Relative Atomic Mass (RAM).
In this way, the relative atomic mass
of any element can be
calculated,
given the percentage of each
isotope.
The mass number of any
one atom will be a whole number.
The relative atomic mass is the
average mass number
for
all of the isotopes of that element.
The RAM is written in green above each
element in the
periodic table.
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Copyright © 2008 Dr. Colin France. All Rights Reserved.
