Page 78 - ICSE Chemistry 8

applied on them. They are thus able to maintain their posi on. This lowest energy
results in the balancing of the inward force exerted by the nucleus
by the outward force of the moving electrons. This in turn prevents
the electrons from falling into the nucleus, thus making the atom nucleus +
structurally stable.

ATOMIC NUMBER highest energy

Fig. 4.12: Each shell (orbit) in an
The number of protons present inside the nucleus of an atom of an atom has fi xed energy.
element is called its atomic number. It is represented by the le er Z.
Each element has its own atomic number.

Since an atom has equal number of protons and electrons, the atomic number of the atom of an element
is also equal to the number of electrons present in it.
Atomic number (Z) = Number of protons = Number of electrons

Each element has a unique atomic number as no two elements contain the same number of protons (or
electrons). Thus, an atomic number can easily be used to dis nguish one element from another.

MASS NUMBER

The sum total of the number of protons and the number of neutrons present in the nucleus of an atom of
an element is called the mass number of that element. It is represent by the le er A.
Mass number (A) = Number of protons + Number of neutrons

= Atomic number + Number of neutrons

Atomic number and mass number are respec vely wri en as subscript and superscript alongside the
A
symbol of an element. An element X is represented as X. This is called atomic nota on of an element.
Z
If the atomic number and the mass number of an element are known, then the number of neutrons can
be easily calculated.

Mass number (A) = Number of protons + Number of neutrons
Thinking Fountain
Number of neutrons = Mass number (A) – Number of protons Calculate the number of electrons,
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Since atomic number (Z) = Number of protons protons and neutrons in krypton ( Kr).
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 Number of neutrons = Mass number (A) – Atomic number (Z)

Number of neutrons = A – Z

Let’s take the example of magnesium atom to understand be er. The atomic number of magnesium is 12
and its mass number is 24.

 Number of neutrons = A – Z

= 24 – 12
= 12

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