Chapter 4, Section 3

Chapter 4, Section 3

Atomic Number

Atoms are composed of protons, neutrons, and electrons. Protons and neutrons make up the nucleus. Electrons surround the nucleus. How, then, are atoms of hydrogen, for example, different from atoms of oxygen? Look at Table 4.2. Notice that a hydrogen atom has one proton, but an oxygen atom has eight protons. Elements are different because they contain different numbers of protons.

Table 4.2: Atoms of the First Ten Elements

The atomic number of an element is the number of protons in the nucleus of an atom of that element. Because all hydrogen atoms have one proton, the atomic number of hydrogen is 1. Similarly, because all oxygen atoms have eight protons, the atomic number of oxygen is 8. The atomic number identifies an element. For each element listed in Table 4.2, the number of protons equals the number of electrons. Remember that atoms are electrically neutral. Thus, the number of electrons (negatively charged particles) must equal the number of protons (positively charged particles).

4.1 Understanding Atomic Number

Problem-Solving 4.15 Solve Problem 15 with the help of an interactive guided tutorial.