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.
In the center of every atom is a tiny, dense nucleus. The nucleus contains two kinds of particles: neutrons, which have no charge, and positively charged protons. These orbitals surrounding the nucleus have different levels of energy - the farther away each is from the nucleus, the more energy it has.