The periodic table is an important tool in chemistry.

It is mainly organized based on two key ideas: atomic number and mass number.

The atomic number, shown as $Z$, tells us how many protons are in the nucleus of an atom. This number helps us identify the element.

For example:

• Hydrogen has an atomic number of 1.
• Oxygen has an atomic number of 8.

Since the atomic number also tells us how many electrons are in a neutral atom, it affects how the element behaves in chemical reactions.

Mass number, on the other hand, is the total number of protons and neutrons in an atom's nucleus.

It is usually shown as $A = Z + N$, where $N$ stands for neutrons.

The mass number helps us tell apart different versions of the same element, called isotopes.

For example:

• Carbon's most common isotope has a mass number of 12 ($^{12}C$).
• Another version, or isotope, of carbon has a mass number of 14 ($^{14}C$).

Isotopes have similar chemical properties, but they behave differently physically, especially when it comes to stability and radioactivity.

In the periodic table, elements are arranged in order of increasing atomic number.

This shows a clear pattern in the properties of elements.

This pattern is known as the periodic law, which says that the properties of elements repeat regularly when sorted by atomic number.

While the mass number is not talked about as much, it is still important for understanding the differences between isotopes.

These differences can be used in things like dating old objects or in medicine.

In short, atomic number and mass number are essential for the periodic table.

They help chemists organize elements and predict how they will behave in chemical reactions.

They are more than just numbers; they are the building blocks of atomic theory and our understanding of chemistry.