The following figure shows the structure of a relatively small atom. What word should be used to replace the unidentified letter X? (A) Neutron, (B) electron, (C) nucleus, (D) proton, or (E) nucleon.
The entire figure represents an atom. Atoms are the basic unit, or building block, of matter. Atoms are composed of three types of subatomic particle: positively charged protons, neutral neutrons, and negatively charged electrons. These particles are found in two main regions inside of an atom. In the center of the atom, we find the nucleus. The nucleus is composed of protons and neutrons. Since these particles are found in the nucleus, they are sometimes called nucleons. Outside of the nucleus is a large region of space where we can find the electrons. This region is often referred to as a cloud of electrons or an electron cloud.
It’s worth pointing out that this figure is not drawn to scale as an atom is roughly 10,000 times wider than its nucleus. Looking at the figure, we can see that unidentified letter X is labeling a collection of two different types of particles in the center of the atom. We have already established that in the center of the atom is the nucleus, which is composed of two different types of particles: protons and neutrons. Since X is referring to the entire collection of particles and not one type of particle in particular, we should replace unidentified letter X with answer choice (C) nucleus.