The atomic number of an element is a whole number because it represents the number of protons in the nucleus of an atom of that element. Protons are fundamental particles with a positive charge, and they contribute to the overall positive charge of the nucleus.
The number of protons determines the identity of an element. For example, hydrogen has one proton, helium has two protons, and so on. If the number of protons were not a whole number, it would lead to fractional or non-integer values for the atomic number, which would contradict the discrete and distinct nature of elements.
Atoms are electrically neutral overall because they have an equal number of electrons, which carry a negative charge, to balance the positive charge of the protons. Since protons and electrons have equal and opposite charges, the atomic number also represents the number of electrons in a neutral atom.
The concept of atomic number was developed based on experimental observations and theoretical models of atomic structure, such as the quantum mechanical model. Through experimental studies, scientists found that the behavior and properties of elements correlated with their atomic numbers, providing a basis for organizing the periodic table and understanding the relationships between elements.