The number of protons in an atom determines what element it is because the number of protons defines the atom's atomic number. The atomic number is a fundamental property of an element and is unique to each element in the periodic table. It determines the element's identity and its position in the periodic table.
Protons are positively charged particles found in the nucleus of an atom, while neutrons are neutrally charged particles also found in the nucleus. Electrons, which are negatively charged particles, orbit the nucleus in electron shells.
The number of protons in an atom determines its chemical properties and how it interacts with other elements. This is because the number of protons determines the atom's positive charge and, therefore, its ability to attract and interact with negatively charged particles (electrons) in chemical reactions.
The number of neutrons in an atom can vary, resulting in different isotopes of an element. Isotopes have the same number of protons but different numbers of neutrons. While isotopes of an element may have slightly different physical properties, such as atomic mass or stability, they exhibit similar chemical behavior due to the identical number of protons and electrons.
In summary, the number of protons determines the element because it defines the atom's atomic number and determines its unique chemical properties, while the number of neutrons affects the atom's isotopic properties but not its elemental identity.