Figure 10 This table lists the symbol, the relative charge, the relative mass, and the actual mass of an electron, a proton, and a neutron. The Model column shows the colors used in this book to represent the subatomic particles. Calculating What is the difference in actual mass between a proton and a neutron?
ddNeutrons
In 1932, the English physicist James Chadwick designed an experiment to show that neutrons exist. Chadwick concluded that the particles he produced were neutral because a charged object did not deflect their paths. A neutron is a neutral subatomic particle that is found in the nucleus of an atom. It has a mass almost exactly equal to that of a proton.
Comparing Subatomic Particles
Figure 10 summarizes some properties of protons, electrons, and neutrons. 
Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom. Protons and neutrons have almost the same mass. But the data in Figure 10 show that it would take about 2000 electrons to equal the mass of one proton. Electrons have a charge that is equal in size to, but the opposite of, the charge of a proton. Neutrons have no charge. Protons and neutrons are found in the nucleus, but electrons are found in the space outside the nucleus.
Problem-Solving Activity 
Designing an Atomic Exhibit
You work as a volunteer at the local science museum. You are asked to design an exhibit that compares the size of a lithium atom to the size of its nucleus. A lithium atom has a diameter of about picometers. The nucleus of a lithium atom has a diameter of about picometers. There are a trillion (1012) picometers in a meter.
Defining the Problem State the problem in your own words. What decisions will you need to make before you can proceed?
Organizing Information How many times larger is the lithium atom than its nucleus? Find several objects that could represent the nucleus in your exhibit and measure their diameters.
Creating a Solution Pick one of the objects you measured to represent the nucleus in your atomic exhibit. Figure out how far away from the object you would have to place a marker so that people could visualize the relative sizes of the atom and the nucleus.
Presenting Your Plan Write a proposal to present to the committee that approves projects. Tell them where you would place the nucleus and where you would have to place the marker. Be prepared to explain why your exhibit needs the space you are requesting.
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