From Simple English Wikipedia, the free encyclopedia
The
mass number
(symbol:
A
) of an
atom
is the sum of the number of
protons
and the number of
neutrons
in the
nucleus
.
[1]
: 20
The mass number is different for each
isotope
of a
chemical element
.
We write a mass number after an element's name or as a superscript to the left of an element's symbol. For example, the most common isotope of
carbon
has 6 protons and 6 neutrons. We write it as carbon-12 or
12
C.
Mass number is not the same as:
- atomic number
(symbol:
Z
), which is the number of protons in a nucleus of an atom
- atomic mass
(symbol:
m
a
), which is the mass of a single atom, commonly expressed in
unified atomic mass units
- relative atomic mass
(symbol:
A
r
, also called
atomic weight
), which is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.
The difference between the mass number and the atomic number gives the number of neutrons (
N
) in a given nucleus:
N
=
A
?
Z
.
[1]
: 44
The mass number is not shown on the
periodic table
.
[1]
: 21
For each element, what is shown is the
standard atomic weight
and the
atomic number
.
- ↑
1.0
1.1
1.2
Moore, John T. (2010).
Chemistry Essentials For Dummies
. Wiley.
ISBN
978-0-470-61836-3
.
