Atoms are built from electrons, protons, and neutrons. Electrons and protons are particles that are electrically charged, while neutrons are uncharged (or neutral). The structure of atoms depends largely on how protons and electrons interact with each other. We need only one straightforward bit of physics as background to understand how protons and electrons interact, Coulomb's Law. If you've encountered Coulomb's Law before, you'll be able to answer the following question easily:

The correct answer is (c). Coulomb's Law describes what happens when two charged particles encounter each other and whether they attract or repel each other. Response (a) is a statement about the speed of light, while response (b) is a statement about Newton's Law of gravitational force.

With Coulomb's Law and a little knowledge about protons and electrons, you should be able answer the following question:

Electrons have a negative charge, while protons have a positive charge, so the correct answer is (a). Answer (b) starts with an incorrect statement, that protons attract each other to hold the nucleus together. Ignoring that problem, the rest of answer (b) might be correct if gravity were a stronger force in atoms than coulombic forces, but it's not. We'll find out soon that electrons do not orbit the nucleus like tiny planets!

Coulomb's Law is the biggest piece of fundamental physics we need to understand what holds both atoms and molecules together. Now that we have reviewed the relevant physics, let's look at the basic components of every atom and molecule, electrons and nuclei.