The atomic number of an element is defined as the number of protons present in the nucleus of an atom of that element. It is the fundamental property that determines what the element is. For example, hydrogen has an atomic number of 1, meaning every hydrogen atom has one proton in its nucleus, while carbon has an atomic number of 6, indicating six protons.
Since atoms are electrically neutral, the number of electrons in a neutral atom is also equal to the atomic number. This not only defines the element but also influences its chemical behavior, as electrons participate in bonding and reactions.
The atomic number is what organizes the Periodic Table—elements are arranged in order of increasing atomic number, which reflects their unique identities and recurring chemical properties.