Which of the following paragraphs provides information that would be MOST useful for students to understand why protons are the most important subatomic particles for defining the atom? The nucleus consists of protons and neutrons. (The ordinary hydrogen atom is an exception; it contains one proton but no neutrons.) Protons carry a positive electrical charge, while neutrons carry no electrical charge.

The atomic number determines what kind of an atom it is. Every atom is associated with a specific chemical element. An atom is the smallest unit of an element, and each chemical element has a unique atomic number. For instance, hydrogen has an atomic number of 1 because every hydrogen atom has one proton in its nucleus. No other element has an atomic number of 1.

Atoms that have the same atomic number but different mass numbers are called isotopes. Carbon-12, the ordinary form of carbon, has six protons and six neutrons per atom. Carbon-14 is an isotope with eight neutrons per atom.

An ordinary atom has an equal number of protons and electrons. So the positive and negative charges are balanced. Some atoms, however, lost or gain electrons in chemical reactions or in collisions with other particles. Ordinary atoms that either gain or lose electrons are called ions.