Every element has a unique atomic number. The atomic number determines the identity of an element and corresponds to the number of protons in the nucleus of an atom. Each element has a distinct number of protons, which distinguishes it from other elements.
However, there are cases where different elements may have the same atomic number. These elements are known as isoelectronic species. Isoelectronic species have the same number of electrons, and as a result, they share the same atomic number.
For example, hydrogen (H) and helium ion (He+) are isoelectronic. Hydrogen has one proton and one electron, while helium ion (He+) has two protons and one electron. Although these elements differ in the number of protons, they both have an atomic number of 1 because they have the same number of electrons.
It's important to note that isoelectronic species belong to different elements and have distinct chemical and physical properties due to differences in the number of protons and the overall atomic structure.