SDI Introduction to Physical Science (SC1-101) Practice Test

The identity of an atom is primarily determined by its number of protons. This is because the number of protons in the nucleus of an atom is known as the atomic number, which uniquely defines each element in the periodic table. For example, an atom with one proton is hydrogen, while an atom with six protons is carbon. The protons determine the chemical properties of the element and its placement in the periodic table.

Neutrons can vary in number within atoms of the same element, leading to different isotopes, but they do not change the identity of the element itself. Electrons, while important for chemical bonding and reactions, do not determine the identity of the atom; rather, they influence its charge and behavior in chemical interactions. The term "atoms" refers to the smallest unit of an element, rather than a specific subatomic particle that determines identity. Therefore, it is the number of protons that is crucial for establishing what element an atom is.