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Mass Number (A)
Atoms are composed of a number of protons, neutrons, and electrons. As explained before,
the masses of proton and neutron are larger than that of electron. Thus, the mass of an atom is
approximately equal to the total mass of protons and neutrons. The mass of atom is stated as a
mass number (A), which is the number of protons and neutrons inside an atom.
Mass number (A) = Number of protons (p)
+ Number of neutrons (n)
Atomic number (Z) and mass number (A) are characteristics of atoms in an element. If X is
the symbol of a chemical element, the atomic number and mass number can be written using
notation below.
Self Practice
After learning about the atomic number and mass number of an element, try to complete the table below.
Element Symbol Atomic Number Number of protons (p) Number of neutrons (n) Mass number Notation
Argon Ar 18 … … 40 …
Kalsium Ca … 20 20 … …
Radium Ra … … 138 …
Aluminium Al … … … …
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