Read the summary below. Choose the answer that BEST fits into the blank to complete the summary. Atoms are tiny particles, which are the building blocks of all matter.

The number of protons of an atom has is called its atomic number, which decides what kind of atom it is. Changes in the number of neutrons in an atom make an isotope and changes in the number of electrons make a positive or negative ion. Immersive Reader
An atom has a nucleus, made up protons and neutrons, which is surrounded by a cloud of electrons.
An atom has quarks and leptons, which make up the nucleus and are positively and negatively charged.
An atom has an atomic mass, which measures the amount of electrons and an atomic weight, which measures protons.
An atom has mostly empty space except for the nucleus, which contains nucleons, neutrons, and electrons.