The element an atom belongs to is determined by the number of protons in its nucleus. The number of protons is called the atomic number and is a unique characteristic of each element. For example, hydrogen atoms have one proton, helium atoms have two protons, carbon atoms have six protons, and so on.
The periodic table of elements arranges elements in order of increasing atomic number. Each element has a unique atomic number that distinguishes it from other elements. For instance, hydrogen has an atomic number of 1, helium has an atomic number of 2, carbon has an atomic number of 6, and so forth.
The number of protons in an atom's nucleus determines its chemical properties and defines the element to which it belongs. Additionally, the number of protons also determines the atom's electrical charge when it is not neutral. An atom with an equal number of protons and electrons is electrically neutral, but if the number of electrons differs, the atom becomes positively or negatively charged, forming an ion.
In summary, the element of an atom is determined by the number of protons in its nucleus, which is known as the atomic number.