Final answer:
The center of an atom is called the nucleus and it is comprised of protons and neutrons. The nucleus is positively charged, with the number of protons defining the element's identity, and the atomic mass number representing the sum of protons and neutrons.
Step-by-step explanation:
The statement that best defines the center of an atom and what it is made of is: The center of an atom is the nucleus, and it is made up of protons and neutrons.
Atoms are composed of two primary regions: the nucleus, located at the center, which contains protons and neutrons; and the outermost region, which holds the electrons in orbit around the nucleus. The nucleus is massive and positively charged due to the presence of protons, which have a positive electric charge, while neutrons are neutral. The electrons, which are negatively charged and have significantly less mass than protons or neutrons, revolve around the nucleus and account for the atom's volume despite the atom being mostly empty space.
Every element is characterized by the number of protons in its nucleus, which is known as the atomic number (Z). The total number of protons and neutrons in the nucleus is referred to as the atomic mass number (A).