CBSE Class 9 Answered

Atomic Number and Atomic Mass Number

• The number of protons present in the nucleus of an atom is the atomic number of that atom. It is represented by the symbol Z.

Atomic number (Z) = Number of protons (p)

• All atoms of an element have the same atomic number. The number of protons and electrons in an atom is equal. Thus, the atom of an element is electrically neutral.
• Atomic mass number is defined as the sum of the number of protons and neutrons contained in the nucleus of an atom of that element. It is denoted by the symbol A.

Atomic mass number (A) = Number of protons (p) + Number of neutrons (n)

The atomic number, atomic mass number and symbol of an element are written as -