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The atomic number is a fundamental property of an element that determines its identity. It is always equal to the number of protons present in the nucleus of an atom. For example, if an atom has 11 protons, like sodium, its atomic number is 11.
This atomic number is significant because:

• It defines the element: No other element can have the same atomic number.
• It determines the element's position on the periodic table.
• It influences the chemical properties of the element.

Each element on the periodic table has a unique atomic number, which allows it to be uniquely identified. For sodium, represented by the symbol 'Na', the 11 protons not only define its atomic number but also contribute to its characteristic properties.

The mass number of an atom is a simple yet highly important concept. It is the total count of protons and neutrons present in the atom's nucleus. If we take our sodium example, it has:

• 11 protons
• 11 neutrons

The mass number is calculated by adding these two values together: 11 (protons) + 11 (neutrons) = 22.
The mass number differs from the atomic number in that it can vary among isotopes of the same element. For instance, although all sodium atoms will have an atomic number of 11, they might have different mass numbers due to the varying number of neutrons. Sodium-22, with a mass number of 22, is just one isotope of sodium. These isotope differences affect the atomic mass listed on the periodic table but do not change the atomic number.

The periodic table is a powerful tool in chemistry. It organizes all known elements in a manageable layout based on their atomic numbers. By arranging elements in order of increasing atomic number, the periodic table groups elements with similar properties together.

• Each row is called a period and represents elements with the same number of atomic orbitals.
• Each column is known as a group, and elements within a group often have similar chemical and physical properties.

For sodium, which has an atomic number of 11, the periodic table provides information like chemical reactivity, typical compounds it forms, and comparison to other alkali metals. The table also helps scientists predict how an element will react with others, making it an invaluable tool for understanding chemical behavior and relationships between elements.