Usage - Pronoun-Antecedent Agreement. A pronoun is a word used to stand for (or take the place of) a noun. A word can refer to an earlier noun or pronoun in the sentence. Example:
A subatomic particle is a unit of matter or energy that's the fundamental makeup of all matter. According to modern atomic theory, an atom has a nucleus, which is its center, or core. The nucleus contains subatomic particles: protons and neutrons. Protons are positively-charged particles. Neutrons are neutral particles.
The nucleus of an atom contains neutrons and protons. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus. Because the nucleus is only made up of protons and neutrons, it is positively charged.