Answer the following question given this paragraph: 

As we saw in the previous chapter, ions are formed when atoms gain or lose electrons. If an atom loses one or more electrons, the resulting ion has a positive charge (more protons are present than electrons). If the atom gains one or more electrons, the resulting ion has a negative charge (more electrons are present than protons). Positive ions are called cations, and negative ions are called anions. Because opposite charges attract one another, cations and anions are held together by strong electromagnetic forces. An ionic compound consists of a large three-dimensional array of alternating cations and anions. For example, sodium chloride (NaCl) is composed of Na + and Cl - ions arranged into a structure like the one shown in Figure below .


Q: When atoms gain or lose electrons, what is formed?

 Choices:

- crystals

- molecules

- gas

- ions

A:
ions
